Battery befuddlement

[Tom _M]

Shed Focus seems to have an electrical issue as went to start it the other evening and was totally dead, not even trying to turn over or anything. Presumed had left something on, but after a quick jump start we were on our way. Thought was okay as used next day on several trips with no issue, but tried in evening again and was totally dead again. Not even enough to open boot catch or power up odometer.

Dug out old mains charger this morning and stuck that on, but it thought battery was already fully charged. Stuck multimeter on and is still showing 12v in battery still. With charger attached it did give enough to power up display and could turn radio on.

Just a bit confused as to why nothing happening when has 12v charge apparently. Any ideas?

I guess new battery would be an answer, but don’t want to overlook something else as seems odd with existing one showing 12v

[GreenV8S]

The type of lead acid batteries used in cars don't like being deep discharged and always suffer some loss of performance afterwards. They can also develop an internal short circuit which prevents a cell from charging at all. I suspect this event has killed the battery to the point that it won't take a full charge. A healthy battery fully charged should read 12.6V while at rest. A battery showing 12.0V is almost flat.

You may have a parasitic load or charging problem which made the battery go flat, but it looks like the battery is toast anyway.

[gt40steve]

Watch the volt meter and put some load on the battery ie try to crank the engine over. My guess the battery voltage will drop like a stone.

[Tom _M]

Thanks. I hadn't realised there would be that little difference between full and 'flat' with the voltage. I just saw 12 and presumed was okay. Interestingly I did accidently let it run flat a week or so ago and had to jump it then, so maybe that tipped it over the edge. It was just the intermittent nature that was a bit confusing, but some fried bits affecting charging seem plausible.