Battery. One dead 2 good.

[Gryphon2]

I have been spending a lot of time this winter trying to understand why when I picked up my new (30 year old ) boat we ended up with flat leisure batteries. They are 3 x 3 year old 142 Ah AGM s. I have come to the conclusion just 1 of the 3 is dead. My question is whether I need to replace all 3 or whether I can fit 2 old with 1 new?
Also how is it possible for 1 of 3 to die , when they are all in parallel?

 

[WoodyP]

First you have to identify the culprit. Then the health of the remaining two. I have had batteries that have failed unexpectedly and only replaced that one while leaving the good ones which are good for a few years more.

 

[plumbob]

I had a similar thing when we bought the new boat. The surveyor tested all the batteries and found 2 had 12.2 volts and 1 had 10.4. The brokers swapped the faulty one FOC . Had I known at the time I probably would have insisted on all 3 being swapped but that was 5 years ago and touch wood they've been fine since then. I think my answer is yes you can fit one but maybe disconnect all 3 and charge individually before changing any of them in case it's a dodgy connection.

 

[Refueler]

Of course if expense is no problem - replacement of all 3 ... but first is to identify WHY one battery died and two did not.

It could be something as simple as bad lead connectors to battery terminals, cable junctions to create the battery string .. undo / remove all connections of cables to cables, cables to batterys etc- clean up - refit and tighten ... Try charging up again and see results.

But if that battery is dead - then replacement of one battery is not an issue as long as the remaining two are of reasonable condition.

The suggestion to charge individually is actually good ... because its possible for the charger to be fooled when set as a block. The bad battery can shoot up to high voltage but be useless in providing power ... unable to maintain charged state.

 

[Stemar]

How they're connected could have something to do with it. If the batteries are daisychained with the + & - terminals coming off the same battery, iirc, that's the one that gets the hammer. + & - connections should come from opposite ends of the chain.

 

[Gryphon2]

Thank you for the replies. I think you are right Stemar as the dead battery is the one that has been taking the loads first. I will definitely follow your advice once I have got everything sorted.

 

[VicS]

How they're connected could have something to do with it. If the batteries are daisychained with the + & - terminals coming off the same battery, iirc, that's the one that gets the hammer. + & - connections should come from opposite ends of the chain.

Discussed and explained here:-
SmartGauge Electronics - Interconnecting multiple batteries to form one larger bank
.

 

[Gryphon2]

That is really helpful. Thank you.

 

[William_H]

The theory of connection of abtteries in parallel is good but I don't think it significant if you have heavy wire interconnections and decent actual connections. As Refueller says disconnect and test all 3 batteries. Or perhaps OP has done that already. He can replace one battery with new or even just remove that bad battery. Or replace all 3. All depends on anticipated capacity needs and willingness to spend money. (eek) ol'will