how fast does your battery discharge ?

[oldnick]

I recently experienced a problem with the battery going flat in a couple of days when the car was left in the garage

having found the problem [ or maybe A problem ] I recharged the battery and , in view of the fact that there is always going to be a small drain because of all the gizmo's fitted , decide to do a test to see just how fast the drain is going to be

car is currently not being used , so left it for a couple of days for the battery to settle down to it's base voltage , then tested the voltage daily for a week ; it drops by .03 volts per day , doesn't sound much but it means that a newish battery which fully charged gives 12.7 volts would drop to 12.4 volts in 10 days , which is about as low as I would want to go and be able to start a V8 on a cold morning , so no leaving it in the airport carpark for a fortnight while you go skiing !

question is....is this a normal rate of drain ? first time I have had a car with lots of electronics , my 20v has just a clock and I can go off for the winter and after 4 months it starts on the button [ so to speak ] ; is this the rate of discharge that others experience or have I got a tiny leak I haven't yet found?

if it is normal , maybe I am going to have to invest in one of those tiny solar panels that plug into the cigar lighter ?

[The_Laird]

Dunno about the values of the discharge, but I've never had a problem starting the car after 2 weeks away on holiday

[David's8]

Ditto what The_Laird says. That doesnt seem right to me.

[sloss]

It's not the voltage that starts the car, it's the Amps the battery can produce at the given Voltage.

[oldnick]

Originally Posted by sloss

It's not the voltage that starts the car, it's the Amps the battery can produce at the given Voltage.

not quite !

watts is the unit of power , so applying ohms law shows power loss proportional to voltage drop !

[sloss]

I'm trying to point out that no matter the voltage if the CCA ain't there then it won't start the car

[ainarssems]

Originally Posted by sloss

It's not the voltage that starts the car, it's the Amps the battery can produce at the given Voltage.

Voltage drop does not mean high discharge rate. You need to check how much Amps or miliamps car is using with everything switched off.

[oldnick]

http://scubaengineer.com/documents/l...ing_graphs.pdf

well , it's not a perfect world and I for one don't have a SOC meter , neither am I prepared to do a temperature related SG test !

but as you can see from the above a simple voltmeter is quite capable of giving you an assessment of what is left in your battery

you can see from the graphs a battery with 12.4 volts is some 50% discharged [ neither quite the 'rest' graph nor the 'discharge' graph in this case ]

bearing in mind the increased internal resistance of both the battery AND the engine in cold temperatures I personally regard 12.4volts as a sensible minimum for cold cranking without adversely affecting the battery ; having said that my 2.8 turbo diesel is fine down to 12.3 volts at , say , 15C ; 12.2 volts , as I know from experience , isn't up to the job

but really I was hoping to ascertain whether others had a drain of the same order as me on an A8 !

[47p2]

You need to find the source of the drain using something like this


Once the car's asleep you remove one fuse at a time and connect the above and take readings, a time consuming job but it will eventually lead you to the drain source

[oldnick]

Originally Posted by 47p2

You need to find the source of the drain using something like this


Once the car's asleep you remove one fuse at a time and connect the above and take readings, a time consuming job but it will eventually lead you to the drain source

yep , been though all that [ time consuming as you say !! ] found the drain , fixed , stuck microswitch

but there is SUPPOSED to be a drain to keep the electronics going , question is , is the level I am now experiencing the normal level ??? I know in the latest mercs it is as high as 50 mA but don't know for the A8

or is there another drain source ? trying not to have to do the whole job again!