lion999bf

Hi guys, my ride is a 2015 W205 that ran perfectly well except for some wear and tear for the past 9 years but problems coming as it entered its 9th year. Just had ECU changed last week as my car kept making noises after thermostat change.

Now I get this battery warning light on saying 'stop the vehicle and shift to P and leave the engine running' about 2 hours after continuous driving on freeway. I ignored it and it never came out again after I reached my destination and none during my city drive. However, it popped out again during my way back on freeway exactly about after 2 hours of non stop driving. I took my car to 2 different mechanics and they said 2 very different point of view. 1st one saying its due to bad battery after they did battery health check. 2nd one say he is pretty sure its alternator problem but to check on alternator they have to remove quite some parts to gain access to it. My alternator now is stock unit, never fail for the past 9 years.

My questions are, if its bad battery not holding charges why I had no problem starting my car? If its alternator problem why is the alternator warning light not coming out? Are they able to comment on battery health just by random check or should they try to charge the battery first to see if it holds the charge?

I had my battery changed exactly a year ago it is an original merc part which I have had 2-3 of them since my ownership they typically lasted about 2 years plus minus for each of the battery and I have confidence in them.

Would like to get some opinion on what is likely to be the problem first before deciding which mechanic to send my car to as spending more than needed is the last thing I want. Why cant it be more straightforward for the computer to tell which is the problem? Thanks in advance!

ygmn

If you drove for an hour with bad alternator your cr would have died due to drained battery as the car would be running of battery power for all the electronics.

Now if you can drive long distances with no issues other then this light I would lean towards bad battery causeing a big drain on alternator and computer telling you to pull over so alternator can charge battery.

Battery is not that old but have it properly tested:
Remove from car and properly charged and then load tested - Auto Zone can do this.