Battery discharging.

[peanut]

ORIGINAL: GMcMillan

Hi,probably not a great idea to disconnect & reconnect a battery terminal to check for arcing as Bosch ecu's don't always like this,

fair point but we are not talking about arc welding here.[8|]
Just touching the earth strap connector to the battery terminal will tell you if there is the tiniest battery drain. Difficult to describe in text easier to demonstate.
You could always disconnect the ecu during the testing. The point is its easy straightforward costs nothing needs no test equipment and works every time[]

 

[GMcMillan]

OK but how do you find the difference between an arc that should be there ie radio memory & clock? After disconnecting the ECU what about system adaption? I know it costs a few pounds for a cheap multimeter but its the only way to reliably trace this fault.

 

[iangray100]

Just tripped over this Porsche technical service bulletin (TSB) in respect to diagnosing battery discharge currents .

If you have a coupe, in the clock you will find "bulb 2" .....its for a cabriolet roof !!!. With this bulb installed the battery drain is 5ma more, when the car is standing, no keys.

TSB mentions 10ma drain but on measuring its 5ma ....

Take the bulb out and you save 5ma drain .....

http://www.pcarworkshop.com/images/0/07/2706_1994_620294.pdf

takes 10minutes ....

My car wont get any were near the drain Porsche recommend for a 964 ...26ma with all the factory accessories only !!

But I do have an aftermarket radio, aftermarket alarm ( Porsche alarm as well but unused as not cat 1 and can't be removed !!) and battery cut out monitor to save a flat battery, the total drain is 55ma.

Before I fixed the CCU with a (17p transitor ) drain was around 1.5AMP !!!!

 

[rjc]

I'd be very suprised if anything under 100mA would lead to a flat battery even after a couple of weeks of non-use, assuming the battery is in good condition, so I wouldn't bother trying to get down to 26mA!