Woodentop said:
Your battery is "flat", 11.5 volts of a resting leisure [C20 rated] battery is zero charged. At this voltage it is in a self destruct mode and urgently needs to be slowly recharged as soon as you can. Leaving them less than 50% charged 12.3 volts, causes this very hard to reverse form of damage and yours is way below that point.
Oh no it isn't.
(i) A lead-acid battery is specified as flat at 1.8Vpc or 10.8V in total. That is why most smart chargers that don't have a 'restart from flat' capability often won't charge if the plate voltage is less than 10.8V.
(ii) The sole purpose of a 'leisure' battery is that its design is such that even if the total plate voltage does go below - and I mean below - 10.8V it can still be brought back, usually if charged at a low limited current for a long period (which is what 'restart from flat' does.) Generally you can't do it with a standard car battery as the internal structure starts to collapse.
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Unfortunately, where you picked up your information has failed to advise you correctly.
1) I was being generous in quoting 11.5 v as flat. Depending on the trace alloying in the lead it varies a bit but flat, and by that I mean empty of charge, 11.9 Volts is generally the figure. See Table 2 in the following independent link.
http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge
2) Leisure battery or not any state of discharge overtime starts degeneration of lead acid batteries. True deep cycling batteries better withstand it, but a budget leisure battery is not quite one of those and as said they only better withstand it, they still are damaged by it. The guide figure for a really good deep cycling battery is not stay below 50% discharged for long, and a starter battery which at the price point, is what we are discussing here, that figure is 80% fully charge. ie 20% discharged.
The info I gave earlier just as this, was stated to try and help you and also to not lead others to be mislead; leaving our batteries so very lowly charged is very damaging to them
Edit: John Wickershams article for this publication fully supports the normally accepted voltage figures I referred to and the irrevocable damage, draining below this causes; see "user guide;
http://www.practicalcaravan.com/advice/33991-how-to-take-care-of-leisure-batteries