Odd battery problem

It's f***ed.

The plates are well sulphated, or you have a dry cell(s).

With some luck and continued charging you might see the volts reduce and get some capacity, but it won't be enough to start the engine.

There are some snake oil additives and techniques to get capacity back, but the highest practical claim I have seen is 30% of original capacity.

Personally I would give up now and get a new one, and this time charge it every few weeks or get a float/smart charger.

There is probably a miniscule current being drawn from the charger but so small that the charger is seeing an open circuit.

Internal resistance of the sulphated battery is rather high.

You might want to try a modest load like sidelights across it and see what the terminal voltage drops to (probably 0V).

High internal resistance?

Cheers, T i m

Can't say I've ever come across a lead acid reading 12v which is actually knackered. First time for everything.

If I left a battery for 1 year and without any charging it gave a 12V reading I would be thinking the same. I assumed the OP was referring to off-charge vs on-charge voltages.

Agreed. I think it *might* be worth leaving it connected to an old fashioned transformer type charger for a while (but not with that much hope). No use trying modern electronic chargers on sickly batteries, they just shut down to protect themselves. Sometimes transformer ones will go to full scale deflection for a few minutes with a battery at 6 volts or so, then settle down as the battery voltage comes up.

I'm assuming the OP has jiggled the clips a few times to make sure there is a good electrical connection: should be seeing sparks.

Maybe it's a 24 volt battery which had dropped to 12 volts during the period of disuse?

If the voltmeter is built-in and includes high value resistors for scaling you often get very high readings at this time of year due to damp on the circuitry. Your true readings could be 8V (battery f***ed) and 17V (charger with no load).

Bill

It isnt

Your battery looks like an open circuit as far as the charger is concerned.

Have you tried measuring its voltage with a modest test load applied?

My little £13 Lidl one has pulse charging for a very low battery. But like all this type with reverse polarity protection, needs a few volts from the battery to pull the relay in. If the battery is too low to do this, jumping a good once across the bad with the charger connected will get it started. Leave it jumped for a short while to make sure the flat battery charges slightly. So saying, a totally flat battery may never recover enough to be useful.

Its open circuit somewhere? I mean if it were taking the load of the charger, it would probably be nearer 15v for a bit then settle to the usual 13.8 or so. Have you looked after the battery?

Brian

Oh, I had a lawn mower like that one of them electric ones with the small lead acid battery. . Brian

High interal resistance, typically due to sulphation. The old school method was to connect it to dc mains via a lightbulb. It sometimes got them good enough to start the vehicle, albeit not good enough for repeat starting attempts.

NT