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Old 10-01-2018, 19:11   #1
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Parallel Battery Charging

I have two 8Ds in parallel and one charger. I've seen info on charging both either of two ways: by having the charger across one battery, with the other paralleled to the first battery, or with the charger across both batteries (charger positive terminal to Pos of one 8D, charger Neg terminal to the Neg terminal of the Other 8D).

Which is better and why? Thanks.
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Old 10-01-2018, 21:42   #2
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Re: Parallel Battery Charging

The short answer is that charging "across batteries," as you put it, is the best way. But it really doesn't matter much unless your charger is cranking out well over 100 amps.

With a pair of batteries in parallel, they will "see" the same charging (and discharge) voltage if the net resistance of the parallel connection is zero. With two strings of batteries, this can be achieved by connecting the charger (or load) to the positive on one battery, and the negative on the other. That way the resistance in the positive connection between the two batteries should be equal in magnitude to the resistance in the negative connection between the two batteries, so they balance out.

Real world, with good clean connections, a short distance between cables, and proper cable sizing, and a charger that either is low amp or that runs a reasonable charge profile, both batteries will be charged at the end of the cycle.
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Old 11-01-2018, 18:47   #3
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Re: Parallel Battery Charging

ideally you'd have a pos and neg bus bar. that came off each end of the bank. with all chargers (AC, engine, etc) and all loads. so everything is across both batteries, not just the charger
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Old 11-01-2018, 19:52   #4
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Re: Parallel Battery Charging

Agreed--there are three positive 4/0 leads on one of the batteries, that's a no-go. It's a bit trickier in that the two 8Ds are the power source for a series-parallel 24v bow thruster (contactor leaves them in parallel when inactive), so I'm having a good think about it all. Thanks.
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Old 11-01-2018, 19:59   #5
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Re: Parallel Battery Charging

in that case I'm pretty sure everything will have to be across the first battery
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Old 11-01-2018, 20:32   #6
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Re: Parallel Battery Charging

As long as I don't run the charger during the brief times the bow thruster is being used (sharp observation, that), I believe I could put it across both batteries,since they are in parallel at rest.

I'm leaning towards putting it across the first battery only, just seems safer and simpler. Just hope the second battery gets a full charge, as there is a fair bit of cabling and contactor between the first and second batteries.
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Old 11-01-2018, 20:49   #7
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Re: Parallel Battery Charging

4/0 copper should have you good for charging up to 100 amps or more.

The contactor is the thing to watch though. They can develop series resistance when they start to fail, and it doesn't take much to interfere with evenly charged batteries, especially at high amp rates.

If it were my boat, I'd maybe check once in a while with a handheld voltmeter, while the batteries are charging, and be sure that the voltages are the same on each bank. Or, measure the voltage across the contactor, while charging, anything more than 50 mV would be troubling.

I would be tempted to rewire the whole boat for 24v in your situation.
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Old 11-01-2018, 21:58   #8
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Re: Parallel Battery Charging

I don’t see anything wrong in parallel charging. But I do suggest that if you have a large charger that has provisions to monitor the battery voltage.... via a separate, light gage wire pair from the charger to the battery terminals, you should use it. This will help compensate for wiring voltage drops in the charging wires that will develop with large currents.
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Old 12-01-2018, 09:33   #9
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Re: Parallel Battery Charging

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I would be tempted to rewire the whole boat for 24v in your situation.
We think alike; I would prefer a 24v system to a 12v system. I call the two 8Ds the 'Machinery Bank' because they power the anchor windlass, generator & main engine start motors (all 12v), and the bow thruster (24v). I would like to go with a larger 24v windlass motor, as I plan on upgrading the anchor chain and anchor size, the 12v windlass I have now would be marginal for that. Going 24v would allow using a larger windlass wrt to the existing windlass cabling. But 24v starter motors and alternators ain't cheap. The separate House Bank is 12v with all the expenditure involved in converting it to 24v as well. I'm an LiFePO4 fan, will go 24v when that happens, but that's a whole different ball of wax.

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The contactor is the thing to watch though.
Funny you mention that. What forced me to learn how the bow thruster system functions was the 8Ds not charging correctly. Turns out one throw of the DPDT contactor had failed in the 24v 'Active' position, which allowed the bow thruster to work when activated, but not relax (deactivate) into the parallel 12v charging position. Weird one, the manufacturer was worse than useless. I'm installing the new contactor today, hope that's the issue.

Thanks for the tips on checking the charging part of the system, I'll incorporate them. I woke up this morning with an experiment in mind: once the system is back on line, check the individual 8Ds' charging voltages with the charger in each of the above mentioned charging configurations, see what we've got.

With the contactor and quite a few cable junctions between the two batteries, I'm starting to think the Across-Both-Batteries charging configuration might yield the most consistent charging voltages between both batteries. Interesting experiment anyhow.

PS: I checked with my battery charger dealer (Sterling PCU-12/40, Compass Marine), he said the charger output is protected from backfeed, will withstand the 24v that would be across it when the bow thruster is activated, if I used the Across-Both-Batteries configuration.
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Old 12-01-2018, 16:21   #10
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Re: Parallel Battery Charging

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As long as I don't run the charger during the brief times the bow thruster is being used (sharp observation, that), I believe I could put it across both batteries,since they are in parallel at rest.

.
no then you are feeding 24v up the charger line to the charger output. to the 12v charger. even when it is off. maybe it can take it. or maybe it'll blow up. you won't know untill it blows.

edit I see you mention you asked. so maybe it'll be ok. I'd still be leary.
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Old 12-01-2018, 16:28   #11
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Re: Parallel Battery Charging

Read the PS in my last message. There's no point in responding to those kind of comments.
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Old 12-01-2018, 16:29   #12
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Re: Parallel Battery Charging

I'd be inclined to removed one 8d, install 2 group 31 agms in 24v. and a dc to dc charger. leave one 8D as 12 house. or whatever it'd doing now, and the 24v thruster bank. or if you have the space. just add the 2 new batteries and leave the two 8d as 12v house.
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Old 12-01-2018, 16:50   #13
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Re: Parallel Battery Charging

My House Bank is a 3P2S Trojan T-105plus setup, so I'm pretty good there. That 2 x Grp31 setup is an interesting idea. It would get rid of the series-parallel contactor folderol.
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Old 12-01-2018, 19:54   #14
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Re: Parallel Battery Charging

so the two 8d's are only for the thruster? and the only reason they go series - parellel is for charging?

in that case just join them together in 24v perminatly and get a dc to dc 12-24v charger and charge from house bank. doesn't need to be big. it'll slowy recharge after brief thruster use. when ever the house is charging. (shore / engine)

I was asuming it was also being used to power 12v loads as well
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Old 12-01-2018, 21:49   #15
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Re: Parallel Battery Charging

Yes, as mentioned earlier (post# 9), the 8Ds power the windlass and generator/main engine starter motors (all of them 12v) when in parallel (99.9% of the time). I guess this was the factory's somewhat cludgy solution for specifying a 24v bow thruster.
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I was asuming it was also being used to power 12v loads as well
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