l drive a 2004 BMW. This morning l drove in the city. l came back and tried to restart the engine. From one second to another the battery was empty. l called Road Assistance and l got power. l drove to the BMW Dealer and they checked my battery. - The battery is doing a great job. - l do not understand what was going on ... ! Can a battery be empty from one second to another? And then doing a good job again?

If your battery is on the way out it will give that sort os result depending on the test use by the dealer. You may have left the lights on or similar and that almost drained the battery until you tried to start it. The alternator will charge flat out when it detects a flat battery and only takes about 20 - 30 minutes to put significant charge into a battery.

l agree with andyhi69 . l would suggest you also check your battery ground.Good Luck.

It sounds like u have a bad connection at the battery terminals, the starter motor, or the ignition switch. A battery does not discharge in one second, then recharge in the next. Check the battery for sufficient electrolyte to cover the battery cell plates. Add distilled water to cover the plates in each of 6 cells. Check the battery for a full 13 volts across the terminals. Start the engine, let it idle; check the battery voltage at fast idle (1200 -1400 RPM) . You should see the alternator providing a charge voltage to the battery at 14.2 Volts DC. A discharged battery will measure something less than 12 volts with the engine off & the headlights on.