Bad day today, absolutely no electrical power at all. The car (2005 E320-CDI) wasn't driven over the weekend and this morning I couldn't even open the trunk (power latch) nor turn the ignition key. After 30-mins, getting into the trunk and hooking up jumper cables, the car slowly started up. Odd that there was no power at all! I check whether any interior/exterior light or door was open, but none.

Could the battery been that weak, near dead, for being only 3-years old? Also noticed that when the engine did startup and charging the battery, the voltage read only 13.4V. That seems low. Should it be 14.5V?

Either

bad battery or draw on the system.

The battery could have a dead cell. I have had them fail in 6 months and one is still going 10 years.

There is an issue with seat relay failing drawing bat power down.

I have same issue with seat relay that kills the batt.

Can you explain, you indicate you have the issue at present. Why did you not fix it? Shouldn't a new seat control module be sufficient?

UPDATE; back from the dealer on this no power/battery problem.

Turns out the BCM had low voltage codes and thus required software update to increase the charging voltage. And from what I can tell with my trusty voltmeter, its was charging 13.4V before and now its at 14.0V

We'll see whether this is sufficient enough of a fix. I was hoping that they would have replaced the batteries since they're so expensive!

since your battery drained down, did they check your car battery to see if its still good?

Well, the service record states that they did recheck the batteries. But that doesn't really show how healthy they are, ie. maybe at 75% of full capacity.

When you really drain the battery it is normal for the alternator to charge at lower voltages. normal charging voltage is 13.8 - 14.4 volts with the battery above 75% charged.