Batteries in Parallel

I've been running 2 29 series deep cycle batteries in parallel for about 4 months now... I have been having some problems with on of the batteries which is 6 months old - it seemed to not be holding charge well and giving out way too early. I checked voltages on it with the fluke and that particular battery read 11.2 and the other battery read 12.8, 2 days after being fully charged. I have been keeping them hooked up in parallel while not in use, and for charging... I contacted a rep at Optima and talked to him about the problems (these aren't optima batteries, I just wanted to talk to someone different)..

anyways, this is what he told me "Paralleled batteries tend to fight each other when they are at rest -- this causes premature discharge and a shortened life span. It's OK to parallel batteries temporarily with the battery switch, while charging, starting and running the trolling motor -- just avoid leaving the switch on 'BOTH' when no power is being drawn.

This is probably where my fault had been - I never thought about each battery robbing each other of power. When I had the same batteries hooked up in parallel, the voltage on the fluke was 12.6... unhooked, well the numbers are above.

I have to take one of the batt's back tonight and replace it because my charger on the battery that is reading 11.2 was flashing "Bad Bat"... I tried to desulfate it with the charger (some option that is built into the charger), and it stated that it can take up to 10 hours.. so this morning is when I got the reading of it being bad.

So in the long run.... unhook your batteries from parallel when you're not fishing.

to give some background... I fish every weekend and charge immediately when I get home. I unplug them the next morning and then it sits until the friday night I'm to go back out fishing. I hook them back up and unplug them right after I get the boat hooked up to the truck and get ready to leave. I guess from now on they will sit unhooked during the week.

Thank you Russ, thats good information. I put a connector between my batts just to make it easier to get in and out of the boat, I am glad I did.

The desulfate option on works on flooded lead acid batteries and can damage AGM(Optima) and maintainece free batteries.Always check the electrolite levels after the desulfation(?) process.

this is why you always should use a battery switch to switch between or combine both batteries.

ben2go said:

The desulfate option on works on flooded lead acid batteries and can damage AGM(Optima) and maintainece free batteries.Always check the electrolite levels after the desulfation(?) process.

Click to expand...

what I have are flooded lead acid batteries... and the levels were good before and after the process

I always disconnect and charge my batteries individually. If you have one battery weaker you also stand a chance of overcharging the other battery. This has always worked good good me.

that's my problem - 3 batteries, 2 chargers. I guess I may have to breakdown and get an onboard battery charger.. I could just use a 2 bank for those, and then put the chargers on the rear batteries

The only problem I have with what the Optima rep said is : How do other applications get away with running the batteries in parallel without such issues? Such as most on highway diesels, the ag industry, and construction equipment almost always has 2+ batteries running in parallel. Reguardless of 12 or 24 volt systems

sparkbr said:

The only problem I have with what the Optima rep said is : How do other applications get away with running the batteries in parallel without such issues? Such as most on highway diesels, the ag industry, and construction equipment almost always has 2+ batteries running in parallel. Reguardless of 12 or 24 volt systems

Click to expand...

Do they run in parallel? My Ford F250 Diesel had two batteries but they were not in parallel

Also, my 253 Mako uses two starter batteries for each engine, again, not in parallel

i'd say the difference between the batteries being in a boat as opposed to a truck - the truck is always running. I doubt it's going to be sitting for a week before it gets current running through it again.

I could leave the chargers plugged up all week, and keep a trickle charge on the batteries... but I don't like leaving them on when I'm not home... I just don't trust stuff like that

That was going to be my question, what would it do if you kept it on a trickle charger with automatic float while not in use? Would it be ok that way?

yea I think it would be fine.. if I left my boat outside, i would leave it plugged up like that. But, my boat stays in the garage and I don't want to put my house in danger of something going wrong.

Chargers are equipped to stay on for long periods of time with a trickle charge in automatic cycle.