Battery self discharge

pessimist

pessimist

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We have a small (44 ah) battery which we use to start the outboard on the dory. Had problems with it last week but it hadn't been used for a while so I wasn't particularly perturbed. Brought it home for charging and 24 hours ago when charging was complete it hat a voltage reading of 13.6. It is now at around 12.9. I've applied no load. Is this reasonable and is there any other test I should try?

Cheers,

Colin
 
If it is a wet cell battery, invest in a specific gravity tester (couple of quid, Halfords). The specific gravity of the acid is the clearest measure of lead-acid battery status.
 
I believe the reduction to 12.9v to be perfectly okay. When a battery is charged (at anything up to 14.6v maybe) the voltage of the battery rises. Turn the charger off and a reading of 13.6v is not surprising. This is what I have seen referred to as a "surface" charge on the battery and before taking a voltage reading once should apply a small load to the battery for twenty minutes or so to clear off this "surface charge". One then gets a "vaild" reading which would probably compare very nicely to your 12.9v reading. 12.9v is after all a fully charged 12v battery.

rob
 
12.9v is a good fully charged reading when off charge and allowed to rest some hours. If you have a light load you could apply, like a lamp or something for say 30 minutes then test again, it should still be up around 12.7/12.8v if the battery is good. Otherwise if it has cells you can top up, use a hydrometer to test, you can get ne in Halfords, or get the battery load tested at a battery supplier not sure if Halfords do it, but they might. Otherwise if you can still hand start the outboard take it to the boat and see if it is OK! Trouble is sometimes all looks OK until you apply a load like the starter then it goes tits up /forums/images/graemlins/frown.gif
 
If you can't test it with a similar load to the starter, which is best, then a capacity test with a medium-high load, like say 100W of halogen bulbs will indicate whether its likely to work. 100W for an hour without dropping below 12V would suggest it's likely to have life left in it. Not precise but a useful weed out of batteries on their last legs imho.
 
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