I have a 2006 M roadster. Lately when I start it the clock is set to 0 and frequently the brake warning light and the stability control light are on. If I restart the engine sometimes these errors go away but not always. I am wondering if my computer is failing, or could it be something like a weak battery. The battery is probably 5 years old, but I keep the car on a battery tender at all times.
I would not hesitate for a minute to replace the battery. Consider this your early warning. Even it doesn't fix it, you will need one shortly. I too have had one of my cars on a battery tender sitting in the garage. Battery still needed to be replaced.
Battery Tenders are great... mostly! However, in time, a battery-tended battery will also need replaced (due to the low-level charge cycles, eventually will kill a battery). Ya, cars will do all kinds of funky things from low battery charge, I second Steven's suggestion.
