mikefamig

Can anyone tell me if my 2005 R230 has a separate backup battery for the SBC system?

Mike.

Rudeney

No, but what it has is two batteries. The smaller lead acid battery under the hood is used only for starting the engine. The larger AGM battery in the trunk is used for operating all the other electronics in the car (including SBC). The BCM (battery control module) is responsible for keeping the two battery circuits separate, but allowing both batteries to be charged by the alternator. Also, if the rear (consumer) battery is low, the BCM can add the starter battery in parallel to keep the consumers (especially SBC) powered. When this happens, the BCM will also shut down certain non-essential consumers, such as the power seat and steering column adjustment, vario roof, cigarette lighter, seat heaters, etc. This is specifically so that the engine electronics and SBC can be powered.

mikefamig

Thanks, good to know. I can imagine this has been the cause of much confusion to guys like me whose knowledge is limited mostly to vintage cars. I was a professional auto mechanic in the 70's but went on to other things always keeping a hand in cars as a hobby. This basically means that I know enough to keep getting myself in trouble. I've learned to move very cautiously with the SL and this forum has been a great help.

I'm aware of the two batteries and have replaced both in the past but did not understand how the SBC was handled. Does this mean that it is possible to lose braking power while you're driving down the road due to a dead battery? Does the car prevent this from happening?

Mike.

kbob999

Rodney has some great answers, but I’ll add mine on this subject.

If the rear battery or system voltage gets too low the SBC pump could stop running. The car attempts to protect you and causes a red brake warning to show on the dash. This happens when the alternator or regulator stops working. It gives plenty of warning before you run out in voltage. I’ve driven 10-20 miles after my alternator has failed twice.

If you do run out of voltage, or the SBC motor fails and the pump stops running, the brakes will still work, but you have to push the pedal all the way to the floor and you have maybe 10% of normal braking available. This is the last ditch fail safe to stop the car. You can also step on the parking brake.

Rudeney

I've had the same experience as kbob - the alternator failed so the car was running strictly from power supplied by the consumer battery, which then drained and switch to the starter battery. i was able to make it home before I lost all power.

ponchik

And even in case of pump failure you will most likely have 2 normal braking attemps as SBC unit has receiver that can provide several brake applications before it runs dry and you will be on your own.