Has my battery died?

[Pitts Pilot]

I think the rear 'Convenience' battery is dying.

I was working on the car and listening to a CD, and after 45 mins a warning message appeared on the Comand screen saying the battery was low. So I checked the voltage with a multimeter and it was 8.75 volts. This after an 8 hour drive! I started the engine and the voltage rose to 12.75, so I know its charging.

How does a 74Ah battery discharge in 45 mins?

What's more I 'conditioned' the battery with an Optimate 4 Charger about a month ago and it seemed fine. So without knowing the age of the battery I have come to the conclusion that its on its way out. What do you think?

My second question is about the replacement. I have no problem popping down to my local MB dealer and replacing the battery like-for-like. But I was reading a post on a German MB forum that recommended a superior Varta "Stop Start" battery: http://www.varta-automotive.com/index.php?id=515&artikelNr=570901076B512&application=PKW&modellinetype=19389

I am about to have Comand Online install some serious audio equipment, so naturally I want the power supply (the rear battery) to be in A1 condition.

My understanding is that the front and rear batteries in the SL are of very different construction as they are designed for two completely different jobs.

I believe that the rear 'Convenience' battery is not designed for cranking the starter motor (huge current), but is designed to service more modest electrical needs, such as the roof, seats, comand, etc. Basically lower current delivery but improved life cycle. (It even has a warning sticker saying "Do not use for jump starting"!) I think they are sometimes called 'caravan' batteries for the same reason.

I currently have a MB badged Varta battery (No. A003 541 73 01) and I would like to know whether the above 'Stop Start' battery is the same type, ie. Not for cranking, but for lower currents, and better life cycle?

Any help and advice on battery types as well as a premium quality replacement would be most welcome

 

[mersum1es]

Yest, 'start stop' class is AGM type battery. A local mechanic of official dealer told me that AGM should be used in main battery because of battery charger control is optimized for them. Don't know how much bulls*** was included, but I bought varta from different storage (>100€ cheaper than in MB) because I have webasto and also has been lot of discussion in Finnish forum that AGM is very suitable for winter 'heavy duty' usage. I read somewhere that auxiliary battery should be same type than main battery?

 

[grober]

Don't quote me on this but 12.75 volts seems a mite low for the alternator to be putting out[ if that's the alternator output you measured? --- normally to charge a battery a charger requires 14.4 volts rising to 14.7 volts for some types of AGM battery. I would get the alternator output checked. That said even a dead battery can summon up a nominal 10.2 volts so are you sure your multimeter is giving yoou an accurate reading? If the battery is "from factory" i.e. 5 years old Renew it.---- In an "electronic heavy dependent" car like an SL 350 2007 I would not hesitate to replace it.

 

[mersum1es]

Don't quote me on this but 12.75 volts seems a mite low for the alternator to be putting out[ if that's the alternator output you measured? --- normally to charge a battery a charger requires 14.4 volts rising to 14.7 volts for some types of AGM battery. I would get the alternator output checked. That said even a dead battery can summon up a nominal 10.2 volts so are you sure your multimeter is giving yoou an accurate reading? If the battery is "from factory" i.e. 5 years old Renew it.---- In an "electronic heavy dependent" car like an SL 350 2007 I would not hesitate to replace it.

+1 for that, didn't notice it was engine running measurement (and if taken from main battery)... there is a charger controller which also control auxiliary battery charge (during the engine run only), and charge is not necessarily on all the time. Engine off and car in sleep, auxiliary battery is isolated from other system.

Edit: SL dual battery info
http://www.mercedestechstore.com/pdfs/R230_adivsor/218 HO Dual Battery System (ACB) 11-28-02.pdf

 

[Pitts Pilot]

+1 for that, didn't notice it was engine running measurement (and if taken from main battery)... there is a charger controller which also control auxiliary battery charge (during the engine run only), and charge is not necessarily on all the time. Engine off and car in sleep, auxiliary battery is isolated from other system.

Edit: SL dual battery info
http://www.mercedestechstore.com/pdfs/R230_adivsor/218 HO Dual Battery System (ACB) 11-28-02.pdf

Thanks for the Link to the battery system info - I never realised how complicated it was!

Yes, I agree the voltage was on the low side. However, it was measured straight after the Comand message and with the engine off (8.75v), and then with the engine on (12.75v).

I attribute the 12.75v to the fact that it was measured across the battery terminals and I would expect a dying 74Ah battery to 'present' quite a tough load for the alternator seconds after it has started the charging process.

I have been on Varta's site looking for a replacement, and I think the LA70 battery, found under 'Leisure batteries/VARTA Professional AGM' is the correct replacement. Certainly the physical size as well as the electrical details are the same as the existing one.

I also believe that an AGM (Absorbed glass mat) type is correct for the rear?

Thanks