Battery Trouble - 2012 Mercedes-Benz SLS AMG Roadster Long-Term Road Test

2012 Mercedes-Benz SLS AMG Roadster Long-Term Road Test

2012 Mercedes-Benz SLS Roadster: Battery Trouble

January 30, 2013

2012 Mercedes-Benz SLS Roadster

This morning I came down to my garage to move the Mercedes-Benz SLS Roadster out of my tandem parking space to let my wife out. The SLS started up with its normal uproarious vigor, as if engaged by the hammer of Thor himself. Seriously, it's glorious.

Anyway, I pulled forward to let my wife out, turned on the map light for some reason, and backed the SLS into its spot again. Approximately one hour later I returned to the garage to go for my first real drive in what could easily become my favorite long-term car ever.

Ruh-roh. The keyless entry isn't working. Try again, again, again. Nothing. OK, pull out the little key within the fob, unlock the door and get in. Everything seems to be normal as all the usual displays are on. I have completely forgotten about the map light. Push the starter button, nothing. Ruh-roh again. I reach back and plug the key fob into its home in the little bin behind the center console. There is what I can only describe as an electrical pop, followed by everything other than the engine coming on in the car. COMAND, HVAC system, everything is running as normal, which makes me doubt the obvious prognosis of dead battery.

What in the world is going on?

I try again and again, with no luck. On one occasion I get a warning that says "Reversing Not Poss. Service Required" and on another something regarding the SRS airbag system, neither of which would show up again on subsequent attempts. After shutting down the car from accessory mode, the keyless entry began working again. I get out and get in, figuring maybe a reboot will help as it has with other cars in the past. Nope, engine still not starting, but everything else is. I still haven't remembered the map light, and it's bright enough in my garage that it's not obvious.

I call Fleet Master Mike Schmidt and describe what's going on. He, like I, suspect the battery from the get go, but is equally perplexed by so many vehicle functions being online and the fact the car started with no problems only an hour earlier. He suggests I go back down to the garage and try rolling down the windows. If they are slow, then it is in fact likely a battery regardless of what may be going on. I do so and sure enough, the windows are slow and COMAND is no longer coming on. The battery has been draining further. At this point I remember the map light, smack myself in the forehead and turn it off.

With no cables and no other car for help, we decide to call Roadside Assistance. The helpful gentleman from Mercedes-Benz of Beverly Hills came promptly. However, his first attempt at boosting the car with his jump starter failed. "Not enough power," he said. Fine, we'll use the ML350 Bluetec he brought. No dice there either, which means I couldn't have done it myself even if I wanted to. According to his battery meter, our 12V battery had a 10V charge, which is apparently not enough to fire up the SLS's massive V8. So the battery wasn't close to being dead, it just wasn't up to the task. Roadside Assistance gentleman thought it was surprising the car had such a small battery.

Not to worry, as the Roadside Assistance gentleman got out a brand new battery and installed it right then and there at no charge. I should point out that I was doing all of this as Joe Blow Consumer and not as a representative of Sure enough, the SLS was finally able to resummon Thor and the V8 fired to life.

I take away from this incident a couple of things. First, it takes a boat load of electricity to fire to life that engine when you consider its remaining charge and that virtually everything else in the car was working just fine. Second, unlike most of our long-term cars, the SLS Roadster came to us after about 14,000 miles in the hands of other journalists and likely celebrities and other samplers of the Benz press fleet not exactly known for taking care of other people's cars. The battery was probably on its last legs and an hour with a map light on knocked the wind out of it.

I'm just happy it happened in my garage on a day I was working from home. I also wish I remember why I turned on the damn map light in the first place.

James Riswick, Automotive Editor @ 15,470 miles

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