Testing a 12v battery

Is this a car battery?

Generally, if it has near the capacity (in amp.hr) which is usually marked on it somewhere, it will be good. Not to say there could be one with another problem, though. You can check this by running a reasonably heavy load like a spare headlamp bulb and timing how long a fully charged battery lasts. A 55w headlight bulb takes about 4.5 amps, so a 45 amp.hr battery would last about 10 hours. A 90 amp.hr one about 20 hours. And pro rata. The test load should ideally be one which would run the battery flat in 20 hours, so does depend on the battery size.

But any half decent battery place will have a tester that tests and calculates this instantly. Even, I'd guess, Halfords.

BTW, many modern cars will run their battery flat in a few weeks, if not driven.

Many thanks. That's what I was hoping for. I've no doubt the supplier could check it but I'm a suspicious person and i.s.t.r. having a battery once that tested o/k for initial voltage but had no endurance.

This was an boat engine battery. They normally hold up o/k over the winter but this one went right down to 0.8v. There must be a drain on it somewhere but I'm buggered if I can work out where.

Many garages and car battery suppliers offer free battery tests so try one of them rather than your original supplier. They certainly won't have any incentive to be over optimistic about the condition of your battery.

Voltage alone isn't much of a test.

I'd be inclined to remove it and keep it indoors over the winter, if not used.

If this is a Calcium battery and not an Indium one, the alternator may not be charging it fully. IME Only lead indium batteries have a decent recovery chance when highly discharged. As it went down to

What sort of 12V battery as it makes a differnce. Is snuff a new SI....

If it was allowed to run flat and not re-charged PDQ, it is likely knackered. And 0.8v is pretty flat. ;-)

After all the replies assuming it was from a car he'll probably now tell us he was talking about one of those 12V batteries you find in lighters and doorbells. ;-)

I use these (there are cheaper ones but this was the first hit)

on a ride on mower and an all terrain vehicle. I also had a key type isolators on a horse lorry once.

Main failure modes, from an electrical only pov, include: loss of capacity deterioration in internal connections creating higher & unsteady resistance self discharge.

If you wanted to self test it you'd need to check for those.

NT

Yes indeed, the newest motors seem to run a lot of stuff when its apparently off. Also of course its worth making sure the battery is charged by the vehicle correctly.

Another thing to look at is that the normal voltage of a 12v lead acid battery tends to be about 13 volts, a bit higher when being charged, but if the voltage drops below 12 when on a decent load, one of the cells could be on the way out which will quickly knacker that cell and make it almost impossible to draw a decent current from it. Brian