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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22755
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Mercedes slk battery going flat when not used. Battery reasonably

Customer Question

Mercedes slk battery going flat when not used. Battery reasonably new previous owner changed it because he thought infrequent use.

Battery charges to 12.5v, when engine running voltage is ~14.4 v

I believe the problem is that it is drawing 1.8 amps when no key in ignition

Any ideas would be helpful
Submitted: 3 years ago.
Category: Mercedes
Expert:  Shantal-Mod replied 3 years ago.

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Expert:  Matt replied 3 years ago.

if its drawing 1.8 Amps with the ignition off then you're definitely right this is reason for the battery drain as a typical current drain with everything off and un-alarmed should be around 0.05Amp

the best way to find what circuit is causing the excessive currant drain is to fit a current sensing multimeter in series with the positive battery lead on the car and the battery, or use a current clamp meter on the battery positive cable.

then remove a fuse and re-measure, if measurement doesn't change then replace the fuse and remove the next.
Once you've isolated the circuit with the excessive drain check the wiring connections and plugs to components and the wiring harness that runs by the engine bay fuse box - the wiring harness chaffs / rubs through and gets damaged - but its not always easy to see so be careful when checking..

Customer: replied 3 years ago.



I have taken the car to a garage, from their sophisticated test kit, they suspect a lazy alternator, it was only just charging and didn't increase with revs. I have read somewhere a faulty alternator can also drain the battery battery is this right?

Expert:  Matt replied 3 years ago.
You're quite right a lazy alternator will just not charge the battery

You can check that the alternator is actually charging the battery in the car by taking a battery voltage reading with the engine off then another with the engine running. The 2nd reading should be higher than the 1st and climbing steadily. And also check that the alternator is secure to the engine as this provides the ground for the charging circuit and make sure that the engine / gearbox earth strap is secure and the contacts are clean and bright.

if the alternator appears not to be working - take it to a electrical specialist and ask them to test it as sometimes you can replace just the diode pack and this will repair the alternator without requiring replacement

however 1.8amp worth of drain could be something other than a faulty alternator so by all means try the test I suggest 1st

but then be prepared to work through the different fuses as I mentioned