Battery charger wiring to parallel batteries

williwaw

I'm upgrading my single house battery to two batteries connected in parallel (Trojan SCS200). Both batteries will be new and obviously the same type. To supply the boat, I'm thinking of taking the negative lead off one battery and the positive off the other.

All was good until I started looking at upgrading the battery charger (currently the factory installed Guest 10amp). My thought was the charger would be connected across the bank, not the individual batteries, so I would need a two lead charger - one for the start battery and one for the house bank.

My local chandlery likes the ProMariner chargers so I looked at the installation guide but to my surprise they specifically state that the charging leads should be connected to individual batteries. I could understand this better if the batteries were mismatched in capacity, type or age. When looking at other chargers I could not find specific advice on this issue.

Given I'm using identical batteries in my house bank, can I connect them to a single charger lead?

Sea Wolf · Posted: Fri Nov 28, 2014 12:50 pm Post subject:

williwaw-

You asked "Given I'm using identical batteries in my house bank, can I connect them to a single charger lead?"

My answer is yes, since they're working in parallel and are in identical condition.

Just watch their condition and state of charge frequently. If they show any variation/differences, the answer becomes "no" and you'll have to get two new identical batteries.
Any differences and the weak battery will pull down the strong one, discharging it and eventually ruining it.

I actually run four group 27 deep cycle marine batteries in one bank in my CD-22, but can disconnect them and run or charge any single or combination I want with switches.
This requires vigilant observation.
I can charge them singly or all at once if I want to with a 40-amp Vector charger that also has settings for 20 and 10 amp charging.
I like this system, but can't really recommend it to those who are "battery shy"!

I know there are others who will not like what I'm doing, but it's been 10 years operating w/o incident, and I feel very comfortable with it. YMMV!!! Laughing

Joe.

Aurelia · Posted: Fri Nov 28, 2014 6:25 pm Post subject:

Agree with joe. One bank is one battery as far as the charger is concerned. I use a total of 4-8 amps of charging for three batteries and am satisfied for our mostly dockless use. That is lower than average i will admit. Our motor does most of our charging while out and at home we keep it topped up at all times.

Greg

Sea Wolf · Posted: Fri Nov 28, 2014 6:44 pm Post subject:

Greg brings up an important point or two, etc.:

1. Keep all batteries topped up in fluid.

2. Fully charged when not under load.

3. Use them in parallel sets, or all together, whenever possible, to keep them even.

4. If you get one set or battery considerably more discharged than others, charge it up before parallel connecting, although if you simply connect them all together while at different levels, they will even out the charges.

This isn't a problem if they're not to uneven, but if one is deeply discharged and the others fully charged, the flow into the discharged battery might be huge and cause problems.

As I said, this may not be for everyone or the battery shy, but with vigilant care is very workable.

Joe. Teeth

Robert H. Wilkinson · Posted: Fri Nov 28, 2014 6:57 pm Post subject:

thataway · Posted: Fri Nov 28, 2014 9:41 pm Post subject:

You do want to use a good 3-4 stage battery charger. Just using the motor rarely gets the batteries to the full charge. There is not the absoption stage where the voltage remains constante, and the charge is tapered off to give the last 20% of charge to top the battery off.

An outboat is not wll regulated in the charging states, and have constant 14.1 to 14.5 volts.

With the low loads one has on the c Dory, and heavy wiring to wire the batteries in parallel, it is fine to connect the two positve and negative poles together, and take the power off one battery.

Yes, treat the "bank" as one battery, but be sure you do not overcharge the start battery--which may be both a smaller battery, and be getting the same charge. You can use the battery charger you have now, just watch the start battery. When you start with high amp recharge rates, then you want a charger which has battery temperature feedback. 10 amps---which is probably 5 amps per battery does not require this. If you have high loads, like an inverter, then you may want a different battery charger, with a higher charrge rate.

williwaw · Posted: Fri Nov 28, 2014 9:57 pm Post subject:

Thanks guys. This is all along the lines of my understanding and I appreciate the validation Smile

Rob, yeah, I have a single engine boat so have one start battery and will have a house bank with the two Trojan batteries.

What baffles me is why the ProMariner people are so adamant that each "individual" battery in a bank must have a charger lead. This is a popular charger so you'd think they have their act together but perhaps its just bad writing. The install guide is here if anybody wants to read it.

Sea Wolf · Posted: Sun Nov 30, 2014 10:29 am Post subject:

Here's another discussion from another site that may also be helpful:

Charging multiple lead-acid batteries with one charger

Joe. Teeth

potter water · Posted: Sun Nov 30, 2014 12:34 pm Post subject:

IF your parallel batteries were relatively far apart...many feet...there may be a consideration to separate charge leads to head off a problem with voltage loss in the long leads between batteries. I don't know anyone who would mount parallel batteries far apart though. You may be over thinking this. If you have a single bank charger, connect it to the positive and negative of one of the two batteries and move on to another project. Be sure to use hefty wire to make the parallel connections between batteries. The same size as the wires that lead off to the house buss at least. It sounds like you don't have a charger that also has a bank to go to the start battery? If so, then, it would be wise to get a two bank charger. DON'T parallel up the start battery to the house. However, a battery management switch that will allow you an emergency start from the house batteries would be a minimum addition if you don't have a separate bank or charger for the start batteries. In fact it would be a minimum addition even if you have a separate charger or bank for the start batteries.