Battery Charging Voltage

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A207 2010 with a battery sensor on the -ve terminal. Replaced battery as it was not holding charge and was the original. New battery and checked voltage engine off 12.4 and engine running 13.9 - 14.1. All good.

With a volt meter in the power socket and whilst the voltage after start is 13.9 - 14. After about 10 mins it drops to 12.4. There is probably about .2v loss at the power socket so driving around it never goes above 12.4 even with every electrical load on.

After a stop with lights on and giving the battery a drain the cycle repeats high volts after start then steady 12.4 at the socket. Battery does not seem bothered about this and car starts no problem.

My 2010 ML after start stays at 13.9. Which is normal behaviour.

The A207 is an E500 with no mention of blue efficiency.

Can anyone advise if this is normal for alternators with a battery sensor?

 

[kid-jensen]

There is a move to reduce the power going into the battery to reduce the load on the alternator and hence save fuel.

Why two cars of the same year of manufacture should be different I don't know, probably one has a more modern charging system.

All the above being said, if both cars start ok, even under difficult conditions, you're probably ok. After all, without a volt meter, you would never know.

The car that charges at higher voltage, without any apparent fold-back will use marginally more fuel...

 

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My 2009 C180K Blue Efficiency exhibits similar behavior in terms of variable charging voltage except the voltage readings are a little higher. When the battery is fully charged I get a rested voltage of 12.6 to 12.7 volts measured at the under bonnet terminals. Charging voltage goes as high as 14.6 volts then falls off to mid 12's.

I understand variable charging is designed to save a little fuel but my experience is it doesn't maintain the battery at quite 100% full charge. I give mine an hour or so on the battery charger once a week and it always needs at least 2 amp hours to top it up.

The fact that a battery starts the car is no indication of 100% full charge rather it tells you the battery is at least in a reasonable state of charge. A variable charging system may keep a battery in a reasonable state of charge but I wonder if there is a compromise somewhere as there often is with other headline MPG technologies.