Parasidic batteries?

[ted655]

n June, I bought 2 marine "start" batts & hooked them in parallel for my diesel engine. Last week we started having "no crank" problems. The relays would just click & if bypassed the bendix would extend but not turn the starter motor.
I feared a engine seize. BUT.. then I removed both batteries to make sure they were fully charged. the first showed 70% ad never charged higher. The second was almost dead,10%. It charged up to 68% and will not charge more either. Whats up? I increased the amperage to 25, but it made no difference.
I thought batts would average each other out? another question, why would the engine not crank off the 70% battery anyway?
The dead one wouldn't sap the good one but blocked cranking? How does that work?
I now have 2 relativly new batts, not abused but did sit unuswd for 4 months. They initially cranked the engine on 2 occasions, then 1 dropped to 10%, the other stayed 70%. but combined would not even spin (or even growl) the starter morto. They both sit @ 68-70% & refuse to budge to a higer charge. Each batt was charged seperatly.
Wal Mart Everstart Marine start batts. Thanks, Ted

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[achris]

Re: Parasidic batteries?

Ted,

Batteries used in parallel should always be isolated when not in use.

The 2 batteries in parallel set up a circuit. Each battery has an internal resistance, this in effect wires up a circuit that will continue to run even when nothing else is drawing current. The difference in the charge state is due to the differences in the initial internal resistances.


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When there is a difference in the voltage of each battery, no matter how small, a current will flow, eventually killing both batteries.

As for not being able to crank the engine, that's because neither battery is able to produce the high current needed to spin an engine, especially a diesel.

Chris..............

 

[ted655]

Re: Parasidic batteries?

Thanks Chris. That answered alot. Screwed up huh. Now what, are both toast @ 70%? I have 2 other boats /outboard motor. Can they be used (salvaged) by using as singles?
I realize % is no indication of current flow .Why have they froze @ 70% of full charge & can they be pushed past by some method?
Thanks,

 

[achris]

Re: Parasidic batteries?

I think those batteries are history... I've never done it, but I have heard of people emptying the acid out and re-filling. BE VERY CAREFUL. The acid is sulfuric, very dangerous. Wear rubber gloves and a breathing mask. Preferably do it next to a swimming pool so if you do spill any on yourself you can just 'fall' into the pool to wash it off.

Chris............

 

[tashasdaddy]

Re: Parasidic batteries?

did you check the water in them. they also don't like to sit. have to be fully charged before storing, and topped off atleast once a month. they don't like being left 1/2 charged or lower.

 

[tashasdaddy]

Re: Parasidic batteries?

achris and i were responding at the same time. i have done what he's saying, but not with good results. the charge does not hold, afterward. your batteries need to be load tested, and then, they are either good, or exchanged with warranty. i have had a group 27 1150 Trojan battery, exchanged 3 times in the last year. they keep getting a dead cell.

 

[Silvertip]

Re: Parasidic batteries?

The short story here is that you bought two new batteries. One had a defect and it killed the other one. You have two junk batteries so they should be covered under warranty. A load test will prove it.

 

[jlinder]

Re: Parasidic batteries?

Two 12 volt batteries in parallel can offer somewhat of a redundancy but of course you need to keep them isolated.

Another approach you could look at is using 2 high capacity 6 volt batteries wired in series. This is common on big diesel trucks. The 6 volt batteries are also used for electric vehicles like golf carts and such.

You might want to think about that approach.

 

[ted655]

Re: Parasidic batteries?

So.... I get the 12V cramking (same or more CCA?) and how about duration of cramk time? I can start engine with 1 in summer, but have to crank longer when cold.

 

[Silvertip]

Re: Parasidic batteries?

You cannot have too much cranking power -- just too little. So buy the largest capacity batteries you have room for. To keep from happening what happened to you, install an on-board charger that you plug into shore power at the dock. The batteries will always be charged and one won't try to suck the life out of the other one. Of course a bad battery is a bad battery and needs to be replaced so the charger can't fix that.

 

[ted655]

Re: Parasidic batteries?

When we bought the boat- she was a dock queen and all was as you described. Now she is a swamp dweller, moored in a bayou and hasn't seen shore power since June.