Havelaar

Went to the store yesterday and came out to a dead battery. Luckily, I have a portable jumper in the trunk. I turned on the Voltage/Amperage readouts in the cluster and took a drive around town to see what was going on. Basically, what I discovered is when the headlights are OFF, I'm getting negative amps and the battery is draining. When I turn the headlights ON, amperage goes positive and the battery voltage hovers around 13.5-14.7.

This is really counterintuitive to me so I don't know where to start troubleshooting. Any suggestions or ideas on what might be going on?

CaliBenzDriver

A flat starter battery is never fun out of warranty when you got ice-cream in your grocery bag


Okay pls show us cluster displays while driving without headlights ON. Expectations are a positive charge current is always flowing in to fully charge battery. 100 % of all vehicle Power is supplied by the battery ALT. while the engine is running... (ECO=Off is obviously a draining stage).

What range of voltage do you see?
​​​​​​What range of Amperage do you see?

What's happening is the alternator voltage is controlled only by the ECU... NOT the ALT itself until control is bypassed to work well.

Crazy Bosch ECU is entirely capable of clamping 90A using ALTERNATOR as a 1kW LOAD - That's one way it can kill any battery, old or new alike until ALT is cooked.

Other available ways is drain while parked... very effective discharge as well.

How old is your main battery ?

Let us know your cluster battery stats...​​​​​​
-- Voltage above 12.5v = no problem
-- Voltage below 12.3V = low power troubles

Havelaar

Thanks for the reply. I went to check it out again and it appears the issue is intermittent. I just now drove around wiith the cluster voltage/amp readout on and everything was fine. No difference between headlights on or off.All within normal ranges.

When the issue was happening previously, with headlights off voltage was around 12.5 and diminishing and amps were ranging from +8 to -18 but mostly in the negative. With headlights on, voltage was 13.5 to 14.7 with amps ranging from +8 to +40. FWIW, I plugged in the icarsoft and didn't detect any codes

I had the main battery checked at Auto Zone and they said it's fine. It is a 5 year old MB battery. It makes sense it might be an issue with the ECU and not the alternator but it's still odd to me that the issue occurs only when headlights are off.

CaliBenzDriver

A -18Amp current out of the battery while driving is a significant discharge. There SHOULD NOT BE ANY DISCHARGE. 100% of all electric power is supplied only by ALT, nothing from starter battery.

Swinging back to +40Amp battery charge means your battery needs charging (10A would be a normal range).

The ECU considers the Headlights a heavy load... it raises voltage to power up the load with decreased current.

Havelaar

Thanks! I'm goign to start by installing a new battery. Probably the cheapest place to start anyway!