How do you limit amps for charging?

[Raftman1979]

I built a 12 volt "generator" for a 1300 mile raft trip. I will be using deep cycles for nav lights, deck lights, music, laptop, 12 volt coffee pot, occasional use of trolling motor for tight manuevers in marinas & locks, etc. but I think I will be using too many amps for solar cells to keep up with during the day, plus I'll be running LED lights when I'm moored at night. The outboard motor I plan to use has no charging system. I plan to use the solar cells to simply slow down the rate of discharge in the batteries, but occasionally I'll have to run the generator for a while when the batteries are discharged to around 50%.

The generator is a 78 amp Chevy car alternator with internal voltage regulator. It's run via V-belt by a 3.5 HP Briggs & Stratton. A house-type light switch energizes the field magnets to get the alternator to start putting out.

Problem is that my bank of two 75 a/h batteries together can only handle 25-30 amp charge. Any more amps would charge them too fast. If I downsize the alternator, then I'm working a smaller alternator 100% which is also not good. I need to know how to keep the charging voltage at the normal 14 volts or so, but restrict the amps so I'm not charging the batteries too quickly.

Does anybody know how to regulate the amp flow to the batteries? Do they sell something for this purpose, like for charging your deep cycles off your truck while you're towing the boat home? I think I saw something that you tap into your trailer tail light circuit and it gives you a measly 4 or 5 amps. (about all you should draw off your tail light wiring) I've also seen ads for "womens' jumper cables" that can "jump start" a car via the cigarette lighter sockets in both cars. Obviously, something would have to limit the amp flow or you'd blow the fuses in the cars, right? Maybe I can get something like that. But I don't know how well-designed something like that would be, since jumper cables that connect via cigarette lighters are obviously intended for someone with no automotive knowledge and likely to be a cheap chinese piece of junk. I need something reliable and I'm not afraid to dump a few bucks if I have to in order to ensure it's something I can rely on. I'm more likely to dump money on hardware that I get to keep than to pay a marina for shore power all night.

Any ideas? Suggestions? I've got all winter to figure this out, the trip from Chicago to New Orleans isn't until summer 2009, but due to my job it might be put off til 2010 if I can't get the time to work on the raft.

[CDK]

The Chevy alternator can deliver 78 amps to a load while maintaining 12 vdc. The internal regulator starts reducing the output voltage slightly above 12 v; when the output reaches 14 v the charging current is zero.

If you use the generator only for charging 150 ah battery capacity the system voltage will very quickly reach the 12 v level, immediately reducing the charging current to a safe value for you batteries.

Regulating the charging current is not difficult if the alternator has a separate voltage regulator. With the integrated type it is much harder and requires some modifications, but I think you do not need to worry about it.

[Raftman1979]

I'd like to use deep cycle batteries. If I was using a car battery or a "starting deep cycle" with more numerous, thinner plates, they can handle a faster charge. But I plan on using deep cycles. Maybe if I use 4 batteries instead of 2, together they can handle a high-amp charge. In that case, they would probably handle my power use for several days before I need to charge them. So since I'm taking on extra weight, maybe I should leave the generator at home and charge up at marinas.

Which leads me to another question: Can I get shore power at a marina when I'm just stopping for lunch and a shower, or would I have to pay them for the plug? I don't plan on spending too many nights at marinas but I'll go insane if I have to eat oatmeal and fish for 6 weeks. Every time I've ever visited a marina, I was using a trailerable boat and only used the ramp. I've never pulled into one just to tie up at a transient dock space for lunch.

[Fanie]

If you use a gas burner and gas instead you could save a lot of battery power. Some of the camping gas units are quite efficient and you can go a long way making coffee and heating your oatmeal ;)

How big are your solar panels ?

[Raftman1979]

Well I'm not planning on spending a whole lot on solar panels. The dollar-per-watt ratio is stacked up against me. I can't afford to cover my entire 8'x12' cabin roof with solar panels, and to be honest, I'm kind of worried how well they would hold up against hail damage. I've been researching this trip and others have mentioned that due to the amount of time I'll spend on the river, and the size of the geographic area I'll be covering, I'm bound to run into bad weather at some point.

I'm not sure what kind of solar panels I'll be using, all I do know is that the total wattage isn't going to be enough to run everything. My stereo alone will probably put me over-budget on amps.

As for the cooking, I'll be using propane on the raft, and might bring along a grilling rack that you place over a campfire, for when we're stopped on a sandy shore somewhere. I might boil water over propane for making coffee. But I'd like to be able to use a 12 volt coffee maker. Using electricity to produce heat is highly inefficient, but if I have an adequate charging system, I'll choose convenience over efficiency in that case.

[CDK]

Here is a cheap and simple current limiting solution for you:
In series with the alternator output, connect an array of headlight bulbs of the old duplex type 12V 50/55 Watts or similar. One lamp can handle approx 8 Amps (both filaments used), 4 bulbs give 32 Amps, that's what you aim at I guess.

The trick is that these bulbs have a very low resistance when cold, gradually increasing with rising temperature, so they limit the current flowing thru them.
With a discharged battery connected, they will glow very faintly for a few minutes only.

About solar panels:
Serious manufacturers (Kyocera, Solarex, Siemens etc) use hardened glass that withstands any hailstorm you can imagine. These panels are used on mountain tops to power TV transmitters and communications transponders in remote areas. Two 50 watt panels on your cabin roof won't cover all your power needs, but vastly reduce the need to use you charger.

[Raftman1979]

Yeah, I'll probably just use a bank of 4 batteries and 100 watts worth of solar panels. It should take several days to run them down, then I'll just head to a marina for the night and plug in. I'll probably want to do that every few days anyway.

[Pericles]

WhisperGen

Page 136 http://www.proboat-digital.com/proboat/e20081011/

P

[goboatingnow]

most batteries can accept charge rates of between C/20 to C/5 in your case between 7.5 and 30 amps, have a look at the battery specs. You can charge faster but gassing will occur. IN pratice your chevy alternator will not put out anything like its rated amperere, youll be lucky to get 40 amps. Also teh standard car regulators generally undercharge and put out even less amps.

I;d simply use it as it is. try it and see if you get warm batteries or too much gassing.

The easiest way to regulate an alternator is via controlling the field current. There are a number of marine controllers on teh market see ample power, balmar etc, but they generally require access to the field wire.

[Raftman1979]

It looks like Ample Power might have what I'm looking for. Thanks.

I can access the field wire on the alternator I'm using. I have the field wire hooked to a light switch that goes to the thing's battery. It won't work without the field energized, but it will drain the battery when the engine is off if I don't flip that switch off.

[pistnbroke]

Unfortunatly all of the corespondents above know nothing of car alternators .You do not need to regulate the current which will be nothing like the alternator full output when charging a battery . the voltage regulator is set to 14.4v and that will limit the charge current ...Its the difference between the battery and alternator voltage that determines the charge current . Your charge will be unlikley to exceed 25a ..Hust connct to the batteries and off you go ...thats what you do in a car ... The rating of the alternator is for supplying resistive loads like bulbs HRW etc ..

[FAST FRED]

Before the arrival of 3 and 4 stage automatic charge regulation, the solution was a gadget called an\ T Mac.

Basically all it was is a manual voltage controller , made up from a heavy adjustable rheostat and YOU watched the charge volts , amps and battery temperature ,,, mostly at the end of the charge.

They used to be about $15 worth of parts , perhaps a google search.

FF

[pistnbroke]

It is unfortunate that when people who think they know about alternators meeetd people who actually know about alternators that there is a conflit....justconnect battery direct to the alternator ..the current will be a max of 25A reducing as the battery charges and no additonal parts are required ...bigger alternators do not give bigger charging currents but they can supply bigger resistive loads .......it works fine in a car for years so whats the problem ???

[Ratch]

I agree with pistnbroke. The amount of amps available is irrelevant the voltage is what matters. The load (battery) and voltage determines the current flow. Google 'ohms law' for details. Of course if you don't have enough amps it will take longer to charge but as soon as you reach the point determined by the volts and load any amps available over that is meaningless.

You may still need to limit the current not for the battery's' sake but so as not to stall the engine CDKs idea with the globes is a good one or resistive wire used in heater elements is another way.

[Fanie]

A LA or SLA battery has very low internal resistance. Any charger voltage exceeding the battery voltage will result in excessive current if the charger can supply it.

[pistnbroke]

Sorry Fanie you are making the classic mistake ...yes low internal resistance of 0.002 ohm will allow large discharge currents BUT when charging the alternator has to overcome the at least and rising 12.5v of the battery with its max 14.4v output . the internal resitance is not relavent when you consider the resistance of the connecting cables and resistance of the alternator internals ..this differecnce of only 1.9v soon reduces to zero as the battery charges to its fully charged state at 14.4v...

[robherc]

Here, let's solve the entire argument at once.

1. The alternator puts out 75 amps at closed-circuit conditions (i.e. 0 volts) and 14.4 volts (regulated) at anything from about 0-10 amps.

Therefore, if you still have 6+ volts energy in your batteries (which would be under 1/4 charge), you should still be WELL under 35 amps transferred from the generator due to the lowered voltage differential (6 volts is a lot more than 0).

If you're still worried about it, buy a 15V, 1F capacitor from Wal-Mart (intended to cusion the load of a too-big amp & subwoofer on your car battery) and install it in parallel with the batteries. The capacitor will soak up any excess amperage until the batteries' charge (and thus resistance) comes up a bit & everything balances out. The capacitor will also help smooth out the draw on the batteries during the surge every time you turn any electronic item on your raft on/off.

[pistnbroke]

I think you should delete all that post robherc every word is rubbish

The alternator output is 75 amps at 14v if you put sufficient load on it with say a carbon pile variable resistor. Alernators usually run to 12000rpm so you need at least 6000 for full output..It is self regulating for current and cannot be overloaded. where you might ( ha hal) be confused is that a professional would short out the voltage regualator for this test .

Your battery is considered fully discharged at 10.8v with a 1/10 amp hour load...whats all this 6v and 1/4 charge crap and that capacitor will do nothing ...

so go find your delete button and remove the techocrap

[Fanie]

Sorry Fanie you are making the classic mistake
No classic mistake.
LA batteries needs to be voltage limited or they may be damaged
SAL batteries should be voltage and current limited or they may be damaged

Both is a function of the charger you use and for the battery type.
An alternator's composition / size & ,materials used determine the max amount of power it can output. It's like a small 24VA transformer can say deliver 2A max at 12V, a 240VA transformer can deliver 20A at 12V. If you add a regulator on both then the regulator would determine what gets put out in terms of voltage and current, if it is designed to do it.

this differecnce of only 1.9v soon reduces to zero as the battery charges to its fully charged state at 14.4v
It is during this time you call 'soon' that the current may be excessive. A large battery may take quit long to get it's voltage lifted, and in the case of a SLA you can damage it if you put an alternator on it.

Alernators usually run to 12000rpm so you need at least 6000 for full output
BS. Alternators are designed to de able to supply full power (voltage and current) at much lower rpm. When you rev the car engine up, only the frequency of the three phase increase.

[pistnbroke]

I think you are on a different techo planet ..jump of and return to earth ...alternator ok for all types of battery you will find on a boat . What your are quoting is techo crap and no help to anyone ...please stick to what you are qualified to comment on or you just confuse people and cost them money ....

Enough qualifications on auto electrices to shut down this site !!!

[Ratch]

Have an idea :idea:

If you add 2 diodes in series and assuming that the alternator is set for 14.4v then you have reduced the charging voltage by 1.2v to 13.2v and this is normally what is used for float charging. Having the diodes in circuit while bulk charging will also lower the amount of amps that can be delivered to the battery due to the lower voltage.

I still think you can hook the alternator up directly to the battery without worrying about all this stuff.

[CDK]

Sorry pistnbroke, you're not only rude, but also wrong. Fanie is correct.
The 14.4 v. threshold from an ordinary alternator can and will damage gel batteries. When charged with more than the current printed on the label (1/10 to 1/20 of the nominal capacity), or over 14,0 v. the safety valve opens because of the internal gas pressure. This ruins the battery and if you return it, the supplier will waive any warranty claim because you maltreated the battery. I learned this the hard way by connecting 20 pcs. 12V/10Ah in parallel with over 400 Ah of ordinary LA batteries. All 20 died within one month, the wet batteries were still in good health..

[robherc]

I think you should delete all that post robherc every word is rubbish

The alternator output is 75 amps at 14v if you put sufficient load on it with say a carbon pile variable resistor. Alernators usually run to 12000rpm so you need at least 6000 for full output..It is self regulating for current and cannot be overloaded. where you might ( ha hal) be confused is that a professional would short out the voltage regualator for this test .

Your battery is considered fully discharged at 10.8v with a 1/10 amp hour load...whats all this 6v and 1/4 charge crap and that capacitor will do nothing ...

so go find your delete button and remove the techocrap
WRONG, and you PROVED IT YOURSELF! Go back to High School and quit trying to insult people who actually have a CLUE what they're talking about!!!

1. If you "short out" your voltage regulator, and put a high enough load on the alternator, the potential voltage drops to (nearly) 0...thus giving you 75 amps at (nearly) 0 volts...check your High School textbooks....somewhere around chapter 1 or 2 "Voltage and Current" and "Ohm's Law"

2. Anytime you wire a capacitor in series with a battery, the capacitor does a LOT of power-conditioning for the battery. It absorbs excess charging voltage to smooth the current taken up by the battery at the beginning of a charge, and it is the first to give out current/voltage for the high-draw "surge" that almost all appliances (even the LEDs he'll be using at night) require to reach their peak efficiency immediately after having been energized. (check your High School textbook again, I'm sure you'll be interested in chapter 5 "Capacitors" and chapter 7 "Initial Loads")

3. I don't know about "deep-cycle" betteries' power levels, but I know I've started my car (over 100 amps to crank it) off a 550CCA battery that was reading below 10V (about 9.8V actually), and it was about -3C out when I did it, so somehow, the <55CCA you're saying it should've had left almost a full Voltaire higher than that doubled itself in a lower-charge battery! (were you thinking LiIon? 'cuz for NiCD, and NiMH the voltage at discharged state is even lower than for Lead-Acid, and I've seen Lead-Acids work SOME down to 8.5-9V...just not well at all)

4. My Favorite "...with a 1/10 amp hour load" now that's just precocious! How, EXACTLY, do you measure a 1/10 amp hour load...with a time machine?!?

I think you are on a different techo planet ..jump of and return to earth ...alternator ok for all types of battery you will find on a boat . What your are quoting is techo crap and no help to anyone ...please stick to what you are qualified to comment on or you just confuse people and cost them money ....

Enough qualifications on auto electrices to shut down this site !!!
Wow, can you just shut up here please? You're beginning to offend the last few examples of intelligence in the "great" human race! YOU are the one confusing people, and I PRAY they have the sense not to listen, or you'll cost them money for sure.

Now, for everyone else:
With 2 deep-cycle batteries in parallel, I think he SHOULD be fairly safe running the alternator. If there are still any concerns, he could simply use a charge controller, or a capacitor, or almost any on the ingenious (if a bit unconventional) ideas I've seen in here. Most of your ideas would work to control the current, but as he already said he doesn't mind paying for a high-quality solution, I think "correct" answer to the original question here would be: "What is a Charge Controller" ... available at any marine supply store/website.

P.S. Sorry everyone else had to listen to that rant, I just couldn't handle reading the nonsense & insults spewing from that individual anymore. :eek:

[robherc]

CDK, thanks for the useful info...I'll think about that & see if I come up with anything else that might help here. :-)

[pistnbroke]

Unfortunatly misunderstanding of alternators and batteries is a problem workld wide.People get very set ideas based on folklore . My knowledge comes from 30 years as a consultant on auto electrical equipment to car makers and canal boats. I have enough qualifications both low level through to degrees in this subject and dont tolerate technocrap....

I/ you should not be connecting batteries in parallel as one runs the other down ..you can get away with it for a time if they are both new and identical and used on a daily basis....

2/ a lead battery is only fully charged when it gets to about 15-15.3v . Unfortunatly it gasses at 80% charged so modern alternator limits to 14.4v. Thus what we call fully charged is in fact only 80% compaired to the odd days of dynamo systems..when we topped up the battery with water every week !!

3/ putting two diodes in series is not a good idea in most circumstances as this drops the voltage to 13v ( 14.4-1.4 ) which is a bit low . If you only charge your battery to 13.6v it will only last 6 months as it will be sulphated for most of the time and this will become permanent sulphation. I cold see one diode being usefull for float charging a small gell battery say 12v/10ah but no good for a starting/aux battery.
4/ if you put a capacitor in series with your battery ( did you mean parallel) then once the capacitor is charged (seconds at most) you will have no charge current...two metal plates seperated by an insulator in series ...??? eh?
4/ the details of load testing the alternator come from the alternator manufacturers with whoom I have worked for 30 years ..you put sufficent load on to pull 75a at 14v and you need plenty revs
5/ I would not use an alternator to charge a 12v 10 ah battery as inital currents could be high but as an alternator was not designed to charge small gel batteries its not important ...anything over 30 ah will be fine.
6/ If you connect 20 gel batts in parallel it only takes one weak one to run all the others down and they dont recover well if over discharged..sillly idea anyway ..must have cost a fortune....
7/ a current of 1/10 of the amp hour capacity means to those qualified in this subject that for example a 100 ah (10 hr rate ) battery would be charged at 1/10 of 100 ah in amps eg 10 amps ...its quite common to quote the charge current as a fraction of the AH capacity in amps

1/10 of the AH capacity in amps woud be a normal charge rate for a fully discharged battery and take 14 hours to charge it

perhaps a trip to the libary for a good auto electrical book would be a good idea for some of you

[Landlubber]

pistnbroke,

Well mate, it seems that the arm chair admirals are at it again......i read many articles on this forum, 99% are crap, there are some very good and knowlegable people here (PAR, Guillermo, Rick in Melbourne etc etc), and there are many comments by people that have obviously had no real boating experience other than a ride on the harbour ferry to visit Grandma.......

1/10 of the capacity has long been the best maximum "normal" rate............

[pistnbroke]

thanks for that Landlubber when I first found this sight I thought great a lot of knowledgable people but as you say 99% write crap to quote yourself ( moderator note that )

[CDK]

Quotes from post #25:

"I/ you should not be connecting batteries in parallel as one runs the other down ..you can get away with it for a time if they are both new and identical and used on a daily basis...."

Solar systems often need 1000 AH or more storage capacity to bridge cloudy days. The only way to do that is parallel wiring. According to your statement, if one cell in one battery fails, you should replace all of them at the same time. But you would be throwing away a lot of good batteries then.
The university in Amsterdam where I studied 45 years ago, had a battery room with several hundred single sell glass batteries, each the size of a large aquarium. Back then they did not even throw the one away that failed, but replaced just the single plate or separator. Most of the time draining and removing the sludge was enough to bring a battery back to life again.

"2/ a lead battery is only fully charged when it gets to about 15-15.3v . Unfortunatly it gasses at 80% charged so modern alternator limits to 14.4v. Thus what we call fully charged is in fact only 80% compaired to the odd days of dynamo systems..when we topped up the battery with water every week !!"

Battery voltage is temperature dependent. To determine the state of charge you need to test the specific weight of the electrolyte. Alternators set at 14.4 volts without temperature compensation cause damage to modern sealed car batteries in a warm climate. Companies like Varta and Exide have reduced their aftermarket warranty period in the Mediterranean from 3 years to 12 months.

"6/ If you connect 20 gel batts in parallel it only takes one weak one to run all the others down and they dont recover well if over discharged..sillly idea anyway ..must have cost a fortune...."

These were new 12V/10AH units, left over from another well paid experiment. I used them in a solar system with a Siemens microprocessor charge controller. These devices must be programmed and have different settings for open, sealed and gel batteries. Because the majority was wet, sealed I did not change the settings. Unlike wet batteries, the gel types do not offer much resistance to an excessive charging voltage because the electrolyte cannot freely move around. Instead, hot spots form, followed by a gas bubble, leaving a 'dead' spot on the negative plate. That is why these batteries only reach their designated lifetime when both current and voltage are monitored. A crude 14.4 volts alternator quickly and silently kills them.

[robherc]

If you're still worried about it, buy a 15V, 1F capacitor from Wal-Mart (intended to cushion the load of a too-big amp & subwoofer on your car battery) and install it in parallel with the batteries.

4/ if you put a capacitor in series with your battery ( did you mean parallel) then once the capacitor is charged (seconds at most) you will have no charge current...two metal plates seperated by an insulator in series ...??? eh?

perhaps a trip to the libary for a good auto electrical book would be a good idea for some of you

Perhaps READING my post before going off half-cocked would be a good idea for you???

[Fanie]

The original question was how to limit amps for charging.

For LA batteries a current limit is not required. LA batteries love the high current bump at the beginning of the charge and the current falls off when they reach 13.8V to 14V.

SLA batteries are different from LA batteries and they need to be voltage and current limited. SLA batteries wants to be charged to 14.6V while their current gets kept below the spec.

An alternator is not really a good option to charge SLA batteries with. Even if you can adjust the output voltage to 14.6V you still have the current limiting to consider.

Whichever way you look at it, SLA batteries needs some electronics to do their charging with.

Despite their seeming drawback with the charging, SLA batteries are in my opinion much better than LA batteries. They are maintenance free, always clean, charge up faster than LA (despite the charge current limit) and if looked after gives years of good service. My personal choice.

Both LA and SLA's doesn't like to be left discharged for some time. SLA's have a better shelf life than LA, but they should be kept fully charged.

Many boaters buy small solars for their boats for only one purpose, keep the battery full. Good idea if the boat is not attended to or used full time, and even then...

So in my opinion an alternator is not good for SLA's.

[masalai]

Simple instructions here http://www.amplepower.com/ps_reports/index.html - - and to introduce the topic in a way most should understand? - - http://www.amplepower.com/pwrnews/beer/index.html - -

[pistnbroke]

Rodec ..perhaps you should read your post no 23 item 2 .above..says SERIES
As for Masalai ....no need to read that stuff I wrote it for them......

any way getting tired of all this ...so many who cannot see the wood for the trees....

To Summerise

Any battery suitable for starting or supplying Aux loads can be charged direct from a vehicle style alternator . outputs vary a little by make usually 14.4-14.7v .charge currents unlikely to exceed 25 A more likley 15 A and falling.
Small gell batts can be floated by putting a diode in series to drop the volts by 0.7v but check the label and your alt output volts ..might not be neccessary ...

All said

[apex1]

Simple instructions here http://www.amplepower.com/ps_reports/index.html - - and to introduce the topic in a way most should understand? - - http://www.amplepower.com/pwrnews/beer/index.html - -
Now kindly be quiet and do some reading (wanking is for when in bed and alone)

Not bad that advice.............:) may I recommend a better?:D

http://www.victronenergy.com/upload/documents/Book-EN-EnergyUnlimited.pdf

and........without being tooo rude..... almost all of the audience providing vicious remarks above, are well advised to have a close look!:D

@ pis..and.broke
Any battery suitable for starting or supplying Aux loads can be charged direct from a vehicle style alternator . outputs vary a little by make usually 14.4-14.7v .charge currents unlikely to exceed 25 A more likley 15 A and falling.
Small gell batts can be floated by putting a diode in series to drop the volts by 0.7v but check the label and your alt output volts ..might not be neccessary ...

All said

Yes... all said.................... much wrong!:!:

Regards
Richard

[masalai]

apex1, Have downloaded the pdf and having a good read (72 pages) - did you write that too pistin? - I'd have thought looking at the beach babes up there would be a 100% distraction? but is there a bar where one can get beer at reasonable prices whilst watching? :D:D Enjoy the good life sir, I'm only jealous....

[apex1]

apex1, Have downloaded the pdf and having a good read (72 pages) - did you write that too pistin? - I'd have thought looking at the beach babes up there would be a 100% distraction? but is there a bar where one can get beer at reasonable prices whilst watching? :D:D Enjoy the good life sir, I'm only jealous....

And.... was it worth reading? ;) have I won the T-shirt?
Our beach babes here in the northern baltic region I think are amongst the most attractive worldwide! And usually nude........
So I am not sure if you can focus on your beer. If it comes to prices and quality, you know Germany has the lowest prices for food at the highest quality level worldwide?!

-did you write that too pistin------ yes mainly;) but everyone is invited to feel recognized.

Regards
Richard

[Frosty]

Pistnbroke is closest. Only I don't agree with 14.4 I set to 13.8 but it could be temperature thing.

You dont have to interfere with the current crossing the slip rings IE feild current. you can put a pot in the voltage sensor wire to the reg. This will fool the reg into thinking the batt is low and boost up the alt output.

It will need constant monitering though keeping voltage down to what ever you want.
With a system as this it is possible to shut down all systems and take them up to 15-16 to clean and de sulphate them up. But you gotta know what you are doing.

If the pot is turned off it will be back on auto again.

Handy for fast charging running the main engine.

Errr--- lead acids only.

[pistnbroke]

13.8 is a bit low .and represents only a 50% charge ..batteries charged to 13.6 only last about 6 months...no doubt you are compensating for your low charge voltage with occasional boosts to 15-16v.

remember that 14.4v is only 80% charged ie 20% sulphate and you need to go to 15.5v for 100 % charge ...problem is the battery gasses and needs weekly topping up like a dynamo charged battery on a car ( 15.3v)

please note that the slip rings carry ROTOR current not field current thats a dynamo term.

Of course you need an alternator regulator with a voltage sense wire to do your variable resistor trick ( most have no seperate sense wire ) and you cannot lower the voltage regulator setting only raise it

[Raftman1979]

I have an electronic "smart" charger. It gives a digital readout and I can switch between charge percent or voltage. While charging, it gets up around 16 volts or so sometimes, but when it reaches "100%" and automatically kicks off, I let it sit for an hour or so and the battery is showing 13 point something volts. I've never seen voltage over 13.6 or so on a battery that wasn't receiving a charge.

I guess when it comes to batteries and charging, if you ask ten guys, you'll get ten different answers. I guess I'll just use my idiot "smart charger" at home and my "I'm too stupid to open the hood" jumper cables that charge a car via cigarette lighter sockets for slow charging from a high-amp source. And if my batteries last 3 years instead of 4, I'll just buy new ones.

[pistnbroke]

thats fine for wet car type batteries but too high for sealed types.. If you stand your battery overnight you will find the terminal voltage drops to about 12.6 ...anything below 12.5 and the battery is suspect ....nice to hear from you

[catsketcher]

On my cat I used to run a wire from the field coils of an alternator to a rheostat. The alternator was run from a belt that ran off the top of the outboard's flywheel. The long belt would moan and the kids would shout that they couldn't do their homework - I told them I needed some amp hours in the battery and I wanted to kick the 400 AH deep cycle battery bank in the guts. Rheostat was set low . I would pike out at about 30 Amps. Then about the third time the belt jumped off, grabbed the alternator and flung it out the engine nacelle. The family cheered. I was too scared to do it again.

So then my mate gets a small diesel and does the same for his lovely cat. He is a sparky and just watches hydometer level and voltage - decides he wants to shove lots of charge in and winds up the field coil level. Top up comes from solar but the majority from the alternator with fiddled field coils given a boost. Like me he tried the normal alternator but it is really dispiriting to be having an engine on and seeing the current drift downwards quickly as the voltage of the battery comes up.

I hope I am not doing something really silly. I want to make another charger out from a proper generator (they are cheap now) with the 240 generator taken off and an alternator put in its place. With no calculations I hope a big alternator will put out some charge at crankshaft revs although I could use pulleys.

I am a little tired of being a mega amp miser - that is the reason for a dedicated fast charger on the cat. My batteries were amazing though. Bought in 1999 - I didn't charge them past 13 volts till 2001 when I finally got the good oil from an electrician. They only died last year. 6 volt deep cycle - lived aboard three years.

Cheers all

Phil

[catsketcher]

By rheostat set low - I meant low resistance - high current.

cheers

Phil

[white840]

You can use SCC2 is a solar charge controller, regulate the power flowing from a photovoltaic panel into a rechargeable battery. It features easy setup with one potentiometer for the float voltage adjustment, an equalize function for periodic overcharging, and automatic temperature compensation for better charging over a range of temperatures... I know a site http://www.zigbee.eu/ that will help you..

[thudpucker]

I've been working on this for awhile. There is some hope out there. My idea was a Weedeater motor, but I think the 3.5 B&S is a better deal.

Among the things I learned so far: the GM single wire alternator is the best. The charge rate is very well controlled.
If you want to monitor; put an Ammeter in between the Battery Plus Terminal and the Charging source. That will tell you the rate of charge. It will be high at first, and as the Regulator shuts down the field voltage the current will fall off to nothing.
When the charge starts up; a Voltmeter across the battery will tell you the Amount of Voltage in the battery. When its at full charge you might see as much as 13.5 but you might see the rate of charge go up to 14.2. It wont stay there though and it don't seem to hurt the battery.

One of those links has a good schematic for what your trying to get.

http://theepicenter.com/tow02077.html

and;

http://theepicenter.com/tow082099.html

and;turbo-charging-gas-engines-weedeater-genset.jpg
33804

[pistnbroke]

I have read what you have written and so that people get properly educated ......please absorb the following

The battery will not be at 100% charge until it reaches 15.3v ..but it will be gassing
the normal 2009 ideas of full charge is 14.4v which is the voltage beyound which it starts gassing ( no maintanece required ie topping up ) but is only 80% of true max capacity
Alternators were not designed to have ammeters which drop the sensed voltage to the alternator and reduce charge rate/voltage

If you charge a 100ah battery at over 25 a it will gass and get over hot ...

your weed eater is too weak to drive the alternator but you must gear the briggs and stratton (3600rpm ) to drive the alternator at 2-3 times that speed for full output ....



if this was usefull please leave rep if not ....ok

[thudpucker]

It is useful. Almost too much info in fact.
I wondered about the GM alternator's minimum and maximum charging Rpm?

The Weedeater will go way up in Rpm, maybe as high as 8000, so if the GM has to run at 2500, it means we can have about a 2" pulley on the Weedeater and a 5 or 6 Inch pulley on the alternator.
The Weedeater should be able to handle that if you let the Rpm build up before you connect the Charge sense wire to the load.

The B&S will do fine with a load like that at 3500 Rpm. It has good torque attitude and good crank oiling.
So if the B&S will be happy with 3500 and the Alternator at 2500, the pulley would again be smaller at the B&S engine.

How about that? Am I in the ballpark there?

[BHOFM]

A 3 1/2 hp motor will not produce 78 amps at 14V!
Find a 55 amp alternator with a built in regulator.

I think the motor will only do about 30amps and you
need to spin the alt. at 2X engine speed.

We built these little set ups for boosters for race cars.
A 5hp is the preferred engine. with a 2X drive, it will
almost turn 3600 at full throttle.

You can easily limit charging by adjusting the rpm of
the engine. You just need a simple amp gauge in the
line.

[thudpucker]

A 3 1/2 hp motor will not produce 78 amps at 14V!
Find a 55 amp alternator with a built in regulator.

What do you mean? Are you saying a 3.5 don't have the torque to run an alternator fast enough to produce 78a @ 14.4v?:confused:

I think the motor will only do about 30amps and you
need to spin the alt. at 2X engine speed.

2X speed, Is this from experience? Did you ever document that or just ran it fast to get the voltage you wanted? I'd be interested in knowing just exactly what Voltage the GM single wire is capable of at the minimum and maximum Rpm's.

We built these little set ups for boosters for race cars.
A 5hp is the preferred engine. with a 2X drive, it will
almost turn 3600 at full throttle.

The B&S run at 3600. so you had a four inch Pulley on the B&S and a Two inch pulley on the Alternator?
Do you have any photos? I'm very curious!:)

You can easily limit charging by adjusting the rpm of
the engine. You just need a simple amp gauge in the
line.

We'd sure like to see some photos of your pretty good sounding effort!:)

[BHOFM]

I have no photos. We tried the smaller motors and they
pulled down badly.

We were spending, in the late 70's about $200 on these.
The 5hp was still light enough to handle on a small two
wheel dolly with the alt. mounted and a small battery.

These were to boost race cars, not really to charge batteries. They were very high compression engines.

We did find that turning the alt. too slowly caused some
over heating problems. We never had the problem when
we went to the 2X drive.

This was before the days of the booster pack you can
carry around.

Check this:

http://www.delcoremy.com/Alt28SI.aspx

[pistnbroke]

alternator starts to charge at about 1500 rpm producing about 8 amps...full output at about 6000 up max rpm usually about 12000 so 2 to 1 up gearing on your briggs should be fine ...or try it direct axial drive as you will not acheive more than 25a and falling charge current however big your altrernator is ...that is if you want it to charge a battery rather than supply lighting loads 100 A at 14v is 1400w thats 2 hp and at 50 percent efficiency you will need 4 hp to drive it ....forget the weed eater even if it works it will not last and makes a lot of noise...

The guy on the raace cars is not trying to do what you want ...he had the voltage regulator shorted out and was only interested in amps to drive the race car starter ...

you must not connect or disconnect anything when its running or you will f44k the alternator and you always need a battery connected

Forget the idea that a 75 A alternator charges a battery at 75A it does not ...about 25A falling for the battery and the rest for the lights air con etc on the vehicle ...

[thudpucker]

OK, I think I see the light now. :)
Thanks all and especially Piston broke for the technical data.

[pistnbroke]

Thanks for the rep thudplucker .....If you have a 3.5 briggs from a mower ..mount the alternator under the mower engine straight onto the shaft ,,makes a very compact unit or run the belts in a box under the motor with the alternator along side ,,you can make a trial set up using some 7/8 ply and blocks of wood at the ends to keep it off the floor ...gets your dimentions and pulley set up right ...
use a standard alternator then you can strart the briggs and switch on the ignition to bring on the load ... if you can get a crank and camshaft gear from a small engine you will have 2 to 1 and could use a toothed belt .

[DaveJ]

With my limited knowledge in this field (I'm avionics, jack of all trades, master of none) i have to agree with alot of what pstnbroke has said, albeit not the method of how it was delivered, but i do get frustrated as well when people make opions not totally knowing all the facts.

Not seeing the schamtics for a car alternator, its hard for me to comment whats really happening inside, but understand engineering principles, and i would have hoped that they design failsafes into the retifier/regulator, so that in the event a terminal jumps off the battery it will be able to deal with this and not selfdestruck, as i have disconnected the positive terminal to prove the charging system is working on afew occations without any adverse effects. So wondering if i just fluked not destroying the alternator or not. The current rating for the alternator is its max output current that it can provide, once again i would hope they have in place current limitors to protect the alternator from exceeding this current, and i do see it, friend has a large sound system in his car that when he cranks it up, the sounds system starts to steal all the power from the alternator and starts to drain the battery (hence thats why they tend to run a second alternator and battery system). The indication he gets is the charge light comes on.

I think alot of people are forgetting, or not sure what is happening in a battery, in theory a lead acid (any lead acid) will last forever, but we all know in practical terms this not not true. I notice some of the number being quoted are based on theory, and practicality proves these numbers wrong, you have to remember, these things are man made, anything man made is dodgy.
A cell in a lead acid battery will provide 2 - 2.2 volts, thats how it is designed the combinatoin of the metals in the plates and the sulphuric acid determin this. You shove 6 cells in series and you get your 12 v battery (well dah).
Its this a chemical reaction that happens where the lead is removed or replaced into the the lead plate, where you get the power from a battery, but because it is man made the reaction deteriates over time and stops, your battery becomes dead.
For the chemical reaction to happen there needs to be enough overvoltage to charge the plate with lead again, this is roughly 12 - 13.2 plus the voltage for the electrolysis action needs to make the chemical reaction to happen. So if the plate has no lead in it, the extra voltage required for the electrolysis to happen is low (this is seen as low internal resistance), as the plate charges up with lead the required voltage increase and the internal resistance increase until the internal resistance overcomes the avaliable voltage and the charging cycle stops. The gassing is the electrolysis action happening, the amount of gassing is based on the current that is used for charging, if there is too much current the reaction will happen too fast and build up too much heat. But because the reaction is happening fast the eletrolysis can't keep up so the plus voltage for this action increases raising the resistance to a point of equriblim (now i know there is an uproar, i hear people yelling out thermal run away) Yes thermal run away is a problem, but only if the voltage and current are allowed to esculate out of control, but because the alternator is locked at 14.4 v this occurance doesn't happen. This is why i agree with pistnbroke about you don't need anything to charge the battery, as this happens all the time in a normal car.
There are alot of factors that make up the battery, the materials used in the plates, how much sulphuric acid is in the electrolite and what not, which determines the charge voltage and current, which is all aim around getting the longest life out of a man made product.
The amount of current is proportional to the surface area of the plates, the bigger the plates, the more current can be provided also the more current that needs to charge it. Also the quality of the materials used in the plates manufacture determines the lenght of service.
But what i believe the biggest thing that is overlooked alot of the time is the wires connecting all this crap up. You can have the best battery and alternator combo but if your cables old and tarnished, this can really effect the charging cycle of your battery. I use a product on the terminals that keeps the mositure out, stop the electrolite build up on the terminals and keep good contact between the terminals and terminal lugs. I believe this is major reason i got 10 years of out a marshal battery for my commodore.

Dave,

P.S. Now i know i'm not the be all and all about this topic, but hopefully i have shed some light on whats going on. Of course, if i'm mistaken, i'm all too happy to recieve criterism.

[thudpucker]

Well I cant be the one to throw the first stone. I have an AA in electronics. I'm amazed at what other people know that I dont.

I had one more little tid bit from my Dad who never understood electricity at all.
When he thought he had an electrical problem, he hung a Hacksaw blade in front of the Generator or Alternator.
If it was charging, the blade was attracted to the armature. Now aint that simple?

DaveJ I had occasion to put some intercoms into a few C-150's. I was pleasantly amazed to find everything in those old airplanes was in great shape.
Because in Aircraft, no hackers, no cheap Chinese wires or terminal ends are allowed. Everything is certified and with that, nothing is intermittent unless it has mechanics tracks written on it.
Not like our boats at all. :)

[pistnbroke]

Well there is no protection for the diodes so if you pull a lead off and the charge current is high its fineto baby

The output current is regulated by the cross section of the core of the stator ...usuallly quoted as the alternator is self limiting for current ...

Whats an AA in electrice ....I know what a degree is and HNC and all those city and guilds in automoblile diagnostics because I got them and more but pray whats an AA ???

[DaveJ]

Accually Thudpucker, i do remember that, but with a screw driver.

As for aircraft, yes there is way more control on what goes into them, I guess alot people don't know that when rating the size of wire for an application on the ground totally changes when you go to put it into an aircraft, as you go up in altitude, the current handling of the wire drops off. When designing wiring looms, you try to make the wires one complete lenght as much as possible, putting splices in (even the sealed type that are used in the aircraft) is introducing another failure point.

[thudpucker]

Well there is no protection for the diodes so if you pull a lead off and the charge current is high its fineto baby

The output current is regulated by the cross section of the core of the stator ...usuallly quoted as the alternator is self limiting for current ...

Whats an AA in electrice ....I know what a degree is and HNC and all those city and guilds in automoblile diagnostics because I got them and more but pray whats an AA ???

AA: Associate of the Arts in Electronics Technology.
It means I have a 2 year degree in Electronics.
I have been exposed to everything from AC and DC physics, to the (1977) latest in Busy-chip development and even some machine level programming in Intel's 8080 processor.
Among the most interesting years in my life was the three years it took me to get that two year degree. I didn't finish all of it though. "Report Writing" and "public speaking" was part of what I did for a living at the time so I skipped that.:p

We'll never get rich doing what we do, but on the other hand, we'll never have a boring day as long as we have work!

[lamiataspaska]

I have a similar problem like Raftman1979. I have a sailboat with 6 gel batteries 80Ah each. I had them all connected in parralel and for a while I thought that everything is working fine. One day I found the batteries almost melted, they really looked as if they've been in a fireplace for a few minutes, two of them were almost round. The alternator also shows 17V.

Now I have them replaced with new ones, but I am a bit afraid to put them in the boat with the same setup.

My engine alternator is rated at 50A, my electronic charger is rated 25A. When I used to charge them with the charger, I noticed that the charging never stops (maybe 480Ah is a bit too much for the charger to detect when they are "fully charged"). When I charge them with the alternator (which was very rare) I didn't use any amp limiting devices.

So the question is if the "too big" battery bank burned my alternator and then it overcharged the batteries? Or did my charger do something wrong to them?
In general is there any problem connecting 6 batteries? Should I separate them in two groups?

[pistnbroke]

for a 480 ah pack alternator should be rated at 4.8 x 25 A which is 120 A . this will give you the shortest charge time . So a smaller alternator takes longer . Your 25 A charger is littlte more than a trickle charger in this situation.
Your problem arrises from the failure of the voltage regulator in the alternator which is normally set at 14.4v. You should do two things ,,,,always check that the warning light is fully out on the alternator ( no glow) and fit a voltmeter so you can always be sure your batteries dont go over 14.4v

I would also check with the maker what is the max voltage for these batteries as some are 13.8v. You could fit a second alternator or a bigger one .It is only a problem leaveing the batteries connected in paralled if they are not charged for long periods. I would be charging them monthly if they are not in use .

[lamiataspaska]

If the batteries have too low resistance and draw too much amps, can this damage the alternator or the volt regulator, or is it just slow charging? The batteries are for 13.8, but I thought it is not such a big difference between 13.8 and 14.4 (learned that I was wrong the hard way). If I disconnect the batteries from the alternator with a switch and the engine is still running, is it ok with the alternator going to keep running without any load?

I can change the alternator to a bigger one, but on the other side it charges the starting battery as well and it is only 60Ah. Isn't it going to fry it with 120A?

[Fanie]

Even a low current will overcharge a battery if it is charging with overvoltage all the time. This is because the internal resistance of a battery goes higher at rated voltage, hence allow the battery voltage to keep on going up.

The alternator should supply to it's max current untill the battery requires less current. I doubt they should get damaged.

[pistnbroke]

lamitaspatia does not know anything about alternators or batteries ...you cannot overload an alternator as it is self limiting for current and there is no way to damage a voltage regulator in normal use ....unless of course you are pulling leads off when its charging ...from a normal alternator regulating to 14.4v you will get a max of 25 A per 100ah of capacity .....if you want to know how to charge starting and house batteries from two alternators its very easy and the whole current capacity of the two can be used to charge the house battery once the start battery is up ...all totally automatic and very cheap.

[thudpucker]

The Alternator works as an Electro Magnet.
The little 3 volt circuit provides the Magnetism to make the voltage.
The 3 volts gets to the Alt because the circuit watching the Battery (the Regulator) sees a "Lo" condition.
The 3 Volt circuit is 'adjustable' and will be shut off when the circuit watching the Battery Voltage sees a "Hi" condition.

SO....the simplest answer to your question on controlling the charge is an Alternator with an internal Voltage Regulator, or get some fancy Voltage sensing circuit, made up by a Tech that will take the time to make sure his work is not fatal to something.

The problem with a series of batteries and ONE Voltage source is this.
Suppose four batteries. Three are good. One seems to need a charge all the time.
The Voltage regulator will keep charging, trying to build up that weak battery.
Meanwhile the good batteries are being "Cooked"

OR....One Alternator/Regulator, and a Rotary switch to send the charge to the battery your Volt Meter across Each Battery tells you one is low.

It gets complex if all the batteries are connected to the same circuit so you have to know to do all this Testing and Recharging after the 'end of day' when those batteries are disconnected from the system.

What your asking for just wont be automatic and Robotic.

[pistnbroke]

" The little 3 v circuit " I have been an expert on alternators for 30 years and never hard such rubbish.

" problem with a series of batteries" God the batteries are in parallel..

"the good batteries will get cooked" NO NO the bad battery will get cooked

" what you asking wont be robotic or automatic" well it is on thousands of british Narrow boats and other craft around the world.

Thudpluckcer best stick to your chosen subject .....

[thudpucker]

The post is for a guy who don't know how to do what he wants to do.
So I made it simple for him to understand.

The (tickler circuit) 3v circuit (which is varied by Time) you've never heard of is the small voltage across the coils to make the magnet for charging. surprised you didn't know about that. It's pretty common and has been in use since the Generators first went onto Vehicles back in the 20's.

It was controlled by Relay points back then, but more recently Transistorized switching circuits.

I did not say batteries in series, I said what I meant, a series of batteries, as the poster defined, he has four IIRC.
To control the charge to each battery separately can't be done if all the batteries are connected.
If you think other wise, I'm your best Skeptic.

His original question was how to, or what was involved to control the Charge. I put it in simple terms for him.
Splitting hairs and slander is not helping the poster.

[pistnbroke]

sorry thudplucker your post is technically inacurate and unhelpfull to the guy....they did not have alternators in the 1920s thay had dynamos ( yanks call generators) and the concept of excitement is totally different .... best delete your post.

[thudpucker]

Here's a little story from the past to explain that "Tickler" circuit.

Way back the Generator had a separate relay the old guys who didn't understand why, just called it the tickler. The points in it chattered all the time. "What the heck's that thing doing?" my Uncle used to say.

So the Tickler points would stick sometimes and really make the Generator charge about to it's max output and burn out lights and other damage not to mention cooking all the water out of the Battery. Usually one end cell.

The Generator is trying to charge the Battery all the time. When ever those points are closed, current is flowing from the Generator to the Battery.
Something has to control that. Eventually pretty sophisticated Voltage Regulators came into existence.

But back then.... to monitor what that Tickler was doing, some smart guys put a Head Light bulb, in those days, and an old Heater Core Rheostat in the line between the Tickler and the Generator field.
So now, you see the Bulb glowing or flickering weakly as the Generator is working.
When you wanted it to charge more you opened the Rheostat a bit and more current would flow. The Headlight bulb would get brighter.
If you wanted the Charge to pickup, like you were going to run an electric winch etc, you just turned the light bulb on a little brighter.
That was real common in the 20's on Machinery and Boats.

[thudpucker]

sorry thudplucker your post is technically inacurate and unhelpfull to the guy....they did not have alternators in the 1920s thay had dynamos ( yanks call generators) and the concept of excitement is totally different .... best delete your post.
Sorry Pistonbroke, my post is technically correct, your interpretation is incorrect.

[pistnbroke]

Its 2010 not 1910 ..beleive me my friend what you are saying is of no help to the guy and I am problably the only person reading this who knows what you are on about ...

you are mixing voltage regulaton with current regulation by sloppy use of terms ....regulation of both voltage and current were neccessary back in 1920 but no current regulation is required with an alternator

[thudpucker]

Enough

[Submarine Tom]

To the original question/post: Get a smaller alternator.

-Tom

[pistnbroke]

Tom please dont join the other guy ..I have told you you need a 120A alternator to charage the bank in the best possible time...and of course keeping the batteries fully charged is he way to a long life ....a smaller alternator will only take longer to charge them and by modern standards 50A is small.. Its the voltage of his regulator going to 17v that has let him down ...
There are 3 systems for this
Use one alternator and switch it to the house bank when the starter is charged.
dont like that as you always forget.

Modern systems use two alternators ..the simplest is to have one for the house and one for the starting ...No problem but you do waste the current available from the start alternator once it has charged the start battery.

the modern way is to use the surplus current from the start alternator to help charge the house batteries ....you need one of these and its automatic.(see pic)

A VSR is not so good because when it connects in the house battery the starter battery discharges into the house battery which tends to defeat the objecet.

[lamiataspaska]

I have some thing that makes the start battery charge first, and then it switches to the house battery bank. What I plan is to devide the bank in two and have a switch to choose which half to use. Those switches have a state 1, 2, 1+2, and unfortunately for me 0. So if I forget it to zero, after the starting battery is charged, the gadget will switch to charging the house battery, but it will be an open circuit. Will that burn the generator?
And is there any famous trick to lower the 14.4V to 13.8V? I can put a super long cable so I get some voltage drop, but it seems like a stupid idea...

[michael pierzga]

Good explanation of alternator controls in Ed Bynes 12v doctor book...I see that its posted on the internet...hope Ed doesn't
mind...http://www.greenenergywindturbine.com/download/alternators_generators/alternatorhandbook_ocr.pdf

[Bglad]

I have some thing that makes the start battery charge first, and then it switches to the house battery bank. What I plan is to devide the bank in two and have a switch to choose which half to use. Those switches have a state 1, 2, 1+2, and unfortunately for me 0. So if I forget it to zero, after the starting battery is charged, the gadget will switch to charging the house battery, but it will be an open circuit. Will that burn the generator?
And is there any famous trick to lower the 14.4V to 13.8V? I can put a super long cable so I get some voltage drop, but it seems like a stupid idea...

Newmar makes a battery isolator (http://www.newmarpower.com/Battery_Isolator_Integrators/Battery_Isolator_Integrators.html) which would solve your switching problem. The mechanism they use to keep batteries separated except for charging causes .7 to .9 voltage drop to the batteries. It would be a bandaid to use and you would still need to check the voltage to make sure it is correct after you installed it. The proper course of action is to install a separate alternator and regulator with capacity and controls required to handle the house bank. Expensive not to if the result is another batch of cooked batteries.

[Landlubber]

"A VSR is not so good because when it connects in the house battery the starter battery discharges into the house battery which tends to defeat the objecet."

...which is why I connect the house batteries to the VSR and they then switch to the start batteries after looking after the house....modern engines start so quickly that there is bugger all drain on the starter batteries, the house batteries are of course usually seriously knocked about

....pistnbroke.....what do you reckon about that...am I a goose?

[michael pierzga]

Gee, I cant remember the original posters name ? At any rate...read Eds description of how an alternator works.. specifically how an alternator senses the state of the battery to determine how much output it supplies and you will find your answer.

[pistnbroke]

Nice to be asked !! yes a good idea and one I had not thought of ...if you have a very big house bank well run down then it could be a while before the strting battery comes in . as you say starting typically only needs about 0.2 AH or less so you got lots of starts in your battery !!

Its easy to up the voltage regulator setting but less easy to drop it . But if we are only talking a 50 A alternator here a big diode in the feed line would drop it 0.7v

And NO lamiaspaska you cannot drop the volts with a long lead thats a resistor and when the charge current drops to zero so will the voltage drop ...

The 12 v doctor information has far too much information for most people to absorb ( the Zener diode in dia 61 is in the wrong place) and the only dual battery system shown is a very dated 1970 dual diode solution ..great reading/reference for an auto electrician though....

[Pandoras box]

Throttle control and a amp meter is a simple answer for the question originally asked.
Just my thoughts, Mike

[Boston]

bookmark for future read
great thread tho
cheers