Electrical drain on battery

mercedes-benz
s-class

#1

If we don’t drive the car in 6-7 days, the battery is dead and needs to be recharged. This is the 3rd battery we have replaced. Our maintenance shop checked it out but couldn’t find a problem. However, it continues to do exactly the same thing. After 6-7 days of non-use, it will not start.


#2

Sounds like a battery maintainer is in order until the problem is fixed.


#3

Ask for a “Parasitic Load Test”, a simple procedure. You can perform it yourself if you have a ammeter that has at least a 1 amp (1000 ma) setting.

Disconnect one of the battery cables and place the ammeter between the battery post and the cable end. Read the draw (load) on the battery. Remove the fuses, one at a time, until the load disappears. Repair the circuit protected by that fuse. You can expect to see 50-100 ma drain reguardless, thats normal. But no more than that…100 ma is the high end of “normal”…


#4

The battery for that vehicle probably has a 50 Amp-hour reserve capacity.

So it is discharging the 50 Amp-hour capacity over 6 days, or 144 hours.

That means the average current drain is 0.35 amps. At 13.6 volts, this is about 4.8 watts.

[b]Guess what size your glove compartment light is?

4.8 Watts.[/b]

See if that’s it.

EDIT: How to check to see if this is the problem.
Open the glovebox and quickly feel the bulb. It shouldn’t get hot until it’s been running a minute or so, so if it’s hot, that’s the problem. Closure switch is faulty. Just pull the bulb and place a small flashlight in the glove box.


#5

There can, also, be a battery drain if some electric motor or light comes on after the car is shut off. Motors or lights can come on minuets, or hours, and go off, anytime after shut down. To check, at intervals after the engine is shut off, go to the car and look for lights, and listen for the sound of a motor (such as the radiator fan running). If you see, or hear something, tell your mechanic.


#6

I thought amps were measured in parallel, not in series. If that’s the case, then we can’t place the ammeter between the post and the cable. Ampheres are current-reading, not voltage- or ohm-reading. Is there another way to measure if the fuses are bad?


#7

Alarms, door-locks, hidden or unnoticed lights…


#8

The amperage meter is placed, in series, between the battery post and the battery cable to measure the total amperage (current) which is flowing from the battery, through the wires and components, and returning to the battery at the other post. Each fuse is pulled, and the ammeter is observed for a drop in amperage. When a fuse is pulled, and the amperage drops, that fuse protects a current drawing circuit. Then, one would examine that circuit to determine WHY it is drawing amperage. A circuit which has amperage, when it shouldn’t, is the one draining the battery.