chiurw

I noticed that over the past year, the Convenience Functions Offline message has been appearing more and more frequently. When I purchased my SL two years ago, this message appeared after about a week but now, it shows up after two days. I have read previous post about this and realize that I might have to replace the rear battery. I haven't checked with the MB dealer about the cost of a replacement but wanted to know if anyone here replaced it with an Optima battery (either red or yellow top), or something else that is equivalent to the OEM.

velvet fog

I had to replace mine last month, expect the battery to cost ~200. I put it in myself so the install cost was zero. The owners manual has the procedure for replacing the battery.

I was unable to locate a non-mb replacement through the usual channels. I think this has to do with the way it bolts into the trunk.

CA_E55

Here is another thought: Something a lot of people are not aware of is that car charging systems are notorious for insufficient charging of really depleted batteries. Your car is a 2006, the battery might still recover with a full external charge. Naturally, car dealers want to sell new batteries and will not make an attempt to really check the battery condition - maybe you have a buddy with the equipment to do that. And if you don't drive the car frequently, a trickle charger - or a one weekly full charge with an external charger - is a must or the battery life is severely reduced.

I realize that if your battery is beyond a certain low re-charge cycle threshold, you have no other choice, but I know from experience that many good car batteries are discarded just for the sake of a small profit for the dealer, a total waste!

erichen

replace the battery and get a trickle charger installed if you don't use the car as a daily driver otherwise, that battery doesn't last very long. Not sure about non MB battery for that slot, it's a small battery and not sure if anybody makes a battery in that size.

chiurw

That is a good idea CA E55, I will remove the battery from the trunk and charge it. If it cannot hold the charge, then I'll buy a replacement along with a trickle charger. The battery looked like a standard size the last time I looked at it but never looked at the tray or tie-down mechanism to see if those are compatible with a standard-size battery.

Are there any special precautions in removing the rear battery? Is there anything I need to "reset" when I reinstall the battery?

E55MKB

Wouldn't it be easier to get battery maintainer (not trickle charger) like Battery Tender plus or Battery Tender Jr before taking the battery out? Amazon have it for $25-$45. Connect it to your rear battery to charge/condition your battery. Battery tender comes with wire looms that can be connected to your battery hence making it easier to connect the charger when the car is not used/driven.

Most of the time, the battery just need a good slow charging/conditioning (with Battery Tender) instead of the fast charging by the car alternator or regular battery charger.

Make sure you get Battery maintainer not trickle charger. Trickle charger is a constant charger while battery maintainer will adjust charging based on the state of your car battery.