Why did my battery go flat?

MoodySabre

MoodySabre

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Both batteries have kept up over 10v all winter. Boat back in last week. I left everything switched off and my starter battery on charge. Went over today. Engine started OK - but no.2 battery had died - 2.5 volts and my smart charger won't even look at it. With the engine running I switched to no. 2 expecting a huge drop in revs as the alternator loaded up - but no change occurred. No.2 was new last year -- Squadron 110AH Leisure.

OK, it might be under guarantee but is it savable?
 
Unlikely. If it has been left slowly discharging down to 10v it will have sulphated as it discharged so some capacity is lost. When you recharge whats left and then leave it slowly discharging again more is lost untill there is no capacity left. Ideally lead acids should be left on float charge when stored, if not recharge frequently and never let them drop below about 12.5v. This could be a useful time to try one of those pulse charge gizmos and let us know if it resurrects it. You may also get some capacity back by a process of high voltage charge and discharge cycles but it will only be parial and the battery will not be reliable so personally I would not want it on board. Next yaer take the batteries of and keep them charged in the garage.
 
At a guess would suspect you have something connected when isolated, memory or several memories or one of those electronic cycling bildge pumps ? if not then one would suspect leakage on that battery circuit or possibly it was not fully charged when you laid the vessel up. Just thoughts and causes.
 
Is it actually at 2.5 volts? On first passage this year found my domestic battery showing under 6 volts after running engine for several hours. Turned out to be a loose battery connection - I think the engineer who worked on the engine over the winter may have disconnected batteries for some reason and not tightened this one up again. The battery wasn't actually flat.
 
Is the fluid level very low? Sometimes I have found topping up the level helps the battery to recover slightly. If there is debris between the plates, tapping may help. Basically it is stuffed, so nowt is likely to make it worse!
 
You need to get some charge into both batteries. Then use them to see if they apparently have enough capacity for your purposes.
If the smart charger won't do its thing because the voltage is too low.

You need and old fashioned charger or connect up the engine battery while on charge to the dud battery via a car headlamp bulb or similar resistance. This will limit the current flow so you don't get a fire or too much current.

Actually it is a wonder you have not had real problems when you tried to charge them on the engine. If the engine battery is OK.
I would find it very odd that the batteries did not come up a little which all seems to point to a fault in the wiring somewhere.
Check terminals or switches.

I would expect the battery to come up in volts with charge but if it is ruined by discharge it will exhibit poor capacity. good luck olewill
 
I agree with William. If the battery is really at 2.5 volts it's effectively dead. If it will accept charge, then you might get it back to some sort of functioning condition, but the chances are that its been damaged beyond recovery. It will have lost some of its capacity at the very least, and might be incapable of accepting any charge at all.

Moral is keep your lead acid batteries charged up! Deeply discharging batteries and not recharging them immediately kills them. Most lead acid batteriers self discharge and need to be topped up through the winter months.
 
We leave the boat ashore in Greece from Mid October till end April. Before leaving for the winter,I charge the batteries up fully and then disconnect them completely ie unscrew the terminals and isolate the batteries. I clean the top surfaces and that's it till we return. I check the voltages when leaving and on return,and up till now there has only been a drop of about .5 V or so in each battery. The engine one has always been easily able to crank it up. Hope this year will be the same.
 
OK thanks everyone - I disconnected it and put a multimeter across it and it was 2.5v. I'm sure that nothing was left on. First stop will be a replacement under guarantee then a fiddle. At the end of the day you need good batteries so I shan't chance a dud on board.
 
William might have hit the nail on the head...modern smart charger reject batteries with very low voltages like that and do nothing. When you have the battery out try connecting it to an old fashioned charger (often called a 'Halfords type') but even Halfords probably sell four stage chargers now! A mega pulse pulser might get you up and running again - they really do work.
 
Squadron recently swopped out a 110 amp battery just outside the warranty for a friend via the Marine Superstore so good guys to talk to.

Pete
 
I had a dead flat battery once and bought a new one. It turned out to be a intermittent short in a live circuit (the wind gen) and later in the trip it nearly started a fire. Worth checking all the circuits for problems.
 
Be very sure nothing was left on. When we bought our current boat I noticed that the engine battery voltage was dropping over just a few days and assumed the battery was on it's way out and it eventually failed soon afterwards. I fitted a new battery ( big 120Ah one designed for HD use on Police/Ambulance vehicles as this powers our windlass too) was still dropping volts over a relatively short time, like a few days at anchor - but why? A multimeter connected as an ammeter between the battery and one power lead confirmed that there was a constant drain of just under 0.5A! It took a long time to discover that the gas alarm was connected (via the battery off switch) to be always on and was taking 0.35A or 8AH per day. The other drain took longer to find and was a lead to the alternator warning light, apparently rigged as an 'exciter' connection and totally unnecessary and this also explained why there was a very faint glow from this light with the engine off, only visible on the very darkest of nights.
 
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