CLS500 Battery voltage

[JeffT]

Had a couple of instances of low voltage (can't use demister, heated seats etc) so been checking voltage at battery using my multimeter or the dash display.
Sometimes first thing its reading 11.8-12.5v. When engine running it hovers around 13.8 - 14.00. Battery is a good Bosch unit about a year old. Have also just replaced the small battery in the engine bay. Is it time to replace alternator?
There is never any issues starting and warning message goes away in a few seconds.

 

[edc]

If you are showing 14v when the engine is running then the alternator is doing the job.

 

[OneForTheRoad]

My car does this also . 5 mins running it returns to normal .

 

[190]

The battery voltage reading will fluctuate depending on the state of charge of the battery. Even when the alternator is functioning OK the voltage will initially be a little lower if the battery is partially discharged. As long as it comes up to 14.0 volts eventually it should be OK.

Mercedes had a habit of using lower than optimum charge voltage on the old basic alternator cars where you might only see 13.8 volts or even a little lower. Some of that might be due to the negative temperature coefficient of the voltage regulator where the charge voltage is deliberately lowered as the the battery gets warm.

Modern batteries like AGM's have different requirements with the charging voltage up to 0.5 volt higher. It's worth bearing this in mind when buying new batteries by making sure the replacement is of the same type.

Some recent cars have smart charging systems that are able to deliver higher voltages when needed and then back off to 14 volts to save fuel. If you monitor voltage on one of these cars then some voltage fluctuation is inevitable.

 

[JeffT]

Thanks for info. Think I will just keep an eye on it for now and maybe get battery tested.

 

[OneForTheRoad]

I've fully charged my agm battery and also fitted a brand new bosch s5 lead acid battery and I still get the convenience function warning - I think its just down to the alarm dropping the voltage a tad overnight , once running its back to normal , so I doubt your battery is at fault , if the car sounds healthy when its turning over and not lazy , id just ignore it tbh .

 

[fatlarry]

I have the same problem, I have renewed the front battery and regulator ( both genuine parts) and it has made no difference. I can only assume it has something to do with the temerature as I did not have the problem in the summer. Gary