Batteries and multimeters

An open circuit using a resistance setting on the multimeter would read "out of range" or a "1" at the far left of the screen (i.e infinite resistance).

Without engine running 12.8-13.2V, with engine running, and charging properley up to 14.2V.

Adam

Should be around 12v with the engine off and nearer 14 with it on. Yours is showing you that the alternator is charging but not at low revs. It's quite common for that to happen and if you spend a lot of time stuck in traffic you may well run the battery down. It could be a sign the battery isn't in it's best condition or that the alternator isn't providing a (very) good charge.

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When the car is running, you should see between 13-14V, ideally 13.8.

Lights do go dim when idling and it's even more noticable on cars that do short journeys.

Good advice, but note that silver calcium batteries have been used on many vehicles since the late '90s and these charge at up to 14.8V.

Chris

My journey to work is short, approx 4 miles. May go to kwik-fit, see if they do free battery checks?

The alternator supplies the power for the headlights with the engine running - although a battery in a poor state of charge will make them dimming more obvious as it's taking some of the output.

The open circuit voltage is dependant on the state of charge and how long it's been left to settle before taking the reading - as well as whether the battery is 'good' or not. So basically pretty meaningless on its own.

The 14 volts (ish) you quote is for a battery on charge. That measurement is useful for checking the alternator - not the battery. To check the battery you need to do this under heavy load.

IIRC, Maplin sell a decent little book about for getting the best out of your multi-meter. Like all tools, the skill is in knowing how to use it.

Please don't...

Chris

Just to expand, Kwik Fit use a discharge tester that puts a large load on the battery, only meant to be used for a very short test, not clamped on until battery flattens so that they can sell you new battery...

Adam

diy-newby ( snipped-for-privacy@asas.com) gurgled happily, sounding much like they were saying :

Free Battery Check, Kwik-Fit style... "Yur, mate, s'fooked, innit? £98 plus vat"

Yes, they will happily test whether they can flog you a new battery when you don't need one.

If it is reliably turning the car over briskly, then don't worry about it.

It is not unreasonable to expect the lights to dim a bit when idling, especially if you have significant other loads on like fan and rear demist. Get back to us with measurements of your battery voltage is with the vehicle idling a) with no lights or equipment on, b) with lights on, c) with lights on and driver compartment fan on at full blast and rear demist on. Also what the battery voltage drops to when cranking the engine, preferably after it's been for a half-decent run so the battery should be moderately well charged.

On your heated seats thing by the way, if you know what the power output in watts of the seat heater is, then take 144, divide it by this power output, and it will tell you approximately what resistance you should be measuring across the plug. So for example it's a 100W seat heater you should measure about 1.4 ohms, give or take. It's only a rough guide though.

I suppose that's why a lot of cars breakdown sitting in motorway traffic jams in high summer,electric fan kicks in kills battery.

Nick

In that case, the alternator never really gets chance to replace the charge lost during starting.

It might be an idea to stick it on a battery charger overnight.

Yes, from P=V^2/R therefore R=V^2/P for nominal voltage of 12V this will be

Or not.

Based on my Nissan Micra.

It starts in well under a second. If that's 100A, that needs about 150 amp-seconds to recharge it.

At 3A, this is only a minute.

Note, that headlights on for 5 seconds may well use the same amount as the starter.

In my case - the battery was indeed broken, and cost 40 quid. Admittedly, it was a smaller battery than most, but I only found it for about 5 quid cheaper.

If the dimming and failed seat heating came about the same time, they are related. The alternator is not pumping out enough electricity to the lights, to replenish the battery and to heat the seat. You may find other power hungry devices not working too; to name just one other, the rear window heater, for example.

Now that you have no idea what "open circuit" means, don't waste you time and money, get an auto-electrician to have a look at your car.

I strongly advise you should go to an evening class to make up what you did not pick up (this, and other stuff) at school. You may get yourself burnt badly messing with car electric if you haven't got a clue of what you are doing!

It needs sorting, then. It can and does on both my cars.