My wife’s 2013 ML 350 (standard gasoline engine) sat for a couple of weeks during the recent holiday period. The wet fill battery was replaced about 2 1/2 years ago (Interstate MT Group 49). Upon return, I did a 2 errand trip. Cranked over a bit weakly on start and 1st stop restart. 2nd stop restart was a no-go. Fortunately, a quick jump got me on the way. At home, I recharged fully….it goes to 12.84 volts with full charge and the Schumacher load tester read 800+ CCA (battery rated at 900). However, i drop down about 25-30 volts overnight, although CCA doesn’t drop much. I’ve read some generic articles on parasitic draw testing….however, I don’t want to actually disconnect the negative cable for the “in-series” voltmeter test for fear of losing my memory settings. I opted to try the “amp clamp” test, but the underseat location makes it a bear to try and lift up the battery enough to position the clamp around the negative cable cleanly for a nice perpendicular placement. With the less than ideal positioning, the Klein amp clamp read about 125-140 milliamp activity. I can’t find a Mercedes’ spec sheet on just how much electronic draw will still occur in overnight “sleep” mode….does anybody know that number/range? I have read generic articles suggesting newer cars might draw 50-80 milliamperes, so, at first thought I am thinking I might have a circuit that is still drawing too much in sleep mode. I did a search in this W166 forum for the subject, but it came up blank. Does anyone know where the negative battery cable connects at the other end in hopes I might get a more accurate amp meter read? I tried the positive cable jump port in the engine compartment as an alternate reading position, but the cable runs in a narrow channel against the fender and I can’t fit the clamp in there either. Anyone know how/where Mercedes mechanics get their test reads?

 

Just in case your car has KEYLESS-GO, try disconnecting the push button from the ignition overnight and check the battery in the morning.

Sis' W166 (2012) was draining the battery enough every night until I asked her to remove the push button, the problem was "solved". Sure I need to get back to it to find out what is wrong with the keyless go module, but she has been driving w/o it for 6 months now, no issues. Something she also mentioned was that if she properly shut down the keyless go (that is lock the car once, and press lock one more time I believe) the car was also OK overnight. I have not verified that behavior, so please take it with a grain of salt.

 

Our car does not have the Keyless-Go feature, however, I have seen videos where the "double-locking" on the key fob is employed as a technique to help ensure the electronics "go to sleep" as much as possible. Since that is such an easy "with and without" test to perform, I am more than happy to give it a try and see if that reduces the voltage loss overnight. Thanks.

 

Not sure double locking will help. These trucks go to sleep after a period of x minutes when being locked. I recall vaguely it's around an hour or two.

130ma isn't outrageous. Especially with driving if it started the first time, it should definitely keep starting.. you wouldn't have a parasitic draw drain it that fast. The only thing that is draining it is repeated use of the starter. However it's also being recharged. Basically parasitic draw means nothing for the use you are explaining.. it is more related to leaving the truck overnight than for short trips.

And the 130ma is reasonable with all of the electronics awake. To shut them off it's pretty difficult and there is a specific procedure. I think you lock the truck with the rear door open, but with the rear latch locked (so it thinks the door is shut). Then you leave it locked for an hour or more. Then you test. The way you are doing it will always show a higher number because electronics are awake .. AND maybe the dome light is even on too, because you have a door open!

I would have your battery/starter/alternator tested first, to rule those out. Try also charging the battery and start the truck, drive it a few miles, shut off, then keep repeating. It shouldn't die at all as lots of people make many short trips and the truck still runs. If it dies, you have nothing to do with parasitic -- you have an alternator that isn't charging, a starter that is drawing too much amperage, or a weak battery.

My guess is starter/alternator/battery -- so have those tested first!