How do you limit amps for charging?

Discussion in 'OnBoard Electronics & Controls' started by Raftman1979, Oct 20, 2008.

  1. Raftman1979
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    Raftman1979 Junior Member

    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.
     
  2. CDK
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    CDK retired engineer

    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.
     
  3. Raftman1979
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    Raftman1979 Junior Member

    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.
     
  4. Fanie
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    Fanie 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 ?
     
  5. Raftman1979
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    Raftman1979 Junior Member

    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.
     
  6. CDK
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    CDK retired engineer

    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.
     
  7. Raftman1979
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    Raftman1979 Junior Member

    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.
     
  8. Pericles
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    Pericles Senior Member

  9. goboatingnow
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    goboatingnow Junior Member

    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.
     
  10. Raftman1979
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    Raftman1979 Junior Member

    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.
     
  11. pistnbroke
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    pistnbroke I try

    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 ..
     
  12. FAST FRED
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    FAST FRED Senior Member

    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
     
  13. pistnbroke
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    pistnbroke I try

    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 ???
     
  14. Ratch
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    Ratch Junior Member

    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.
     

  15. Fanie
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    Fanie 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.
     
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