Battery Drain

Hi Hope someone can point me in the right direction.

Connected my car battery the other day after a full charge as the cars not been out for a while and when doing so noticed it was sparking more than usual.

now i have had a multimeter on it and when testing the current to the car fuse box with the starter motor disconnected  its pulling around 3A which I'm guessing is way to high as nothing is switched on .
then I've pulled all the fuses and I'm still getting the same reading. :dontknow:

any ideas would be appreciated

Where did you measure the current?

hi paul thanks for the response

i measured across the positive battery terminal and the red wire going to the fuse board then i tried measuring from the positive on the battery again and the wires going to the starter and alternator which gives the same reading

hope that makes sense

Did you disconnect the red wire so that your multimeter formed part of the circuit? Or is your multimeter one of those induction pick up type ones for measuring current ?

Hi ade thanks for your post.

I have a meter in series with the circuit between the battery and the red wire going into the car.

You using the 10A unfused connection then - are you absolutely sure that you have the meter on the right scale - I have seen people do this before and the actual current flow is 30 or 300 mA.

3A is a lot of current to be flowing - if it was a short then I would of thought that you would smell some burning or hear it.

If you have pulled all the fuses then there is very little that can draw current (unless there is a short), next step would be to pull all the plugs from the back of the fuse box, including the main battery feed.

I would use the voltmeter not ammeter,
with a fully charged battery and not connected to car check voltage (around 12.50v) then connect to car and see if voltage goes down, if so start disconecting different items untill voltage suddenly goes up.

But he has current drain - so using an ammeter is the best possible way of determining it.

The voltage will not necessisarily go down if the current source (i.e. the battery) can continue to supply the current at that voltage (ohms law V=IR)

lhasadreams said

But he has current drain - so using an ammeter is the best possible way of determining it.

The voltage will not necessisarily go down if the current source (i.e. the battery) can continue to supply the current at that voltage (ohms law V=IR)

yes i uderstand this (as im a Auto electrican) in my years working on all types of 12v systems the best way to check on a car for a battery drain is with a voltmeter. you my need to stand a while waiting for voltage to go down but that also tells you how bad the drain is, and you can see instantly when you have found the problem circuit.

rosie81 said

I would use the voltmeter not ammeter,
with a fully charged battery and not connected to car check voltage (around 12.50v) then connect to car and see if voltage goes down, if so start disconecting different items untill voltage suddenly goes up.

lhasadreams said

But he has current drain - so using an ammeter is the best possible way of determining it.

The voltage will not necessisarily go down if the current source (i.e. the battery) can continue to supply the current at that voltage (ohms law V=IR)

You're both half right. The problem with measuring voltage is for a small current drain, the voltage difference will be very small indeed, and clouded by other issues such as removal of surface charge from the battery, battery issues themselves which can cause them to drain themselves, etc.

3A is a large current to be having a permanent drain of, for example its the equivalent of both brake lights permanently lit. So I'd double-check that this measurement was done properly.

What I've done before is to measure the current drain across each fuse holder, this is handy for finding which specific item is causing a current drain. And also measuring the current drain directly at the battery, by removing the terminal (the whole thing) and connecting the ammeter in 10A range in series.

Items which typically cause drains are:

The battery itself, self-discharging
Stereo (sometimes not wired up properly)
Alarm
Interior boot light
Alternator 'leaking' current across itself

If you suspect the battery is self-draining (easy to detect, by leaving it disconnected and measuring the voltage at regular intervals, eg every hour, for a couple of days) then as well as replacing or fixing the battey, suspect the alternator too. Often a faulty alternator initially shows up as a flat battery, or a battery seemingly unable to keep charge.

Yes you will be needing a digital voltmeter which can read to 100th of a volt (12.50) analoge would difficult. the problem with using an ammeter is you keep disconecting the circuit every time.
Not too much of an problem on older cars but sometimes things will power up for a few seconds or even minutes (radios etc)