Battery drain
 

Recently, i just put a new battery in my benz. after one week or so of inactive use of the car, the car wont start the next time. Since the battery is brand new, i wouldnt think the battery is defective. My mech told me it is possible that my alternator is bad or I have something that is draining it. What are the things that I can check besides the alternator for this battery drain? All suggestions appreciated. Thanks

Seeing if the alternator is charging is straightforward. Put a good voltmeter on the battery with the car off. A charged battery will be at about 12 volts. Start the car... with the car running, the voltage at the battery should be 13.5 volts or better. If the battery voltage doesn't increase after the car is started, your charging system isn't working.
If the charging system is OK, you probably have a large parasitic current drain somewhere in the car. To isolate it, put a good ammeter between the battery and one battery terminal. If you have a high drain, you will see it pretty easily. Now start pulling fuses one at a time until the drain drops significantly. That circuit has the offending component.
Hope this helps.

check to see if your trunk light comes/stays on...

if you have an aftermarket radio, this could also be a culprit.

check your convenience relay.

Could very well be time to change the voltage regulator brushes in the alternator. Easy, two screws, and cheap, $35 or so. Depending on how long it's been since you checked or changed them, you may just want to order a new one and change it. Keep the old one if it's good as an emergency spare.

But as the others have suggested, make sure you don't have a constant drain from a malfunctioning circuit.

Len

Thanks guys I will check those out. On my previous battery which still seemed t be new, this was happening but I though it was time to change the battery and didnt mind it. At that time I still had my stock radio and such. Now that I put a new battery, couple of months old, same thing but have an after market unit.

If you have an ampmeeter, pull the + lead off, and put the + lead from the meeter and the neg on the batt post. Turn it on(amps not volts) and that will tell you right their if you have a large drain. Anything under .003 is acceptable. You may go as hight as .006