Nait Sirhc

So my battery "visit workshop" red gauge display came on a few weeks ago. I replaced my Aux battery with a standard 14-BS ready-to-rock AGM ordered from some battery place on the Internet. Installation was simple, jumped the connector cables to my GF's Passat while swapping the batteries so I didn't lose all my settings.

Two weeks later, I lent my friend my car. He left the lights on and the car was completely dead the next day. Now, it seems like I've done this before and the car fired right up the next day, albeit with the "Battery Protection: Convenience Features Disabled" message. So I'm thinking, oh well maybe he drained both. We tried to jumpstart the car by connecting his to my aux battery; that didn't work. Jumped it to the rear (actually to the cables coming off the battery that normally go to my amp), car started up fine.

When I reconnected the cap, it sparked something decent and the cap's readout instantly jumped to 12.3. This is when I remembered you're supposed to charge caps with a resistor. Think this could have damaged the main batt? This was two weeks ago, mind you.

Fast forward to today. I haven't driven the car in 2 days, I go out and it lights up the big WARNING BRAKE PEDAL MAY BE REDUCED message. Car cranks once weakly and then won't try it again. Now, at this point I'm inclined to believe something is wrong with the aux battery. Or the main battery. Or something is leeching power that shouldn't be. I didn't leave the lights on overnight, I know that much.

I have a Kicker subwoofer in the trunk, an Alpine amp, and a 2 farad cap. Amp runs to cap, cap runs to chassis ground and to main batt positive. These have been in the car for 4 months or so and I have never had any problems.

So what do you think the culprit is? Aux batt? Main batt? BCM? Something being left on?

Nait Sirhc

Also worth noting is that my little battery charger is a 2/6 amp, and when I try to charge the main batt with 6 amps, it'll jump to "overload" after a second or two. 2 amps works fine, but it's gonna be 3-4 hours before I can start the damn car.

NORCAL///AMG

May want to load test your main battery....it may have died since the lights drained it to zero. Regular batteries - especially old ones - do not like to be drained to zero. New batteries or deep cycle batteries are the only ones that fare well when drained to zero.

GL

AMGSpartan

Hello Nait Sirhc,

Something is amiss with your battery charger not being able to charge at the 6 amp rate. The main battery is very large and should be able to accept a 12 or even 15 amp charge rate.

As NORCAL suggests, your main battery may have 'died' since it was run down to nothing when the lights where left on overnight. So I'd be looking at the main battery as the culprit....how old is the main battery? If more than 4 years old, I'd just replace it if load testing is not possible short of involving the dealer.

Peter_02AMG

You try disconnecting your stereo stuff in your trunk?

Nait Sirhc

That's what I'm thinking. All the stereo stuff is disconnected; I have to disconnect it in order to charge the battery (I just disconnect the 0ga wire going from the cap to the battery, and then connect the jumper/charger cable to that. It's 60A fused)

Anyone know offhand how much it is to buy a battery from the Roadside Assistance guys? I just want this done and I have no way to get around to buy another one myself.

Peter_02AMG

I don't know about your area and E55 but on E63 MB Roadside assisance charge me $224 minus $40 discount from MB which automatic deducted so it about $184 plus tax and he come out to your house and install it.

Nait Sirhc

Car started up fine today (after charging @ 2a all night). Reconnected the cap (charged it via test light first) and it's reading 12.1v with the car off. That seems quite low (don't our batteries normally read 12.6ish?) so I think the main battery is going bad.

AMGSpartan

Hello Nait,

At rest a fully charged lead acid battery should read 12.7 volts. Your reading of 12.1V corresponds to barely 50% charge. See attached chart.

930chas

How old is your main battery? At this point I would definitely just get a new one and call it a day.

Nait Sirhc

Yeah, I guess that's the plan. Just a huge pain in the *** to pull the sub/amp/cap out.

Nait Sirhc

So the night before last, 8 months after this thread was posted, my battery finally went bad. Systems started dying one by one, then the SBC went out "BRAKE FUNCTION REDUCED" and I lost the brakes almost completely. Limped the car home and had roadside assistance (who was actually one of the local MB techs) come out and swap the trunk battery last night. $270 later and the car seemed to be fixed. One thing I noticed, however, was that my cap's digital readout only read 11.8 volts, even with the car running. Normally when the car's running it'd read something like 14.3, and with the car just recently shut off it'd read 12.5-12.7. I figured it may just need to charge, but it seemed as though the car wasn't running off the alternator.

Last night, I drove downtown and suddenly the nice blood red VISIT WORKSHOP battery light comes on. So I start heading back home and then got a new message... "BATTERY ALTERNATOR: STOP CAR!" alternating with "BRAKE SYSTEM DISABLED!" (even though they were still working). Called roadside assistance again, the tech told me to meet him at the dealer (a couple blocks from where I was).

Dropped the car off at the dealer and I should hear back tomorrow. Once I parked it, it wouldn't even crank over. Yikes. Good thing it didn't leave me stranded. He thinks the BCM may be bad or need reprogramming. I'm assuming this will be covered under the extended warranty. Very odd, though. Any ideas?

harjothundal

Sorry to hear about your troubles my man! I too was searching this forum as I received the "conv functions temporarily disabled, battery low" message... The heated seats shut off, as did the radio... A few minutes later after freeway driving, another message popped up saying "convenience functions are available again"... Here's one thing I noticed though, lately I've noted that the dash display lights (backlight) dims back and fourth when the car is idling... When I got the "functions disabled" message, they were dimming quite heavily... Then when the functions were back on, the dimming was completely gone... Weird... Drove the car a few hours later with no problems, stereo cranked and all...

Please do keep us updated with your cars diagnosis!

Thanks!

AMGPilot

You have a alternator that needs to be replaced and probably the BCM needs to be replaced also

linus69

Where I am Winter comes early and stays late, so I keep a battery maintainer/tender on every vehicle and piece of equipment we own. I just picked up this one for our 2019 GLC300.

https://tools.woot.com/offers/noco-m...attery-charger