ua549

AGM batteries have been around since the 1980's.

whoover

Yep. I've used them in solar power setups for years. But MB only switched to them when they added auto start systems for emission control. The higher number of starter cycles would kill a flooded battery prematurely.

ua549

Conventional flooded lead/acid batteries had a very short useful life of 2 or 3 years. I started using AGM replacements in my cars in the early 80's and never regretted it.

cetialpha5

They were also in the W211. Mine is original in that car, it's 12 years old at this point. I don't think they last as long in the W212 because MB doesn't seem to charge them up as much as they did in the past. Not fully charging a battery is what will kill it prematurely.

ua549

IIRC my E55 had 2 AGM batteries side by side in the trunk.

whoover

Some models had them because of SBC brakes. I believe that they were specified as part of getting certification for brakes that needed a working electrical system to operate. The W211 had Sensotronic brakes through 2006, but they were dropped because of reliability issues. The batteries stayed on. Other models of the same time, like the W220, never had SBC brakes. They had a mix of flooded and AGM batteries as delivered. I had a 2006 S65 that came with a flooded-cell but when it died, the MB replacement was AGM.

It was the adoption of auto start that triggered the wholesale switch to AGM.

I agree that the charging system in the 213 doesn't do a complete job. After an hour on the highway, I can connect the CTEK and the battery will continue to accept charge for 24 hours. All I can think is that the supercap auxiliary "battery" needs to be coddled so the alternator output is throttled. I know that these secondary battery replacement devices are a major reliability problem.

ua549

Most charging systems are set to charge a battery to 80%. Too fast a charge after 80% will reduce battery life. Thus a charge from 80% to 100% is done very slowly at a trickle rate.