Electrical drain

[Jonny-Jimbo]

Hi all,

One of 'moderns' has developed an electrical drain... What is the best way to try to identify it? Disconnect the battery at the -Ve lead and use a voltmeter to see what the draw is (if any), then pull fuses until it drops to zero? Or leave the battery on and use a voltmeter across all the fuses to see what's flowing?

[efp]

I’m no electrician, but think you need to measure the current draw across each fuse with an ammeter, rather than volts. Something might leap out as obviously sucking current.  It’s not simple on a modern as some circuits continually draw a little, like alarm, and some take up to 30 minutes to go to sleep after stopping.
But check the simple first? Anything you’ve fiddled with recently or has been acting strangely?  Are you sure the interior/ boot/ glove box lights go off when closed?

Went through this recently on my Merc, turned out to be the alarm siren unit in the whelarch: internal rechargeable batteries within had died and leaked, eating up the circuit board and shorting, trying to continually sound a non functioning siren..
Killed the battery and locked me out with no functioning keys. What a game that was..
Hope yours is simpler.

[JohnD]

JJ, if you disconnect the battery there will be no volts to draw or drop!
And disconnecting the battery on moderns can cause problems with alarms etc.
Might be better to use the resistance mode, with a lead on the earth terminal, and probe the circuits looking for a low or earth.  Trouble there is knowing what the various circuit resistances should be.

To measure current needs the multimeter to be in continuity, which means breaking the circuit to do so, unless you buy a clamp meter, that puts a loop around d the wire to measure by induction.  But they are costly:  
https://www.screwfix.com/p/kew.....znu9cCFSSJ7Qod_74D5w

John

[RobPearce]

Current clamps are good but they tend to be designed for much higher currents than you're probably looking for.
As EFP says, you need an ammeter, in series, which means disconnecting things and that, as John says, can be problematic on moderns.

It's probably the central locking or alarm module, especially if it's ever been near a VAG factory. Or it could be that the battery has failed without warning, as happened to my modern last Christmas.

[Jonny-Jimbo]

I should have put more detail in... when i say 'modern' I mean a 1994 BMW. I have not fiddled with anything, as AJ has only just bought the car - and decided she wanted a V8.

I checked it with the multi-meter in series, with the battery disconnected, and it showed no draw even down to the milli-scale.

It's a brand new battery, 2 week old Yuasa 110 Silver.

[Clive]

I would be checking the battery. Is the same as one of your other cars?? The timing looks about right, and although unusual a duff battery is a possibility.
Zero current doesn't sound right either, would expect something to register, maybe 2-5mA? No expert here but radio etc all have a tiny tiny draw.

[efp]

Quoted from RobPearce

It's probably the central locking or alarm module, especially if it's ever been near a VAG factory. Or it could be that the battery has failed without warning, as happened to my modern last Christmas.

In my albeit limited experience of moderns, the dangers of disconnecting the battery, at least briefly, seem to have become a bit overblown and the stuff of urban legend. I’ve yet to have had any problems other than the audio losing its settings and the windows needing retraining. But the owners manual should give you an express warning if it’s an issue on your car. Maybe I’ve just been lucky..

Yes, a multimeter (on the right settings!) across the fuse base should give you the draw, which should be nothing or just a few milliamps at rest. But one fuse can be feeding several circuits so it’s only a starting point sometimes.

No help in your immediate problem JJ, but if like me you are forever juggling batteries of suspect vintage, I heartily recommend getting something like this, which can measure the actual cranking amps left in a battery, not just the voltage which is a pretty useless indicator of its health

[Jonny-Jimbo]

I've got a fairly good battery charger than can test the CCA of a battery.

As I said the battery is brand new and only 2 weeks old at most.

Yesterday the battery voltage was at 12.6V (parked, nothing turned on), and today was 12.5V in the same condition.

One of the problems is that the battery is under the rear seat on the drivers side, and the fuse box is in the engine bay on the passenger side between the primary and secondary bulkheads. My intention is to monitor the battery voltage over the next week or so, taking a reading each day.

[Hogie]

Hi JJ,
           what was the temperature around the battery overnight. Cold weather with reduce the battery volts.

Roger

[efp]

Hope my responses didn't sound condescending J-J, not intended, just had a similar issue recently. I so much more prefer our ancients where few problems can't be solved without the right AF spanner and a large hammer..

[sparky_spit]

Quoted from Jonny-Jimbo

I checked it with the multi-meter in series, with the battery disconnected, and it showed no draw even down to the milli-scale.

Hi Jon - you may actually mean something a bit different to what you have written there?  If you disconnect the battery, and put a multi-meter in circuit in series, then you will see no current draw. This is because there is no battery voltage being applied to a fault/load/something switched on to create any current draw to measure.

Or am I being too literal and you actually mean something else?

[bxbodger]

All you need to do is disconnect the battery positive, set your meter to the amp setting (careful with the leads) and connect it between the battery and positive cable, and you should see a current draw. Some of it will be the radio, some of it will be whatever is draining the battery. You then just pull fuses one by one until you see the draw drop. That will lead you to the problem item.

[RedRooster]

Quoted from bxbodger

All you need to do is disconnect the battery positive, set your meter to the amp setting (careful with the leads) and connect it between the battery and positive cable, and you should see a current draw. Some of it will be the radio, some of it will be whatever is draining the battery. You then just pull fuses one by one until you see the draw drop. That will lead you to the problem item.

2nd that.

[Jonny-Jimbo]

For the last 5 days the battery voltage has sat at 12.5 or 12.6... this would suggest to me that there is not an electrical drain and the previous owner just had a dead battery and didn't know what to do with it.