2008 W221 S320 CDI - Proper way to change rear (consumer) battery and should it be a AGM or Non-AGM?

[Rick]

Hi All,

Hope you are well.

My dad's 2008 S320 CDI (has no stop/start feature) is showing 11.8V with the car off which indicates a low battery. The car's engine fan is also coming on randomly on cold start-up and a Yellow coolant light displays in the dash.

Before we chase the engine fan problem, we'd prefer to change the rear battery as 11.8V is low indeed and the car still has the original MB battery in the rear (we think it could be the low battery causing the engine fan issue)

I want to know if I can simply disconnect the - cable then disconnect the + cable and remove the battery or should I first disconnect the starter battery (in the engine bay) and then disconnect the rear battery? Can I get away without providing aux power to the car whilst the battery is disconnected?

Also, I have checked various sites and they all are saying that the car requires a AGM battery at the rear - can anyone confirm if the original MB battery at the rear is supposed to be AGM or non-AGM? I rang Mercedes but they did not have that information on hand, as usual.

Finally, if the battery is supposed to be AGM, then I take it that no coding is needed to tell the car that a new battery has been fitted to it?

Thanks.

Hi,

My (S211) E240 needed a new battery a month ago and having researched the advantages of the AGMs versus the ridiculous price (€240) I went for a traditional lead acid battery.

Any low battery voltage issues that I had have disappeared and it and my auxiliary battery now both show a healthy 12.6v on a cold morning.

If I had Stop/Start I would have gone the AGM route but the 100Ah I bought instead works perfectly and will probably do so for years.

Incidentally, I simply swapped the old for the new and didn't recode the new battery. The radio held its settings but I did have to 'reteach' each of the electric windows (all the way down, hold switch for 5 seconds, all the way up hold switch for 5).