Battery Sudden Death

[C00P]

Caravan has a 120 Ah battery. Hooked on to a solar panel via a charge controller. Also connected (I think) to a battery charger with a "float" feature.
Caravan is currently parked in shed with no skylight. Panels only get illuminated by fluoro's when we are there working on things (Usually only on weekends).
Noticed during the week when I dropped in that a light in the caravan had been left on, so turned it off. Charger was connected to 240 and was switched on and was indicating "float".
This weekend turned a light on- nothing. Measured voltage at Battery terminals 0.75V.
Measured output at Battery Charger- 13.8 (or something like that).
No circuit diagram so dunno how the charger is connected to the battery.
Disconnected the battery from the caravan circuit and connected my 5-stage battery charger that I use on car batteries. It failed to indicate any charging and then after a couple of minutes, gave a signal that the manual says it gives when it has gone into "power save mode" because it hasn't been connected to a battery for 2 minutes. Made sure the charger was connected to the battery. It was.
Can anyone help me understand what has happened? Battery is non-spillable (completely sealed) 120Ah of unknown age- came with the van when we bought it.

Puzzled

Coop

[bee utey]

This battery may be rescuable, you'll need to get its voltage above the point where the battery charger will recognise it. Use jumper leads to a good battery for a minute or so then see if the charger takes over. I recently had a car battery go dead flat twice in a row because of an unidentified lamp staying on and each time it came good with the jumper leads treatment. It's still going well a month later now that I've fixed the guilty circuit.

[C00P]

Thanks Beeutey,
I'll give that a go. I thought the solution might be something like that, but wasn't sure how to go about it.
Cheers

Coop

[DeeJay]

Either that, or connect to an old type charger -ie non electronic, but preferably a decent amperage, not a trickle type. Once over 10.5 volts you should final charge with an electronic charger.

[C00P]

Thanks DeeJay, I figured I'd connect another battery and employ an ammeter to measure the current while they are connected to see what is happening. (I've got an old shunt-connected ammeter used for measuring the output of alternators and so should be able to cope with the load).
I'll let you know how I get on,
Cheers

Coop

[C00P]

Tried using a larger battery charger which I thought had no electronics but it very slowly lifted the voltage to 1.4 volts at the rate of about .1 of a volt per minute. Gave up on that. Left the multimeter connected and used a 75Ah bettery with built in voltmeter and hooked it up to the dead battery with jumper leads. That dragged the donor battery's voltage down to about 11.5, and the dead battery began to show signs of life with the voltage steadily rising towards 11 volts. The healthy sparks as I made the connection convinced me that a fair current was flowing, but I didn't have my shunt meter so couldn't tell how much. When the recipient battery reached about 11.5 volts (about 5 minutes) I disconnected and watched the voltage rapidly plummet while I hooked up the battery charger. Managed to catch it at about 7 volts and it began slowly rising again. Have left the charger connected. It can manage about 5A so should take 24 hours to bring the battery up to full charge (or somewhere near it).
Thanks for the tips.
Cheers

Coop

[C00P]

Left the charger connected for about 36 hours and found it with the "float" light illuminated, so looks like we've had a successful resuscitation. I doubt it will have the same capacity as before, but should be able to get some use out of it.
Thanks again for the suggestions,
Cheers

Coop