Leisure batteries flat...

[Captain Crisp]

Hi, my 'leisure' batteries were both totally flat when I visited the boat (on the hard) today... I had stupidly left the main switch on 'both'.
1) will I be able to save them? And how?
2) there was nothing switched on... It may have been a bilge malfunction as some rainwater has got in... But, is it possible that there's a fault in the circuit that is somehow draining electricity..?
Thanks!
Crispin

 

[black mercury]

Only way to find out is to charge them and see if they recover. If you have a voltmeter with an amps setting you could probe between battery post and disconnected terminal to find out if there is a draw. After charging them of course. If batteries not too old and haven't been left discharged for very long they should be ok.

 

[Pye_End]

I did the same last year - not a twitter left in them. Left them on charge for a few days and they have been fine. Doesn't need much of a load to do it, given plenty of time - eg VHF or FM radio left on.

 

[MoodySabre]

I left the fridge on by mistake but fortunately it switches off if the voltage drops to 7 volts. The batteries came back from there. A good charger should get them back

 

[neilf39]

If the characteristics of the batteries are not identical through age, Internal resistance, number of cycles they have been through, then I think one can discharge into the other until they balance out but it could be they never quite balance as the characteristics change as the SOC changes so there is always a slow discharge. I might be totally wrong though as the internet seems to imply they would reach an equilibrium. If you charge them up, turn the battery switch to off and put an ammeter between the positives is there a current flow (assuming negatives are still connected)? If so that is what would happen with the switch to both but nothing else was doing anything. Of course it could simply be something you have onboard that takes a small current even if everything is apparently switched off. Again you could check with an ammeter by taking the main power out cable off the switch and putting the ammeter between the switch and the cable you disconnected. I assume it would be less than 10 amps and use the 10 amp setting first in case it is significant (above the max of the lower ranges).

 

[Boathook]

I had the same problem after my solar panel regulator failed and left the batteries at around 11.5 volts for the winter. Could get them to hold full charge after that even using a smart charger so had to bin them. Only 2 years old.

 

[Refueler]

There are two things that are BAD to do to a flat Lead Acid :

1. Allow it to freeze !! Its instant death ! Only charged Lead Acid can survive minus temps.
2. Charge at high rate such as car alternator ... it literally buckles the plates. Especially Leisure batterys as the plates are very soft.

The battery should be slow charged to bring it back up .... and ideally in a warmer area away from winter. If its a battery with screw plugs for the cells to allow top-up ... get yourself a Hydrometer that tells you the charge density of the cell.
You can pretty well ignore those White - Green tell tales ... they are only good if battery is in good overall condition.

 

[jwilson]

There are some items on many modern boats that are often directly battery connected, bypassing breaker switches. Some boats also often have gas detectors and electric auto bilge pumps direct to battery, allowing them to still be on when everything else is switched off. Eberspacher heaters are often fussy about battery voltage when starting up, and it is thus common to minimise drop in cables and switches by wiring them direct to battery.

I don't have bilge pump or gas detector direct to battery, but I once turned off everything and came back a week later to nearly flat domestic batteries. Someone who shall be nameless had set the Eberspacher programmable timer to go on twice a day for two hours.

Once a typical leisure battery has gone down to below around 9v even once I think you'll find a distinct loss in capacity, though it may work on again at that lower capacity for a few years. Charge it/them fully and try it with a known test load.....

 

[ghostlymoron]

My Eberspacher had two fused supplies: a 20a to the glow plug and a 3a (I think) to the controller. The latter connected directly to the battery so that heater can go through its shutdown sequence even if the main switch is turned 'off'.

 

[Finbar]

You may find a modern smart battery charger will refuse to have anything to do with them, where a simple old charger can bring them back. Did for me anyway a few years ago and still going strong

 

[VicS]

There are two things that are BAD to do to a flat Lead Acid :

1. Allow it to freeze !! Its instant death ! Only charged Lead Acid can survive minus temps.
2. Charge at high rate such as car alternator ... it literally buckles the plates. Especially Leisure batterys as the plates are very soft.

The battery should be slow charged to bring it back up .... and ideally in a warmer area away from winter. If its a battery with screw plugs for the cells to allow top-up ... get yourself a Hydrometer that tells you the charge density of the cell.
You can pretty well ignore those White - Green tell tales ... they are only good if battery is in good overall condition.

Those "magic eyes" are only looking at one cell anyway.

 

[lw395]

Main thing is to get them charged up ASAP.
I stupidly flattened my bike battery by leaving the heated grips on, about 4 years ago.
Down to 2volts!
I charged it up with a 15V supply with a current limit until it would start so I could ride home.
I'm stll using the same battery, which is a fair life.