Have an EM1 that keeps chewing through batteries every couple of weeks.
It will be fine for the first week or two, then the battery starts to get weaker and weaker (not hold a charge) until finally it won't even have 1.0v in it after running for over half an hour.
Car runs well and starts right up if jumped with a booster pack, alternator tested ok and is putting out 13.5 to 13.8, new terminals installed on both the negative and positive sides with a new ground wire on the negative.
I have gone through 5 batteries already and the autoparts store doesn't want to keep warrantying them for me.
Any ideas on what the problem might be?

 

Sounds like you have a short somewhere.

 

Are the batteries you keep replacing testing bad or are they just dead?

Do you have any aftermarket electrical mods on the car e.g. alarm, hardwired radar detector, stereo, amp, HIDs, etc?

Check the 7.5a fuse #15 in the under-dash fuse box.

Has anything been changed lately?

 

Car sat for a while, that's really the only thing that's changed.
The batteries are testing bad and won't hold a charge at all.
The car does have an aftermarket alarm, but it is currently disconnected and still doing the same thing.
The alarm was installed professionally and the car did not do this even with the alarm connected and operating as it should before being stored for a while.
Checked fuse #15, it is a 10amp fuse in the car, tested ok with a multimeter.

 

Do you know how to do a parasitic drain test at the battery?

You quite likely have a major short somewhere in the car that is draining the battery rapidly to the point of the battery internally shorting itself out.

A parasitic draw test will tell you how much current draw you have on the battery when it's sitting.

If I recall an alarm will have a few milliamp draw at all times so is expected, half amp or more and I would be concerned. I am just not sure where the safe and not safe line is. I don't know if 200 milliamps is too much etc.

 

I will have to conduct a draw test once I replace the battery yet again, but the weird thing is that the car will be fine for a week or two and then it very rapidly gets worse and worse until the battery is dead and will no longer hold a charge.
It's not a slow process, it will go from totally fine to absolutely dead within a couple days (after a few weeks of being fine, car being driven short distances once or twice per day).
The current battery won't even hold 1.0V after the car being ran for 40-45mins.

 

Running a battery down to zero volts even just one time is prone to ruin it and it won't charge up again.

Take the bulb out of the trunk light. Do you have an aftermarket stereo with separate amplifiers? Those are often the cause of battery drain, either being faulty or improperly installed so the remote wire does not shut them down when the head unit is off.

 

The only aftermarket part of the stereo is the headunit, and that shuts off properly after the key is taken out of the ignition.
There are no lights being kept on when all the doors are closed, I've checked.
The alarm main power wire fuses are pulled, and it still does the same thing.