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Battery Voltage Drop - How Much ?
perksy - 5/12/10 at 05:27 PM

Quick question please lads

What sort of Voltage drop should you see when a Multimeter is connected between one of the Battery cables and its Battery Terminal ?

ie how many Amps OR Millivolts ?

This would be on a modern petrol engined car with an ECU



Cheers
Perksy

matt_gsxr - 5/12/10 at 05:36 PM

Under what conditions?

Engine off, cranking, engine running

BenB - 5/12/10 at 05:48 PM

If you're connecting in between the cable and the connector (ie in series) then you're measuring AMPs. If you're connecting it in parallel then you're measuring volts.
Either way it depends on the situation as already said.

perksy - 5/12/10 at 05:50 PM

Sorry

This would be with the Engine turned off, no lights or radio on etc

RazMan - 5/12/10 at 05:53 PM

Quiescent (open-circuit) voltage at full charge: 12.6 V
Fully discharged: 11.8 V
Charge with 13.2–14.4 V
Gassing voltage: 14.4 V
Continuous-preservation charge with max. 13.2 V
After full charge the terminal voltage will drop quickly to 13.2 V and then slowly to 12.6 V
Open circuit voltage is measured 12 hours after charging to allow surface charge to dissipate and enable a more accurate reading.
All voltages are at 20 °C, and must be adjusted -0.022V/°C for temperature changes.

[Edited on 5-12-10 by RazMan]

russbost - 5/12/10 at 06:04 PM

Razman, I think you've misunderstood the OP's Q - tho' your voltages are spot on & I'm sure will be useful info for others.

I think the Q is "if you look at the change in voltage when connecting direct to the battery post as compared with connecting to the other side of the connector, what difference would you expect" - if that is indeed what the OP meant then the answer is virtually no difference at all other than tiny fractions of a volt - if there is any significant resistance at the battery connection point (-ve or +ve), which would need to be there to produce any significant voltage drop, then you are certainly going to have starting/cranking probs amongst others!

RazMan - 5/12/10 at 06:20 PM

oops! Of course you are right on both counts Russ.



Ignore what I just said then - it all depends on the size of the cable, but as Russ said, you should only see a fraction of a volt drop. So if you are seeing more than that you have a high resistance somewhere so start checking tightness / cleanliness of earth terminals and connectors.

perksy - 5/12/10 at 06:28 PM

quote:

if that is indeed what the OP meant then the answer is virtually no difference at all other than tiny fractions of a volt - if there is any significant resistance at the battery connection point (-ve or +ve), which would need to be there to produce any significant voltage drop, then you are certainly going to have starting/cranking probs amongst others!




By tiny fractions of a Volt would that be 0.1 volt or ?

Obviously there's things like the digital clock etc

But just wandered what these should all add up to




Just trying to help a mate out with a Battery that seems to discharge overnight


Thinking it might be the Battery about to Die at the moment as the Alternator seems to be doing its job



Thanks

russbost - 5/12/10 at 06:47 PM

Ahhhh! Different Q entirely!

You don't want to be looking at voltage drops, look at current drawn. Disconnect one terminal & put a 10A ammeter in series with the disconnected lead DO NOT under any circumstances forget you've got it on there & try & start the engine or you'll have a fried ammeter!!! Exact amount of current drawn will vary according to radio/clock/alarm etc but you would expect probs less than 0.3 Amps being constantly drawn (& imho that would be quite high)

If you're drawing more than that try pulling fuses till the current draw goes away. If it still doesn't try disconnecting the alternator as you could have a diode leaking current. HTH

Daddylonglegs - 5/12/10 at 07:38 PM

Sounds like the battery is on it's last legs to me.

Best thing to do is stick an ammeter in-line with the battery supply cable and see what sort of current it is drawing. Shouldn't draw more than a few hundred milliamps I wouldn't have thought, even with an alarm fitted.

[Edited on 5/12/10 by Daddylonglegs]

RazMan - 5/12/10 at 07:44 PM

If the battery is going flat overnight then there must be a circuit still live which is taking a significant load - either that or the battery should have been given the last rights months ago

DRC INDY 7 - 5/12/10 at 07:59 PM

With no load from the battery to power the systems and ecu that has gone to sleep with an IOD (ignition off draw test) then you are looking at 0.01 amps if you are pulling 1 amp upwards then you need to check each circuit ie pulling the fuses till the amps drop

perksy - 12/12/10 at 09:32 PM

Thanks lads

It was the draw test info i was after


Appologies, Just re-read my original question and it was the ramblings of a muppet

That'll teach me for posting a question after testing the latest batch of stella artois


Cheers
Perksy