Voltage drop test for parasitic battery drain
 

Anyone using this as an alternative to current leak test? Looks like a better method and going to try it on the cdt which I still suspect of having a faulty circuit with battery going flat in 4 or 5 days. It may just be be the battery of course, but battery holds charge when disconnected.

strange this is the second time in only a few days I have seen the mention of a drop test, I had to look it up first time I saw it.

A similar test, is with engine off, turn on everything electrical and then press the horn.
Do headlights dim and horn sound meek?

macafee2

I understand the theory is that when looking for a source of current drain on a car with complex electronics its a better way than the old current drain test as you don't have to disconnect battery before testing and risk reactivating computerised modules when the circuit is bridged with the meter. Makes sense.

Please tell us where we can read this theory.---:shrug:

lots of information out there

try these to start (you will find much more on google):

https://www.youtube.com/watch?v=lRcj1fQcWwU

https://www.autonerdz.com/yabbfiles/...crossfuses.pdf

I by preference would initially use a dc shunt to test for a parasitic battery drain, once fitted there is no need to disconnect/reconnect until testing is completed, the car can be started and run with it fitted and there is no risk to your meter (connect it to the shunt, set to mV and read display as amps - i.e a reading of 1mV = 1 amp).
With the shunt fitted you can also accurately measure starter motor draw and alternator output etc.
If the parasitic draw measured is very small, then you can connect your meter conventially and measure a ampage directly knowing there is no risk of meter damage.
The shunt I use is a Briggs and Stratton item as below and is a valuable part of my toolkit.
(Sorry about the quality of the picture, I can't get it to upload any clearer than that)

Yes, I think it's the same but the method in the video measures the drop across each and every fuse to locate the leak more precisely. The tables remind be of trig tables we used at school. But not so complicated.

This was how I was going to address the problem.

https://www.the75andztclub.co.uk/for...d.php?t=303962

But I can't find anybody who knows how to drive it in software, nor a Rolls Royce Phantom, that I can fiddle with (legally anyway) :D

The problem with the voltage drop method is that with a shunt value of 1milliohm, a discharge current of 10mA would give a voltage drop of 10uV, which cannot be resolved with a 3.5 digit 200mV digital multimeter.

Hence use the BMW part, which has been designed to do exactly what we want.

Yes that looks right Mark.

From the table you would need a meter capable of accuracy down to 0.1mv to detect 30mA drain on a 20A fuse.

I'll try it out and report back.

Also not sure if having the bonnet open keeps the car awake.

As mentioned you can check the current drain without disturbing the various ECUs - Harry on here had a good method.

I followed French Mike's suggestion of using a second battery wired in parallel (with the Ammeter in series with the second battery) so that once the ECUs have gone to sleep the Ammeter can be switched on, the car battery terminal disconnected and the second battery would provide the power.

https://i.imgur.com/QRNqbYs.jpg?1