red battery display

[Niro06]

Hi, over the last couple of mornings i get the red battery come up on the display ( 2007 w204 c320), i press ok and it goes off, then a few minutes later the radio will switch off and heater will still work but not on full fan. If i stop turn off and restart everything is all ok again. car always turns over no problem even on these cold mornings. I guess it's either battery or alternator related but can anyone let me know how to test to to determine which is most likely. also does my car have two batteries, it has main battery in boot but after a bit of reading there may be a small battery under the bonnet? is this likely to be the problem?
Thanks
Scott

 

[OneForTheRoad]

Easy test is use a multi meter on the battery with engine off . Then again with engine on.

12.7 and 14.7 are ideal figures.

 

[DSM10000]

Do you have a voltmeter?

If so check the battery voltage with the engine off and no load, then start the engine and measure the voltage again.

A normal battery reading is around 12.8 V and with the engine running around 14.4 to 14.8 V

 

[ALFAitalia]

12.8v would be a bit higher than average....12.6 to 12.7 is deemed to be 100% charged on a Lead acid battery....a bit more on an AGM. And you need to check battery voltage at LEAST half an hour after switching the engine off or you will get a fake high reading while the battery chemistry settles after charging.