A First off, recognize that modern automobiles equipped with numerous electronic processors and gadgetry experience a parasitic current "draw" on the battery when parked with the ignition turned off. BMW publishes service information showing the timeline for the current draw after shutdown. After 60 minutes the parasitic loss should be about 30 milliamps -- or 0.03 of an ampere per hour -- or less. It would take several weeks at this loss rate for a good battery to fail to start.
The fact that every battery that's been in your car has discharged to the "no-start" point in just a few days indicates the parasitic current draw must be significantly higher. A stuck relay can draw half an amp per hour and flatten a battery in a couple of days. A small lamp -- glove box, trunk or the like -- draws more current and will deplete the battery faster.
Since the shops haven't been able to find the leak, so to speak, you could try the do-it-yourself method that involves connecting a homemade ammeter -- a tail light bulb's terminals connected between the positive battery cable and positive battery terminal. If there's a significant current flow, the light will at least glow dimly. If it does, unplug each fuse and relay, one at a time. If the light goes out as you disconnect or unplug a fuse or relay, that's the circuit where the current is flowing.
Don't overlook the possibility of the alternator or charging system not adequately recharging the battery as you drive. The cumulative loss over several driving cycles can leave the battery significantly discharged when you park the car. Normal parasitic loss can then deplete the battery in just a few days.
Q I'm having trouble with the passenger side original rear strut on my '96 Camry with 500,198 miles on it. I've had a few timing belts replaced along with one fuel pump, oil pump, water pump and radiator. Unfortunately I've not replaced the struts. When driving down the freeway in the left lane, the right rear strut makes no noise, but when driving in the right lane it clatters very loudly to the point I have to turn the radio up. Is it not safe to drive it this way? My friends say to drive it until my knees start to hurt. It runs down the freeway fine but handles more like a boat. I hate to put too much money into it.
A Are you telling me that your car has more than half a million miles on the original struts? That's incredible. If you're going to keep the car, have new struts/shocks installed ASAP. Safety is the biggest concern. Dead shocks no longer properly control up and down wheel movement and load transfer. That means significantly less efficient "contact patches" between the tire tread surface and road. And that means the vehicle will no longer ride, stop or turn well.
Congratulations on reaching the half-million mile club; that's an amazing achievement. Do your car a favor -- give it a new set of struts.