Battery keeps going flat

mollycat

Mutton_Geoff wrote: »

To the inexperienced, a multimeter is useless, as DaveyJJ says, it's cold cranking amps you need to measure which involves measuring current, so you'd need a current clamp for the multimeter.


Measuring voltages across the terminals will not tell you much about the state of the battery. Just get it tested by a place with the correct gear and replace yourself or have them do it.

Just for clarity, didn't suggest simply measuring voltages across the terminals.

Not sure why suggesting the OP learning something fairly straightforward like checking CCA is a bad idea?

AdrianC

So, umm, how do you plan on checking the battery's maximum current delivery ("CCA") with a ten quid multimeter?

Without letting the smoke out of the multimeter, obvs.

mollycat

AdrianC wrote: »

So, umm, how do you plan on checking the battery's maximum current delivery ("CCA") with a ten quid multimeter?

Without letting the smoke out of the multimeter, obvs.

Are you saying you can't check cold cranking amps with a multimeter?

I could provide dozens of you tube links; or you could just search for them yourself.

AdrianC

mollycat wrote: »

Are you saying you can't check cold cranking amps with a multimeter?

Yes, I am doing exactly that.

You measure the current being drawn by putting the meter in series with the load and battery. Multimeters have small fuses in them to protect their internals. Cheap multimeters normally measure a max of 10A, and only that by bypassing the fuse. Put one of those straight across a heavier draw than 10A, and watch the smoke come out. Even a cheap battery for a small petrol car will have a CCA rating around 350A, up to around 1,000A for a big diesel van battery.

If you put the meter between starter and battery, you aren't measuring the battery's maximum current delivery - you're measuring how much the starter's drawing - and that's probably <100A for a small petrol, up to maybe 250A+ for a bigger diesel.

You want to measure what the battery CAN deliver, you need to put a substantial load across it.

£15 or so will buy a 100A battery load tester.
https://www.amazon.co.uk/dp/B01F98LUMO/

You want to check the maximum deliverable current from a battery? You're spending around twice that.
https://www.amazon.co.uk/dp/B07MPNWQMR/

mollycat

AdrianC wrote: »

Yes, I am doing exactly that.

You measure the current being drawn by putting the meter in series with the load and battery. Multimeters have small fuses in them to protect their internals. Cheap multimeters normally measure a max of 10A, and only that by bypassing the fuse. Put one of those straight across a heavier draw than 10A, and watch the smoke come out. Even a cheap battery for a small petrol car will have a CCA rating around 350A, up to around 1,000A for a big diesel van battery.

If you put the meter between starter and battery, you aren't measuring the battery's maximum current delivery - you're measuring how much the starter's drawing - and that's probably <100A for a small petrol, up to maybe 250A+ for a bigger diesel.

You want to measure what the battery CAN deliver, you need to put a substantial load across it.

£15 or so will buy a 100A battery load tester.
https://www.amazon.co.uk/dp/B01F98LUMO/

You want to check the maximum deliverable current from a battery? You're spending around twice that.
https://www.amazon.co.uk/dp/B07MPNWQMR/

Appreciate the time taken to post what you have; given the amount of internet posts, youtube videos and other tutorials available, I contend that the use of multimeters to test CCA is valid.

Unless you want to tell all those guys they're wrong?

Back to the OP, he/she has a (probably) goosed battery they are trying or hoping to "drive" back into a usable state charge wise....presumably to cope with the rigours of winter.

I don't think the purchase of a cheap multimeter to easily determine if the battery is a no hoper is a bad idea as an alternative to his/her idea of just purchasing a replacement.

Paying for someone with the "right gear" to check it is a false economy if the £10 multimeter gives an obvious low reading on the simplest across the terminals check.

Off out for a pint with one of my old apprentice colleagues; he's a time served mechanic, if he tells me i've been talking peesh I'll be right back to say so.

AdrianC

mollycat wrote: »

Appreciate the time taken to post what you have; given the amount of internet posts, youtube videos and other tutorials available, I contend that the use of multimeters to test CCA is valid.

Unless you want to tell all those guys they're wrong?

Well, the first google result is...
https://www.youtube.com/watch?v=tsJUuLu1cw0

And from a quick watch, he's not measuring current in that video. He's measuring voltage drop. And even then, he's not measuring the battery's maximum output, just what the starter's pulling.

Oh, and the multimeter he's using...?
https://www.amazon.co.uk/dp/B0002YFD1K
£360

mollycat

AdrianC wrote: »

Well, the first google result is...
https://www.youtube.com/watch?v=tsJUuLu1cw0

And from a quick watch, he's not measuring current in that video. He's measuring voltage drop. And even then, he's not measuring the battery's maximum output, just what the starter's pulling.

Oh, and the multimeter he's using...?
https://www.amazon.co.uk/dp/B0002YFD1K
£360

So you can use a multimeter to check the CCA of a battery then?

That will explain why every single person in those you tube videos, my dad who owned a susscesful garage for many years, my brother who ran the garage after my dad died, myself, my mechanic mate who i had a few pints with earlier today, and numerous others who all use a multimeter to check CCA don't end up with a smoking, melted lump of plastic in their hand after doing so.

Back in the real world...OP, hope you got it sorted.

AdrianC

mollycat wrote: »

So you can use a multimeter to check the CCA of a battery then?

No. You can use it to measure voltage drop while operating the starter.

From that, you can come up with a guesstimate of how much current the starter is drawing.

That's not the same as the battery's CCA capacity.

Jackmydad

My newish 30 quid multimeter does min / max readings.
And it's reading voltage so hopefully no chance of melting the meter.
Worth having at least some idea about voltage, current, and resistance though before using a meter.
It's not complicated.
Oh and "I had X changed 12 months ago so that should be fine" doesn't work in the real world.
I've changed (non car, but that makes no difference) electrical stuff in the past, and had it go wrong after a short period, done the same again, made all the checks I could think of, had another guy look at it as well, changed the part again and it's been fine ever after.
In general electrical stuff is pretty reliable. No knowing though.

Jackmydad

AdrianC wrote: »

No. You can use it to measure voltage drop while operating the starter.

From that, you can come up with a guesstimate of how much current the starter is drawing.

That's not the same as the battery's CCA capacity.

Yes, but to be fair it gives a decent indication of the batteries condition with a cheap bit of kit.