Charging 3 battery banks with 2 engines

[crook038]

I am helping my father setup a new power distribution system on his Mainship 36 DC and have a couple of questions. He will use the boat for weekends away with the occasional week or two trip. While away he will not have shore power as he will be on a mooring. The largest load is an AC/DC refrigerator, with secondary loads consisting of navigational electronics and pumps (freshwater/macerator). I have drawn 2 crude diagrams for the charging circuit. The first is with a battery isolator and the second is with strictly integrators. I am leaning towards the second due to overcharging of batteries through the use of the isolator. The big question I have, how would I use the first setup and effectively charge each battery without overcharging any one battery. I appreciate any input.
Sean

[rasorinc]

Both AC and refrigerator draw a lot of amps. The AC much more and really requires a generator as battery banks do not last long. Check out specs of motor home AC for draw data. Both amps and watts--to kool that size boat minimum 3500 but 5,000 for ample- Generator size--Subare/robin gas or diesel.

[crook038]

I just re-read my post, I should probably clarify......the refrigerator is a dual voltage unit, it can run on DC power or off of shore power (AC). We just took the genset out of the boat (Onan 6.5Kw) We are going to see if it is worth getting it running or seeking an alternate. For the time being we would like to be able to run on the house bank while at the mooring and recharge as necessary from the engine(s). Thank you for the quick response, it is appreciated.
Sean

[gonzo]

An isolator will not overcharge the batteries. It is a passive element consisting of diodes or switches. If there are diodes, there will actually be a voltage drop.

[Ike]

Besides if you use a good three stage or four stage charger then you should not have an overcharging problem. I would suggest either getting the Genset fixed or replacing it. You don't want to be fooling around with a portable genset on a boat.

[Bglad]

Too complicated. Combine enough batteries in one bank to run your house loads as long as you need to plus some extra on one engine, cranking battery on other engine combiner between. Big house bank may run windlass ok. You probabaly won't need air-conditioning. Carry portable generator aboard in case you get in a pinch. LPG and small inverter for cooking and coffeemaker, LED lights in fixtures you should be good to go.

Instead off all the combiners buy more batteries and/or HO alternator(s).

[Joakim]

Your systems seem quite complicated to me. What are your goals? Why do you have two separate house battery banks? Why do you have two separate starting battery banks? Does windlass battery need to be electronically separated?

At least during charging all battery banks can be united into one. This can be done using battery integrators or just simple relays. The alternator will normally (with standard regulator) just give about 14.4 V (or as much as it can). This is not considered overcharging, but it is more than optimum, if engines are running for a very long time. If you just unite all batteries together during charging, they will all see the same voltage and charge well.

For shore power charging you can skip charging of starting batteries. They will always be fully charged anyway, unless they are defective. If for some reason you need to charge them, you can connect them to house batteries with a jump cable or a manual switch, which is also good to have, if you have problems starting with a starting battery.

If you really want to keep all the battery banks you have, I would do something like the following.

1. Connect the starting batteries through a diode to the alternator (port to port). Starting batteries are always fully charged. No need for 14+ V charging. If you like, use a battery integrator or a relay instead.

2. If you like, put a manual switch between the starting batteries in order to use the other battery as help in starting and to handle one alternator failure.

3. Connect one house battery bank to each alternator.

4. Put a battery integrator or a relay between one house battery bank and the windlass battery.

5. Connect the shore power charger to house battery banks (one circuit for each or one charger for each).

6. If you like, put a battery integrator or a relay between house batteries.

The relays can be operated by the charging lamp circuit.

[crook038]

Thank you for all the input. Gonzo- my understanding of an isolator is that it will electrically isolate each bank of batteries so they will no back charge (try to equalize) between the banks. This is great for isolation but when an alternator is pumping out 70 amps and only one bank of the three needs a charge, my understanding is the isolator does not reduce amps across the 3 banks, so you can overcharge the other two banks.
This is my understanding and I may be wrong, also maybe a different type of isolator has that capability…..I couldn’t find one in my 2 days of searching. Ike- We plan on using a good 3 step charger on shore power but that doesn’t help while on the mooring. We are going to work on rebuilding or replacing the genset, but that will take some time…..not before the next boating season begins……and let me tell you, they built that Mainship around that Onan Genset……not user friendly to work on or remove!
Bglad- I would really like to simplify the setup, I think the run from battery location to the windless will be quite long causing enough voltage drop that we may as well put a battery right at the windless with a small gauge(relatively speaking) for charging only. What I would really like to find is a charging system like the one I have depicted below. Thanks for all the input, keep the ideas flowing.
Sean

[Joakim]

Originally Posted by crook038

This is great for isolation but when an alternator is pumping out 70 amps and only one bank of the three needs a charge, my understanding is the isolator does not reduce amps across the 3 banks, so you can overcharge the other two banks.

Alternator just tries to keep to the voltage at about 14.4 V. Then each battery takes all the current it can at that voltage. If this current is more than the alternator can provide, the voltage will be less.

So if one battery is empty and one is full, the current will be mostly to the empty battery. It can easily be that house batteries are taking 100 A while starting battery only takes 0.7 A. This can happen, if the batteries are totally connected together while charging (via a relay or an integrator) or are both charged through the same isolator. When you are charging there is no point of trying to isolate the batteries. You need isolation only during battery usage and storage.

While charging batteries do not "see" each other and there is no current from battery to another battery. It is just as they would be connected to the charger alone (as long as the charger has enough capacity). Thus there will be no overcharging due to other batteries.

[pistnbroke]

I have 40 years experience as an vehile/boat electrician and I would not even try to unravel those diagrams ....

You have forgotten one thing KEEP IT SIMPLE STUPID..... KISS

fit another alternator to one of the engines and use each alternator to feed its own bank ....cheaper ...more efficient ....KISS ...

[bulk-head]

I agree Brokepiston. With twin engines, twin aternators powering twin battery banks the system should be super simple. Simple Dedicated systems are easy to build and service. A crossover switch to connect them together if needed. Better to spend time ,money on added battery capacity .
If an additional heavy load battery bank must be charged.. Anchor winch ...then add another dedicated alternator to one engine .

[Bglad]

You are on the right track but you haven't simplified the battery arrangement. Make a diagram with two battery banks(house and cranking) and dedicated windlass battery using charging relays between. Batteries do not necessarily need to be in the same box together as long as they are within a few feet of one another you can use buss bars to combine them. Put overcurrent protection on windlass battery so it's charging conductor doesn't burn up trying to run your windlass. Get it up when you can. When you start hearing purring you will know you are on the right track

[mydauphin]

If he is going to be out for a week or two, does that mean he will have a generator on-board? Perhaps a generator recharging house battery is way to go.

[Bglad]

Originally Posted by mydauphin

If he is going to be out for a week or two, does that mean he will have a generator on-board? Perhaps a generator recharging house battery is way to go.

He won't need a generator if he calculates how many batteries he needs to last between his visits to the boat. While aboard he will need an adequate charging system (alternator) on his main engine(s) to recharge the battery bank in a reasonable amount of time.

[crook038]

I have not posted in a while. We did the upgrade on the Mainship 36dc using balmar external regulators, balmar centerfielder, blue sea charge relays and blue sea remote battery switches. The 2 engine alternators are regulated by the centerfielder thru the balmar regulators based on the designated battery type. When charging the charge relays close to charge the 2 house banks and the start battery. When each house bank or start battery reaches full charge, the charge relay opens and that battery or bank is isolated from the charge circuit. The system has worked very well, the house banks have had no problem keeping up with the refrigeration. When out of port the starboard engine is run in the morning to generate domestic hot water and this in turn also tops off the battery banks. Thanks for all the input it was helpful.
Sean