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I was traveling for work for a few weeks so I unplugged my main battery, and when I got back and plugged it back in, I got a message saying "auxiliary battery malfunction".It's still there after driving the car for a few hours. The aux battery is less than a year old so it should still be good.
I couldn't put a trickle charger on it because I live in a condo and the aren't any outlets in the parking lot by my car.
How long should it take to charge the auxiliary battery with driving?
In my years of experience....no time to recharge it. Buy a new one. Seriously. I have taken them out of the car and used a charger...never seems to work.
What a pain these batteries are. Like I said, I replaced the battery less than a year ago. It shouldn't need replacing again this soon.
Should I expect this every time I travel and leave the car for a few weeks?
It seriously depends on how windy it is....seriously. Our GLS will go for ever in the garage. Out in the weather the wind (and the car trying to level itself) will drain the battery.
I was traveling for work for a few weeks so I unplugged my main battery, and when I got back and plugged it back in, I got a message saying "auxiliary battery malfunction".It's still there after driving the car for a few hours. The aux battery is less than a year old so it should still be good.
I couldn't put a trickle charger on it because I live in a condo and the aren't any outlets in the parking lot by my car.
How long should it take to charge the auxiliary battery with driving?
you can't charge flat batteries with unlimited current supply.
Alternator is not a battery charger.
It's a chassis power supply that can maintain batteries already charged up.
When you disconnected your chassis MAIN battery AUX supplied the car vital circuits for couple hours and got flat dead.
NOTE: Disconnect BOTH!!
When power is reapplied to chassis, a flat AUX will blow its protection 7Amp fuse. Check it out after charging or replacing AUX Batt.
Personally I've installed a motorcycle battery disconnect on my AUX... very handy for experimental reboots.
> ALT Is a regulated voltage supply with nearly unlimited current (200A).
> Charger is a limited current supply with controlled voltage.
Last edited by CaliBenzDriver; May 12, 2025 at 02:29 PM.
Also, depending on the model, the aux battery isn't actually a battery. In some models it's just a capacitor for short term energy storage and voltage stabilization. A capacitor doesn't mind getting fully discharged, though,, but you should also not attempt to charge it. Generally it's better to leave the car on a battery maintainer if parked for an extended period. Some models also have a standby mode that turns off all consumers (telematics, cellular connection, alarm system etc.) to be able to park the vehicle for an extended period. Nice thing about the standby mode is also that it tells you how long you can leave the car before the battery needs to be charged. For example I can leave mine for over 20 weeks if I fully charge the battery beforehand.
Last edited by superswiss; May 12, 2025 at 05:49 PM.
As a point of reference, my battery from sept '12 finally went kaput last month. Thats quite a life. I placed it on a Noco charger with desulphation in order to see what would happen. While the battery did charge, It only lasted a few days before it died again. This indicative of there actually being a bad cell which the car can now no longer charge even though the charger did. You may be able to charge it but if it was seriously low, it needs a force charge or to be stabilized in parallel with another battery (like people do these days with drill batteries) before charging. If you discharge a few times, consider it dead.
Over 5Month storage with battery connected is the best but then security is disabled.
It's not like there's much security if the battery is disconnected. Car can be broken in, or even stolen with a flat bed. I'd assume if somebody leaves their car for an extended period that it'll be stored in a secure location.
2015 SL400 (M276 Turbo), 2014 C350 Sport (M276 NA), 2004 SL500 (M113), 2004 Audi TT225 (BEA)
Originally Posted by Pyronaut
What a pain these batteries are. Like I said, I replaced the battery less than a year ago. It shouldn't need replacing again this soon.
Should I expect this every time I travel and leave the car for a few weeks?
What's the warranty on the battery? Full-size batteries from Sam's Club can come with a 36 month, full replacement warranty. However, the smaller batteries (like for a riding mower or generator) come with 6 months.
you can't charge flat batteries with unlimited current supply.
Alternator is not a battery charger.
It's a chassis power supply that can maintain batteries already charged up.
When you disconnected your chassis MAIN battery AUX supplied the car vital circuits for couple hours and got flat dead.
NOTE: Disconnect BOTH!!
When power is reapplied to chassis, a flat AUX will blow its protection 7Amp fuse. Check it out after charging or replacing AUX Batt.
Personally I've installed a motorcycle battery disconnect on my AUX... very handy for experimental reboots.
> ALT Is a regulated voltage supply with nearly unlimited current (200A).
> Charger is a limited current supply with controlled voltage.
Good to know that I need to disconnect both next time, but what a pain. The aux battery is under the centre console and I have to spend some time getting everything out and hoping no connectors or wires break in the process.
I guess I'll have to find a place with a trickle charger to leave it next time to avoid all this mess.
Last edited by Pyronaut; May 12, 2025 at 10:41 PM.
What's the warranty on the battery? Full-size batteries from Sam's Club can come with a 36 month, full replacement warranty. However, the smaller batteries (like for a riding mower or generator) come with 6 months.
Not sure what the warranty is. I got it off Amazon so I'm not expecting a lot of support from the seller in this regard.
Interesting. I'll give charging it outside of the car a try and see what happens.
There are just a few tricks like trickle charger to keep ownership painless.
Another good one is to dump engine oil regularly at 5kMi not 10kMi and don't forget transmission + differential service. They are too expensive to fix after being neglected.
I saw Cali mentioned the motorcycle disconnect. That can always be a good idea as well, as there’s plenty of devices out there that turn off your battery circuit when the battery gets low. I have one of these on an old land cruiser where i couldn’t find the parasitic draw first time around…a project for this summer. Some people even have fancy remote disconnects for security.
I saw Cali mentioned the motorcycle disconnect. That can always be a good idea as well, as there’s plenty of devices out there that turn off your battery circuit when the battery gets low. I have one of these on an old land cruiser where i couldn’t find the parasitic draw first time around…a project for this summer. Some people even have fancy remote disconnects for security.
well help you troubleshoot it as if it was just another MB... they use the same technology from the same suppliers.
Is this a "keylessGo" or a "key in ignition" only ??
Reboot chassis
scan for faults
measure Amp draw with a DC Amp clamp meeter
Find CAN-B bus bar : unplug doors modules...
Of course if you have a schematic it will come in handy... checkout "All Data DIY" for diagrams.
well help you troubleshoot it as if it was just another MB... they use the same technology from the same suppliers.
Is this a "keylessGo" or a "key in ignition" only ??
Reboot chassis
scan for faults
measure Amp draw with a DC Amp clamp meeter
Find CAN-B bus bar : unplug doors modules...
Of course if you have a schematic it will come in handy... checkout "All Data DIY" for diagrams.
I appreciate that. Its a 2000 model so is should be simple. I have four very large technical manuals (FSM) for that one. The draw was not consistent so the likely culprit is a door switch or overhead light. That truck also has some rust and previously had a small stereo upgrade prior to my ownership. I used a probe but really should have used my ammeter. Its not at my house so I have not dug in. This is also an unfortunate case of where the auto disconnect works so it was easier to let it be.
IH8mud is a forum for those and its very comprehensive and kinda unmatched in how much technical knowhow is there. We do good things here, but they are different genres and build philosophies.
We bought my wife her 2023 SL55 a couple months ago. It had been sitting on the dealers lot for about 45 days and was showing the low battery notification on the dash. We took it to the dealership for a diagnostic test and it came back that the battery was still good. I put a pigtail on the battery and it’s been hooked up to the trickle charger ever since and is fine.
When I first hooked it up it took 3 days on the trickle charger before the LEDs on the trickle charger to show fully charged. Now it takes only a couple hours to show full.
A trickle charger is the answer to battery woes. I have 11 of them hooked up to various cars and toys in my garage.
A trickle charger is the answer to battery woes. I have 11 of them hooked up to various cars and toys in my garage.
Yes but ...........
Its ridiculous that a "modern" vehicle should REQUIRE trickle charging if it is sitting for less than 3 or 4 weeks. I find that totally unacceptable but it appears to be the new norm. On my 2013 ML the auxiliary battery was not much bigger than the 9 volt battery you would use in a smoke detector. Once I realized that the mini bat was sitting in a compartment originally intended for a battery that you might find on a garden tractor I immediately swapped in the real battery. Maybe its too soon to declare victory but my frequent transmission shift errors have completely disappeared with that simple change. These cars are marvels of engineering but, like everything an engineer touches, sometimes they go too far.
Its ridiculous that a "modern" vehicle should REQUIRE trickle charging if it is sitting for less than 3 or 4 weeks. I find that totally unacceptable but it appears to be the new norm. On my 2013 ML the auxiliary battery was not much bigger than the 9 volt battery you would use in a smoke detector. Once I realized that the mini bat was sitting in a compartment originally intended for a battery that you might find on a garden tractor I immediately swapped in the real battery. Maybe its too soon to declare victory but my frequent transmission shift errors have completely disappeared with that simple change. These cars are marvels of engineering but, like everything an engineer touches, sometimes they go too far.
Modern connected cars are like smartphones. They maintain a constant cellular data connection, so you can always reach them for remote start, remote lock etc. Therefore they are always partly powered on for the telematics and online functionality. This uses battery over time and drains it. If the car is parked in an area with a weak cellular signal then it uses more battery as it has to boost the signal more. So eventually the battery will be drained after a while. That's what the standby mode is about I posted above. It turns off the cellular connection and telematics among other things, so the car can be parked for an extended period knowing that you won't need to use the online features and as shown with standby mode turned on, you can leave the car for 5+ months and the battery won't be dead.
Having said that, there is an other aspect on the newer cars with retractable door handles. Seems if the key fob is too close, it remains in constant communication as the car has to determine when to extend the door handles. Some people have resorted to putting the fob in a faraday pouch, but you can also just turn it off. The steps for that are in the owner's manual. The fobs are technically supposed to go to sleep after they've been motionless for 2 minutes, but seems that's not happening with the retractable door handles, and it keeps the door modules powered up, which then drains the battery.
Last edited by superswiss; May 13, 2025 at 11:50 AM.
Its ridiculous that a "modern" vehicle should REQUIRE trickle charging if it is sitting for less than 3 or 4 weeks. I find that totally unacceptable but it appears to be the new norm. On my 2013 ML the auxiliary battery was not much bigger than the 9 volt battery you would use in a smoke detector. Once I realized that the mini bat was sitting in a compartment originally intended for a battery that you might find on a garden tractor I immediately swapped in the real battery. Maybe its too soon to declare victory but my frequent transmission shift errors have completely disappeared with that simple change. These cars are marvels of engineering but, like everything an engineer touches, sometimes they go too far.
I wholeheartedly agree, it is pretty ridiculous to have to do that when the car sits for a bit.
And also that a fairy new OEM aux battery effectively permanently dies when fully discharged once.
Last edited by Pyronaut; May 13, 2025 at 11:47 AM.
I’m also on FerrariChat and it’s a common problem with Ferraris too. Almost everyone keeps their cars on Tenders when not being driven. The guys that live in condos with underground parking will put a Powerstation (large battery) on the passenger floorboard and hook up a Tender to that. When the Powerstation gets drained they take it inside their condo to recharge, then put it back in their car.
A good alternative to the faraday pouch, is a minimum of 20 feet of separation of the fob from the car. Prior to separation, I always depress the lock button, pause, then press it again.
The reality is that with ever more complex and always connected electronics, and the need to electrify more and more parts of a car, the batteries continue to be the weakest link in all of this. We are at best 1-2 decades away from commercializing some of the potential breakthroughs in lab environments that we seem to read about every few weeks. Over the last decade we've come up with sophisticated energy management solutions and been shrinking the transistors on chips to make them more energy efficient that we've been able to make current batteries last longer than ever, but when it comes to the battery itself, the best we've come up with so far are lithium based batteries that if you care about performance and lifetime only have 60% useable capacity. Don't charge them above 80% and don't discharge them below 20%. Given how heavy batteries are and how low their energy density is, that's a lot of dead weight, literally. Even though combustion engines at best achieve 30-40% efficiency, the energy density of the fuel is so high and refueling is so quick, it doesn't matter from a usability standpoint, but when the battery dies in our devices it takes time out of one's day to recharge them and worst case the battery is completely dead.
I’m also on FerrariChat and it’s a common problem with Ferraris too. Almost everyone keeps their cars on Tenders when not being driven. The guys that live in condos with underground parking will put a Powerstation (large battery) on the passenger floorboard and hook up a Tender to that. When the Powerstation gets drained they take it inside their condo to recharge, then put it back in their car.
Problem with exotics is they are mostly driven on weekends and in the winter, are garage queens, so the battery never really gets a chance to charge much. Plus the starter uses energy to start the battery. So chances are, the battery never got charged more than what the starter drained?
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