What should my battery voltage read

[McDonaldajr]

Just bought a 2 year old E class diesel estate 2.0 e220d. Whilst sitting in my drive, learning all the new shiny buttons and updating the maps etc, I got a message "12V Battery See owners manual" and an email from Mercedes Me saying "the starter battery charge status of the vehicle KR21CXC is critical". I started the engine and let it run for a bit. Later I measured the battery voltage using my multimeter (in situ, but with the ignition off) and it read 11.6 volts. Took it to MB and they "checked" the battery and said it was fine, just needed charging. I have since driven it some long distances (600 miles), but when I use the dashboard display to read the battery voltage before starting the car it generally reads 11.8 volts, which still seems low to me. It rises to 14.8 when I start the car, so the Alternator is working fine.

I realise that ideally I should disconnect the battery and measure the voltage without the current drain to get a proper reading, but just wondered if those of you who have the ability to read the battery voltage on your dashboard before starting the car also experience the same sort of reading, i.e. 11.8 volts?

Thanks, Anthony

 

[Flyinspanner]

Maybe it’s been sitting around for a while / short journeys.

just give the battery a good long charge overnight in the first instance.

 

[McDonaldajr]

Maybe it’s been sitting around for a while / short journeys.

just give the battery a good long charge overnight in the first instance.

Tried that. Trickle charged for a long while.

 

[jasonh]

Under 12V under light load is low. I'd expect to see 12.1 at least with the ignition on with a reasonably healthy battery.

Whilst age kills car batteries you can wipe one out in a single go by letting it discharge excessively. If the car has been sitting for a long period prior to your purchase (6 weeks could be enough) that would have damaged it.

I'd get MB to load test it - just takes a few seconds with the correct tool.

 

[190]

I realise that ideally I should disconnect the battery and measure the voltage without the current drain to get a proper reading

You don't need to disconnect the battery, just open the bonnet and leave the car alarm off for 10 mins or so to be sure it's gone to sleep, then measure the voltage. If you so much as open a door the car wakes up and starts to draw 4 amps or more, which means just playing with it on the drive is running the battery down. It seems a dumb design to me if I can't clean my car inside and out without discharging the battery.

 

[AMGeed]

These figures show what state of charge the battery has.

 

[markjay]

Just bought a 2 year old E class diesel estate 2.0 e220d. Whilst sitting in my drive, learning all the new shiny buttons and updating the maps etc, I got a message "12V Battery See owners manual" and an email from Mercedes Me saying "the starter battery charge status of the vehicle KR21CXC is critical". I started the engine and let it run for a bit. Later I measured the battery voltage using my multimeter (in situ, but with the ignition off) and it read 11.6 volts. Took it to MB and they "checked" the battery and said it was fine, just needed charging. I have since driven it some long distances (600 miles), but when I use the dashboard display to read the battery voltage before starting the car it generally reads 11.8 volts, which still seems low to me. It rises to 14.8 when I start the car, so the Alternator is working fine.

I realise that ideally I should disconnect the battery and measure the voltage without the current drain to get a proper reading, but just wondered if those of you who have the ability to read the battery voltage on your dashboard before starting the car also experience the same sort of reading, i.e. 11.8 volts?

Thanks, Anthony

What you have experienced is normal, the 12v battery gets drained while you're sitting in the car playing with the gadgets.

At 11.8v, you'll need to take the car for a long drive, or use a trickle charger, e.g. CTEK.

If charging the battery while driving the car, note that the alternator will show 14.3-14.8v when the battery is below 80% capacity, after that the battery management module will reduce the charge level and the alternator voltage will drop to 13.3-13.7v. At this point, the battery is being trickle-charged by the alternator until it reaches 100% capacity (in order to prevent battery damage). If you don't see the alternator voltage drop, then the battery is still below 80% capacity.

If the battery voltage remains low even after a long drive, then Your battery is probably tired, but even so it will still start the car for a long time, months or even years. The first sign of trouble will be when Stop/Start no longer works.

But, again, most likely all the car needs is a good long drive to fully charge the battery.

 

[Flyinspanner]

Maybe whip it out and get it stress tested at Halfords/garage/motor factors near you.

if you need a new one check out Grangemouth/Newcastle MB they do good deals.

Tayna online are good prices and deliver to your door.

 

[AMGeed]

As above, get the battery load tested.
I had a Bosch battery showing fully charged, but it failed the CCA when load tested and was replaced under guarantee at ECP.

 

[Uky]

My recently purchased second hand E-class coupé C238 exhibited the same problem. If I sat in the car with some electrical consumer turned on, the car would text me with a "critically low battery level". The car did however start without problem. BUT: neither the ECO-drive nor the keyless boot entry did work. When turned off, the battery voltage was some 12.4V and the "ME" App said "Partially charged". When at idle, the voltage was at 14.4V. This did not change even after a long ride. The workshop tested the battery and charged it. For a short time "ME" reported "Charged" but when I came home after a 50 km drive the problem was back.

So I filed a warranty claim with the dealer who accepted replacing the battery under warranty. After replacement, the battery is at 13V whith everything turned off. No error messages. "ME" says "Charged". Keyless boot access works and (unfortunately) the ECO drive as well. I just have to learn to turn it off each time I go for a ride...

The battery test results were inconclusive. There was indeed start capacity left but not in the vincinity of the specified 800A, rather 740 or so, not enough for a formal warranty claim but replacing the battery made all the difference.