W123 parasitic draw
 

Today I had to swap a spare battery to start my 300cd (for the first time in years; cranked right up).

Before attaching, I put a quality ammeter (fluke 87V) in series and got a parasitic draw of 20mA. Each second as the clock ticked it would bump to 23mA.

Ive always read that <50mA is acceptable in “modern cars”. That is, cars with electronics and modules that may be on or may be in a sleep mode.

My cd has a quartz clock and a digital Becker radio that has memory.

20mA therefore seems a bit high. Ive not messed with a w123 battery in over a decade, and have never put a meter on before because Ive never had an issue.

23mA seems high. Has anyone measured before?

I should check my 240d as the battery keeps VERY well.

But does anyone have a data point??

Sounds fine, it would take close to two months to drain the battery at that rate. Try unplugging your alternator and then measure parasitic draw. Most of that leakage is probably due to the alternator diodes.

I measured 20mA last month on my new 300d. New to me I mean. It seemed like a lot for an older car but I realized there was an aftermarket stereo in there with a standby mode.

Check your radio power antennae. Disconnect the harness from the motor and monitor your battery charge over a period of time.. a week...

Also if your 300D has the Klima AC servo, then it might be stuck in the 'ON ' position inside the head of the unit and/or your AC computer behind the glove box may be shorted...

you can bridge the battery termionals and methodically remove fuses in order to try to observe a draw. Do an archives search.

My thoughts on this after speaking with other MB owners, well one anyway.

Cars fitted with OM606 engines have similar parasitic draw regardless of spec, however those with higher specs have higher rated batteries which helps overcome this issue.

Example, a/c fitment requires additional fans which tend to run after vehicle shut down.

I can't remember the exact figures but my 210 with a/c factory spec 120mah battery, v E300 no a/c 80mah.

Thanks; yes aware of how to do a test and isolate.

The value I observed doesnt seem high by any standard other than the fact that this is a fully mechanical W123 with the only draws being Becker radio memory and the clock...

Batteries are rated in AH, not MAH. A typical battery for a 201, 123 or 124 Diesel will have a 70AH capacity, enough to last 145 days @20 ma (actually, longer, due the the Peukert effect.) Sometimes batteries are rated in hours of reserve capacity, measured with a 25 amp load. So a similar battery might be rated at 120 reserve minutes. That works out to more than 100 days @20ma, but will actually be much longer due to Peukert's effect. So you have 3-5 months before this draw becomes a problem. Most of these small losses are going to be leakage through the alternator diodes. If you're really worried, replace the alternator. Just be warned that even brand new diodes will have some leakage. Better to find something else to worry about.