'94 reg W202 C220 battery drain

[DIYspanner]

2.2 petrol

Basically there's a drain somewhere albeit the 670A battery has been tested and only has 407A capacity with a "BAD & REPLACE" on its test earlier this week.

I've tested the alternator B+ terminal for the diode pack and it appears to be working.

Anyone Merc gurus who can say what are the likely causes of a battery drain are on one of these motors?

 

[Harry Bloomfield]

That reads as if the battery is near dead, why are you looking for an issue with the car causing a discharge, have you checked what the level of discharge actually is?

The way to check it is - avoid cranking or turning any loads on throughout the procedure.

Park and lock car, with bonnet open to give access to the battery. If there is a bonnet open detector, bypass it, so the car thinks it is closed.
Disconnect battery negative main terminal, fit an easy to disconnect jumper lead between battery -ve and the -ve lug.
1 Connect a DC ammeter across the same connections, set on 10 amp range.
Remove the above link, so all current goes through the meter, wait and watch the meter for maybe 10 minutes
If the value is too low to read, repeat from 1 above, but using the 1 amp range.
If the value is too low to read, repeat from 1 above, but using the 0.1 / 1000mA range.
If the value is too low to read, repeat from 1 above, but using the 100mA amp range.

If that last range shows less than perhaps 50mA, then you do not have a discharge. Keep in mind that disturbing the power will wake up the cars electrics and it will take a few minutes for it to go back to low power sleep mode.

 

[DIYspanner]

Thanks Harry!

I'd looked at the car last week and had to jump start it. When I left it, the battery was at 11.8v. I went back the next day and all the electrics were dead and the battery was 0v.

I took the battery off and charged it to full which took 22 hours. Let it stand for 3 days and the voltage dropped to 12.46v, hence my suspicion of a drain.

Got it tested at a local factors.
670A rated battery measured at 407A.
State of health: 60%
State of charge: 82%

Put it back on the car today and it cranked and the meter showed a 10.2v cranking voltage. I've disconnected the battery black to stop any drain for the time being.

 

[Burnerman]

Mercs of this era suffered significant battery drain courtesy of the security system.....if the car was left for two weeks, no way would it start.
John

 

[DIYspanner]

Mercs of this era suffered significant battery drain courtesy of the security system.....if the car was left for two weeks, no way would it start.
John

So if the car is not used often, the remedy is to keep the battery disconnected (negative cable off) and take it for a 10 mile spin every week to stop it seizing up. Its kept in a garage so not hassled by the weather.

 

[Harry Bloomfield]

So if the car is not used often, the remedy is to keep the battery disconnected (negative cable off) and take it for a 10 mile spin every week to stop it seizing up. Its kept in a garage so not hassled by the weather.

Some cars fine tweak the engine settings as they drive the first few miles, then forget them if the battery is disconnected. The 'relearn' is barely noticeable, though something to be aware of.

My car is used irregularly, lives in the garage, so I have a charger connected to boost the battery for 20 minutes, once per day. I use a Smart Plug plus Alexa to do the timing.

 

[JohnD]

So if the car is not used often, the remedy is to keep the battery disconnected (negative cable off) and take it for a 10 mile spin every week to stop it seizing up. Its kept in a garage so not hassled by the weather.

A short run on a cold engine is damaging.

A battery charger is more suitable.

 

[Burnerman]

On the Merc I had to look after there wasn't any power nearby so I had to disconnect the battery and run the car occasionally - relying on jump leads if I'd overdone things. Nothing came to any harm over the 12 months I was responsible for the car.
John

 

[JohnD]

I was thinking of condensation leading to corrosion in the engine and exhaust, and in some cases fuel contamination of the engine oil which does not get hot enough to boil it away.

OOI, I once read the "pilot's notes" for a Hurricane, among the effects of a relative who had flown in WW2, and it recommended adding petrol to the engine oil in very cold conditions, to make it thin enough to flow. It said it would harmlessly boil off at operating temperatures.