Red battery light
 

Hi all - I’ve been monitoring an intermittent red battery light for a few weeks in my 2007 S550. It tends to come on 10-15 seconds after startup on a cold engine, and this maybe once every 3 or 4 days. I’ve been pulling down some Autel live data and today finally caught it right after it happened. The error code is P0620 (Operational Fault of component ‘G2 (Generator)’. First assumption was the alternator is faulty but now wondering if it’s the battery. When the fault happens the Vehicle Power Supply Voltage reads around 11 to 11.4. But then after a minute or two of driving it goes up to the mid 14s and stays there for rest of the drive. When I turned the car off after getting home the battery voltage showed 12.3. Any ideas? And which battery is this even checking - the accessory one in the trunk? By the way, all driving and starting behavior appears totally normal to me during all of this.

So I would check your serpentine belt for slippage on that alternator. It could be a pulley or a tensioner or a loose belt. So check the components. I would also check the date of the large trunk battery. They typically last about ten years. BOTUS has written posts about how to revive a dried out glass mat battery if you want to play with it. I just replace my consumer battery every 7-10 years when I get symptoms of low voltage. The battery is fairly expensive and very heavy.

Great, thanks! I checked the trunk battery and it is from 2016 (front starter battery is from 2021). I can’t tell if anything is slipping with the serp belt. I’m guessing at this point it’s worth replacing the trunk battery and see how things go from there before taking it in to an indy shop. Will report back after a while if things stay settled.

Quick update: I replaced the trunk battery a couple weeks ago (very easy to do) but the red battery light still comes on intermittently. So next I’ll probably take it to the shop and see what they say. I’m assuming I’ll be in for a new alternator/voltage regulator. Will come back here once it’s fixed.

I will add be careful when replacing this battery...it is super heavy, and the space very tight. I got a finger pinned by the battery. Not only was it painful, but I spent several long minutes trying to figure out how to remedy the situation, as my finger took up all and more of the tiny bit of room to move the battery. I was lucky that I ended up with only a bruised and cut finger...could have been worse.

yes, you’re definitely right! It was really tight trying to get it out.

Final update (I hope!). The battery light started coming on every time I drove it. I took the car in and got a new alternator. Seems to have solved the problem.

Very cool. Sad part is that flakey alternator probably killed that 2016 consumer battery. Since it has been replaced, you should be good for at least seven more years.

That's good to know , it looks like you solved the problem

BMWs and Mercs often had a replaceable diode pack - which is the bit of an alternator that normally gives up - it was actually cheap - I thought I have read these are still available for the 221 but most want to sell you the whole thing?