Hello.

I forgot to turn off the parking lights (because i need to turn off AUTO mode every time i put my car in the parking, because there is a mirror facing me in the entrance to help to align the car properly and it blinds me if i do not turn off the lights. I can not remove the mirror because it is a public paid parking space (automatic robot thing)). Normally i turn the lights back to auto when car is parked, but forgot to do so this time.

So battery died completely and i was not able to start the car.
So i used a jump starter. When i connected it - the parking lights went on for 4-5 seconds, then something clicked and they went off. Then in a matter of 1 second they turned on again, and click and turned off again, and on and off, and it repeats forever.

I was able to start the engine and run the car fro about 2 hours on highway. But the battery did not charge at all and big red battery error icon without any message did not go away. So i suppose i need a battery change now. My questions are

1) is it normal for the car to turn on and click and go off and on and off again and again when power source is connected? If its not normal what can be wrong?
2) Do i need main battery swap or auxiliary too?
3) What is auxiliary battery used for?
4) is there any switch or other way to turn off the lights completely?
5) Why do you think my battery failed to recharge? Is it true that once battery is fully discharge it will never be able to charge again even a bit?

Thank you

Some thoughts. Full disclosure, my personal bias is to replace parts in situations list this. Batteries are wear and tear items. On a 7 year old vehicle, batteries are beyond their average life expectancies, notwithstanding the unintended battery drain incident. It's time to replace them.

- a totally flat battery would benefit from being replaced
- a flat battery incident would benefit from having the auxilliary battery replaced
- replacing both main and aux batteries will get you back to more of a known condition, from where you can commence additional troubleshooting

The aux battery in some or most M-B vehicles is used for the transmission gear selection function. It is also be used for ECO start-stop in some vehicles, if your vehicle is so equipped. Some models have a capacitor rather than a battery for this, and some have a second, larger aux battery.

Thank you.
Of course my first plan is to replace the battery. Thank you on advise to replace the secondary one too

Any thoughts about "not charging even a bit " part? I was thinking alternator might be bad but I could run the car for 2 hours. Which would be impossible with bad alternator. AnD what about turning on and off again with clicking? Any fuses that connect battery to alternator went bad?
Or I can only know after I replace the battery?

At our house in FL, my 03 VW Passat hoopdie sits for months during the summer months Something drains the battery dead. I've learned the hard way that a dead, dead battery will stay dead. You can try to charge it by connecting jumper cables to a good battery then connecting the battery charger to the good battery. It will get current flowing through the bad battery and charge up. It may even appear to hold a charge, but it won't last. Second replacement battery got a charger/maintainer for when we're not there. Only real solution to your problem is to replace the batteries. Do them both and you should be good to go for 5+ years.

A full drain will kill an otherwise weakened (old) battery. In a normal world you can pull the battery out and drive down to Autozone for a capacity test, but the battery replacement for the X166 is such a PIA once you get the old one out you might as well replace it. (If you can borrow a tester, problem solved!)

Either way best to replace if your battery is more than 3 years old.

Thank you for all you advise.
I will replace both batteries.

Meanwhile, i feel a little worried (suspicious) that something else could be wrong. Even if the battery is totally drained, it should be able to have at least some charge (at least to blink an interior light) after driving 200 km on highway.
and that Turn On - Turn Off after every 4-5 seconds also makes me worried.

1) if your battery is near end of death, it won't charge from the alternator so seems normal to me.
2) if your battery isn't near death, it could be your alternator that is dying/dead - but at 2013, I don't think it is, but it could.

Like everyone else said, it's time to change the battery - but I would suggest you buy this CTEK charger to 1) test the battery and 2) to see if it can trickle charge it back to life (might take a long time). The CTEK charger can recondition the battery possibly. It can also test the alternator. This is the only trickle charger I recommend and you can do your own research - but this is what MB uses and rebrands Mercedes (they use a cheaper model and use 5amp but very similar).
https://smartercharger.com/collectio...est-and-charge