2013 MCS hardtop with 42k miles. Today I was sitting in the car playing with the customization settings (auto lock the doors, etc.) The car was “on” but the engine wasn’t running. The yellow battery icon showed in the instrument cluster. I tested a few more times and it seems like the light will come on if the car stays on for a minute or so without the engine running.

I put a multimeter on the battery with the A/C, lights, and fan on and got 12.1V. 42k miles isnt unreasonable for a new battery but shouldn’t the voltage be lower under load if it’s time for replacement?

Bonus question: the tail lights used to turn on when I unlocked the car and now they don’t. All the lights seem to work otherwise. Is there a setting to make them do that again?

 

if you are testing the battery while the engine is running, you wont get an actual voltage of the battery because the alternator is also charging the battery and supplying voltage to the various elements in the car.

bonus question question, did that happen after you were messing the with the setting?

 

 

12.1 is pretty low for the small load you are putting on it with just the ignition on. I don't know what voltage triggers the battery warning but that could be it.

You might try a code scanner that can report for you the voltage the computer thinks it sees.

 

That battery is about 5 years old (assuming it's original). I'd go ahead and replace it, even a marginal battery can cause odd electrical problems in these cars..

 

The engine wasn’t running at any point. And the light thing started a while ago, before the battery warning started. I just hadn’t sat down to look at it. I replaced my fog and driving lights with Ziza lights from ECS but the battery and light thing was happening before I did that.

I didn’t mess with the settings until it started happening. Which setting, anyway? The one with the unlock symbol is set to the light symbol and my DRLs are on.

Huh ok I just looked it up and I guess the voltage should be higher. Can Autozone pull codes effectively or do I need a specialized scanner?

 

Replacing the battery solved the external light problem. Apparently the car will do things like that (no unnecessary electrical load when the engine is off?) without giving a low battery warning. Thanks!

 

Yep, when the battery starts to go all kind of things get wacky.