[soundquest]

No joke -- at lower speeds, if I step on my brake pedal or use my blinkers... all my dash lights light up like a christmas tree (or reboot) and my power steering eats $hi#* and gets all hard to steer. Battery is from 2011 (but still does not drop below 12.01 volts (until after two days it will go down to 11.97 volts). Battery cables and terminals are tight and clean. Any ideas? Anyone?

 

[TinLizard]

No joke -- at lower speeds, if I step on my brake pedal or use my blinkers... all my dash lights light up like a christmas tree (or reboot) and my power steering eats $hi#* and gets all hard to steer. Battery is from 2011 (but still does not drop below 12.01 volts (until after two days it will go down to 11.97 volts). Battery cables and terminals are tight and clean. Any ideas? Anyone?

A fully charged lead acid battery should be 12.6V, 11.9 volts is close to being completely dead. Seven years on a battery was a good run and I would be looking to replace it.

 

[avlcat]

The battery is most likely close to its death, but this alone does not explain electrical issues when car is running. Alternator on running car provides all the energy to the electronics and electrical power steering.

I would stop by at PepBoys, O'Reily or Autozone (assuming OP is in US) to get electrical charging system tested. They do it for free and would be able to tell you if the alternator is dying.

Talking from experience - had similar issue on my other car and it was dead battery which affected and killed alternator; had to replace both.

 

[Mike C]

Get your battery and alternator checked. When the engine is idling even a good alternator will not charge much, and your engine is probably idling when you use the brakes or turn signals.

See what happens with the engine idling and your headlights are switched on to high beam.

I also have a MB SLK, with electrical devices everywhere, and less than perfect battery causes heaps of fault lights, the codes generated generally centre around low input voltage into the various vehicle computers.