So I've had this 2004 SL500 for a couple weeks, and it started with the error warning "Electrical consumers offline". Strange, it had only been 2 days since it was driven, so supply battery shouldn't be way down. Tried to drive it a bit, throttle was unresponsive. Shut it down and fired back up for a reboot, and ran fine, error message gone. So I took it back for checkout (bought it from a Merc shop), and what I wanted checked was lost in translation between the girl in front and the tech- enough about that. So on the way home, I stopped a couple times, about a half hour first leg, came back out and fired up, all OK, made a phone call, and instruments suddenly bottomed out, then rebounded, SRS light came on, then I got an warning that the top was partially open which it was not. Instruments bottomed a couple more times. Got the big red battery light- car has both batteries replaced just before I got it, "Mercedes" batteries, and I checked both after the first episode and 14+ while running. Thought I'd get it moving, then the battery light went out and it settled down a bit and the SRS light went out. Stopped to eat, fired up and it's still having a spaz attack, AC started blowing dead hot, SRS again etc. Shut it off and rebooted, AC back to normal, again with the instruments. Got home and shut it off, checked service battery and 11.98 V(!). Start bat 12.7. Started it back up and 14+ at both. So I have a good NOCO charger with low amp and AGM setting, overnight came up to 12.56, short drive and it was 12.10(!). Charged it again and let it sit a couple days, service is 12.08V. Charged it for a couple hours, came up to 12.26, so I'm thinking it should charge up while driving, went to lunch, it's fine, stopped at H Depot, came out and spaz attack again, see the shop, error saved, SRS light on and off, top open light again. Get home and 12.08V. This thing had a LOT of work done on it just before I got it, including new suspension and brake modules at many $. Why is it not charging the serv battery properly? I am aware the low voltage makes these things unhappy, but it shows 14+ when running, idle with AC on. New batteries can be bad, seems like it show be charging up better? I let it sit for a couple hours to settle down and checked amp draw at the neg cable, .19 to .2 amps which seems pretty good. At this point, I am not impressed, and driving my XK8 that is bulletproof WTH?

 

As you said, it all suggests an intermittent charging issue potentialy? Can you stick you meter on (use the cigar lighter) and drive around to see if the 14.1V drops down as you drive, could be a loose cable or perhaps your voltage regulator is on its last legs?
I have a permanent tiny 12V digital display wired into my ashtray to keep an eye on the voltage to ensure car is charging etc. or you can buy a plug in cigar lighter 12v display cheap on ebay,

 

Yeah, going from the XK8 to this has its downside for a gearhead, the Jag has that gorgeous dash with a full set of good size guages, including a pretty accurate voltmeter and clock- the dash pod on the SL is pretty boring. I need to put the scanner on it (icarsoft MBII coming today) and see what the stored code is etc. I have a really good scanner for my Powerstroke Diesel and can navigate it pretty well, but I would have to get the Mercedes bundle, and the icarsoft seems better for this car. Batteries are new and connections very clean, but it sure doesn't seem like the service battery is taking the amount of charge it should. I have been testing directly on the battery post and it shows 14+ V when running, even with the AC, stereo and lights on. Cigarette lighter voltmeter is interesting, but with a supposedly superior vehicle, I shouldn't have to lol... Thanks!

 

Just ordered a lighter socket charger with volt/amps digital readout, be here tomorrow. When it has done this, it is fine at first, does it after fully warmed up, then parked for a while hot. Acts up after sitting hot- hmmm

 

Battery control module may be bad

 

BCM was just replaced- but of course that doesn't mean it's good. It has only been spazzing out after sitting a while when fully warm, so maybe some heat soak involved. Drove it 3 times yesterday, but all short trips, was fine. Scanner showed up last night, and I ordered a phone charger with amps/ volts indicator so I can watch it while driving. Gotta say, getting out of my XK8 with that awesome dash with a full set of guages, including a nice big and quite accurate voltmeter, and then looking at this quite plain dash is disappointing

 

I believe you can scroll through the menu on the dash and get the voltage, not sure if it’s while running though

 

Probably, should have seen me looking for the dipstick That little phone charger had a bright digital readout so I can see it going down the road, and I needed a charger anyway. Thanks to everyone!

 

Voltmeter in dash: use starter key. Turn one step and press stalk between the dials three times (not too slow). Voltage will show up and stay up while driving until you toggle it away with the 'master' steering wheel button. Use the arrows key above it for more fancies.