BMW 325i, Battery dieing but testing fine?

I drive a 2003 BMW 325i. I have had my car not start a few times now. Each and everytime, getting a jump helps it start right up.
It seems as though I have a bad battery BUT i have had it tested multiple times and it shows up fine. NOW, I have NOT cleaned the cables/terminals just yet.
They aren’t noticeably bad. I understand they could still be bad enough. Other notes that might be helpful is the fact that i know for a fact i have a fuse blown for some curtesy lamps. Would this drain the battery though?

Alternator is fine. Battery is fine. Starter is fine. Any clues?
Thanks for your time. I will also update once i’ve cleaned and replaced fuses but hopefully there might be another option or 2 that i can consider with all of your help. Thanks,

The battery needs to be properly “load” tested in order to find out if it’s good or not. Battery connections on the outside can look good but it’s inside where the actual connections are…is what counts.

It is possible you have a battery drain issue, and the battery is resilient enough for now to recover, but the more times it happens the less likely your battery will remain in healthy shape. If you are a DIY guy this may hep.

If not make sure glove box light, trunk light, hood light etc. are actually shutting off.

Anytime that kind of trouble happens the first thing to do is clean the battery connections. You would be wise to have the charging system load tested and also check for a parasitic current drain on the battery while the car is parked. Winter is coming.

The trouble could also be a bad connection to power near the distribution panel under the hood. That would make it seem the battery is dead but it is really alright.

I’m assuming you mean by “not start” you mean “not crank”. It is possible – though not a common problem – that the battery would load test ok, the battery connections and wires are ok, still the car wouldn’t crank, but could be jump started.

Possible causes

  • Connections at starter motor corroded.
  • Starter motor not properly grounded, either due to corrosion of the starter case to bell housing, or a problem between the transmission and chassis ground.
  • Starter solenoid contacts on the fritz.

The reason why it would crank with a jump start is that the voltage from the jump-starting car (that is running) is close to 15 volts, but the voltage from a good battery is at most 12.5 - 13 volts. That extra 2 volts can make the difference between cranking and not cranking. I’ve heard tell that some road-service providers, tow trucks, etc, use a special starter gadget to jump start cars that uses even higher than 15 volts.

A mechanic would do a voltage drop test at all the interfaces to determine the cause.