Batteries and multimeters

Not sure if my battery is a little flat. Starts ok but I have noticed that the head lamps dip when the car is idling and it looks like the original battery (car is 6 years old).

Going to buy a multi-meter this lunch time to test the voltage. What

*should* the voltage be of a good battery. I have checked around on the web and some say high 12's, another post I have read says 14!

Could someone post the range for a *good* battery and what the minimum volts should be.

Also, another reason for buying a multi-meter is my heated seats are not working. Have been told to check the plug which goes to the seat and check the ohms, if this is an open circuit, then the heating elements is knackered. What is an open circuit, what would this show as on a multi-meter?

Thanks in advance.

Reply to
diy-newby
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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).

Reply to
Sandy Nuts

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

Adam

Reply to
Adam Aglionby

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.

Reply to
adder1969

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Section 6. HOW DO I TEST A BATTERY?

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Reply to
Mike Barnard

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.

Reply to
Conor

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

Reply to
Chris Whelan

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

Reply to
diy-newby

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.

Reply to
Dave Plowman (News)

Please don't...

Chris

Reply to
Chris Whelan

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

Reply to
Adam Aglionby

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"

Reply to
Adrian

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.

Reply to
Vim Fuego

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

Reply to
Nick (Scots)

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.

Reply to
Conor

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

144 (since 12^2 = 144), you can substitue your own value if you know the voltage. Conversely, if you can separate the seat wiring from the rest of the car wiring then you can probably measure the resistance directly using your multimeter as the seat heaters will more than likely be purely resistive ie no inductance or capacitance.
Reply to
Robbie Mayhem

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.

Reply to
Ian Stirling

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.

Reply to
Ian Stirling

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!

Reply to
Lin Chung

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

Reply to
Dave Plowman (News)

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