Car wont start

"Will not start" can mean a number of symptoms. Do you mean the starter would not crank the engine, it cranked too slowly to start, or it cranked normally but the engine would not run?

If the starter was cranking slower and slower over a period of days or weeks, that suggests the battery was not being recharged while you were driving. That's a charging system problem, not a starter problem. Charge the battery at a slow rate for an hour with a portable charger, then have the charging system tested. You can start the test yourself with an inexpensive digital voltmeter. Measure the battery voltage with the engine running. It must be between 13.75 and 14.75 volts. If it is low, suspect the generator or voltage regulator. If it is okay, that only means it is okay to perform the rest of the tests, but that requires a professional load tester. Your mechanic will measure the full-load output current and "ripple" voltage. There are six diodes in every AC generator. If one of them fails, you will lose exactly two thirds of the generator''s capacity. 30 amps from a common 90 amp generator is not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks.

All AC generators produce three-phase output. One phase will be missing when a diode has failed. That causes output voltage to drop quite a bit during that missing current pulse. That is where ripple voltage is a clue. That is the variation between the highest and the lowest voltages the generator is developing. With a failed diode, ripple voltage will be very high. Some testers actually measure the amount of ripple voltage, but knowing that is of little use. Most testers just show it on a relative scale. "Low is good; high is not good".

Just to add to this one on that car when you change the battery you have to have the battery module computer reset to tell it you got a new battery. Otherwise the battery can be damaged. Also your going to have to take it somewhere that knows about BMW's and have all the computers scanned. To see what is going on with your key issues. A car like that is a really hard to work on for a diy-er it takes special tools professional scan tool etc. Just to get a starting point on what to look at where to go.

Thanks Saturntech9.

Is it always the same key that gives you trouble or both of the keys?When battery's go bad in cars like that sometimes they have issues with recognizing keys etc It might just be that the keys need to be just reprogrammed to the immobilizer.

If one key always works and the other two don't then I would have to say it really sounds like a intermittent bad key issue.