Battery conundrum

[Alf Roberts]

I don't suppose there's really an answer to this but it's an interesting one all the same.

 

I have a pair of US2200 6V batteries, about 4 years old, paralleled with a pair of Trojan T105s. A few weeks ago I suspected that these were dragging the whole bank down so isolated them.

 

I measured the voltage this morning and one is at 4.7V and the other 6.2V

 

So one has had it the other hasn't.

 

And a 6V battery isn't much use.

 

What shall I do?

 

See this as a lonely hearts column for single 6V batteries.

[Nickhlx]

I think the safe answer must be to replace both if they are the same age, or all if the other two are also the same age, and if you think the "good ones" are actually still OK, perhaps use them for the starter battery, the bowthruster or keep as spares? I am hesitant mixing old batteries with new ones, as well as different types and certainly different chemistries, as inevitably their "resting" voltages will be different and dragging the better ones down faster than otherwise..

 

Nick

[Paul C]

There's something to be said for combining (in series) the best 2 batteries together (lets call them #1 and #2, #1 would be a new battery); then having this in parallel with the next best 2 batteries (ie #3 and #4 in condition). Obviously the battery showing 4.7V has a dead cell, so should be fixed/replaced.

 

The way to avoid issues getting out of hand is mid-point monitoring, something the Victron BMV702 can now do.

[Alf Roberts]

Yes, the conundrum is whether to replace the duff 6V with a new one.

 

I have no problems with adding a new pair to the bank (as I did 18 months ago).

 

If they weren't so heavy I'd offer it to someone in the same situation - or put the call out for a matching 2nd - but the chances of getting it within reach are small.

[Paul C]

Yes, the conundrum is whether to replace the duff 6V with a new one.

 

Can you clarify - do you mean

 

1) whether to replace it (the one battery) or not

2) whether to simply slot in the replacement one in the knackered one's position without further rearrangement of the other batteries?

3) whether to replace 2 batteries even though only one is knackered (and the other is part way through its life)

 

And to clarify my previous post, the point I was making is its more important to match the pairs of batteries which are in series to make a "12V pair" than match the paralleled "12V pairs" in their performance.

Edited October 4, 2014 by Paul C

[Alf Roberts]

I agree i'm reluctant to replace one and get an ill matched series pair. I'm also reluctant to buy two to replace one.

 

There are two more so I don't have to rush to a decision although leaving the part used one at 6.2V won't do it any good.

 

I think I'll have to replace the two eventually.

 

What I really need is someone nearby with exactly this problem so we could toss a coin for it.

[Detling]

If the batteries are the same age the others will probably not last that much longer before developing problems. Manufacturing tolerances these days give similar products similar life expectancies ( think of 4 light bulbs in a single fitting, one fails after 3 years the others will usually fail in the next 6 months).