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      09-04-2019, 11:29 AM   #1
msdss
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Battery Tenders / Long Term Storage

I'm planning an extended trip. I've seen here and there on the forums that if you are going to leave one of these cars for any long period of time, that it is a good idea to leave a battery tender on it to keep the battery health.

I've read that the battery in these cars are crazy expensive, and if I use the wrong charger it can destroy the battery, and they are something like $1200 to replace. I have a battery tender for my Ducati, and a Dewalt charging station that I've used on another car of mine, but I'm not sure what makes these chargers safe or not safe to use on this car.

Last edited by msdss; 09-04-2019 at 11:52 AM..
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      09-13-2019, 07:31 PM   #2
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Very good question. It must have the ability to maintain the charge, not just charge continuously. Also, must have the ability to charge AGM type batteries.

Here is a solid recommendation for a "smart" battery charger.

CTEK (56-864) MUS4.3 12 Volt Fully Automatic 8 Step Battery Charger

So, this will get you in the right direction. The cool thing with our BMW's is that you can charge from under the hood...and not the truck (where the battery actually lives)...


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      09-14-2019, 02:34 PM   #3
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+1 for the CTEK unit. I've been using it on my GT4 during the wintertime. What convinced me? Finding the Porsche's battery dead after a few weeks of sitting. Where's the battery? In the front trunk. How do you open the front trunk to access the battery to recharge it? An ELECTRIC switch next to the driver's seat. A switch that won't open the front trunk when the battery is dead. Turns out I had to connect an external power source to a special metal connector in the fuse box to be able to open the front trunk with a dead battery. Then I had to jump start the car with jumper cables, because the roadside assistance guy's battery charger didn't have the required juice to restore the Porsche battery. The official "Porsche Roadside Assistance" guy turned out to be a local third-party guy who had never dealt with this type of situation before. I googled it and we figured it out together. Nice enough guy, but he had no clue. Not very reassuring, Porsche.

So get a quality battery maintainer like the CTEK. Well worth the money!
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      09-14-2019, 02:51 PM   #4
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thanks for the replies. I'll start looking into ctek. I went and looked at the battery. it's some massive lithium ion laptop looking thing. I absolutely do not want to piss away money fixing that. much less finding myself locked out of my car because of it.
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      09-16-2019, 11:07 PM   #5
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It's not a "lithium ion laptop battery thing," it's a pretty standard (for German cars) AGM battery. There's "a thing" attached to the top that is part of the SOC (state of charge) monitoring system, and somewhere in all the mess there will be a pyrotechnic charge that fires in the event of a severe accident (this is the battery disconnect system since it isn't up under the hood like in most cars).

They are still not cheap batteries, plus if you want to not prematurely kill the replacement battery, you need to have the dealer do the swap (or someone who has access to Rheingold so they can run the "new battery who dis" procedure on the car's software). This dance is required because the car "learns" the battery's characteristics over time and adjusts charge voltage based on temperature and other factors. If you just change out a dead battery for a new one, but don't "reset" this table of learned charging factors, you end up killing the new battery much more quickly since the car ends up overvolting it (new battery doesn't need as much voltage to charge as the old one did).

The CTek chargers are solid, as are the actual "Battery Tender" brand. Just hook your tender of choice up to the posts under the hood - do NOT try to attach directly to the battery, as this would bypass any protection in the car and is the opposite of what you want to do.
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      09-25-2019, 08:28 PM   #6
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Quote:
Originally Posted by ZPrime View Post
It's not a "lithium ion laptop battery thing," it's a pretty standard (for German cars) AGM battery. There's "a thing" attached to the top that is part of the SOC (state of charge) monitoring system, and somewhere in all the mess there will be a pyrotechnic charge that fires in the event of a severe accident (this is the battery disconnect system since it isn't up under the hood like in most cars).

They are still not cheap batteries, plus if you want to not prematurely kill the replacement battery, you need to have the dealer do the swap (or someone who has access to Rheingold so they can run the "new battery who dis" procedure on the car's software). This dance is required because the car "learns" the battery's characteristics over time and adjusts charge voltage based on temperature and other factors. If you just change out a dead battery for a new one, but don't "reset" this table of learned charging factors, you end up killing the new battery much more quickly since the car ends up overvolting it (new battery doesn't need as much voltage to charge as the old one did).

The CTek chargers are solid, as are the actual "Battery Tender" brand. Just hook your tender of choice up to the posts under the hood - do NOT try to attach directly to the battery, as this would bypass any protection in the car and is the opposite of what you want to do.
Hmm. I have a bit of a dilemma. I live in a condo in downtown, and I don't have access to an electrical outlet. Considering the various battery sub-systems you've mentioned, I'm guessing I can't take the battery out and run the tender out of my condo unit (or anywhere else for that matter)?


For everyone else looking for the specific CTEK mentioned above, apparently it's been replaced by a successor, the CTEK 40-206 MXS 5.0-12 Volt Battery Charger. This is according to various reviews/answers on Amazon. I believe one of said reveiewers called CTEK to confirm, but you might want to do the same to be sure.
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      09-26-2019, 06:15 PM   #7
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I personally use a ctek multi us 7002 for my personal '17 m2 which has an AGM battery. I'm not sure if the M2C has a lithium battery, but if it does you'll need a lithium ion battery tender.
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      09-26-2019, 08:25 PM   #8
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Quote:
Originally Posted by F87source View Post
I personally use a ctek multi us 7002 for my personal '17 m2 which has an AGM battery. I'm not sure if the M2C has a lithium battery, but if it does you'll need a lithium ion battery tender.
I guess only the m3/m4 got a lithium ion? m2c is just an AGM.
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      09-26-2019, 10:02 PM   #9
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Quote:
Originally Posted by msdss View Post
I guess only the m3/m4 got a lithium ion? m2c is just an AGM.
Yeah I wasn't sure about that, the good thing is that you have a wider range of trickle chargers available and theyre cheaper.

Also Don't forget to only connect the trickle charger via the underhood jumper terminals, that way the IBS can monitor the charge and you won't risk damaging the sensor.
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      10-02-2019, 03:56 AM   #10
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Quote:
Originally Posted by gphung View Post
Hmm. I have a bit of a dilemma. I live in a condo in downtown, and I don't have access to an electrical outlet. Considering the various battery sub-systems you've mentioned, I'm guessing I can't take the battery out and run the tender out of my condo unit (or anywhere else for that matter)?
You could take the battery out and put it on a tender outside of the car, but that will then piss off all of the systems in the car since they lose power. They should mostly (if not fully?) figure that out when you hook it back up to the car though.

The concern about attaching the tender directly to the battery is only a concern if the battery is still in the car. You don't want to do that because it can damage the car's charge monitoring stuff (in theory, although I don't know if anyone's actually done that).
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      10-02-2019, 09:45 AM   #11
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Quote:
Originally Posted by ZPrime View Post
You could take the battery out and put it on a tender outside of the car, but that will then piss off all of the systems in the car since they lose power. They should mostly (if not fully?) figure that out when you hook it back up to the car though.

The concern about attaching the tender directly to the battery is only a concern if the battery is still in the car. You don't want to do that because it can damage the car's charge monitoring stuff (in theory, although I don't know if anyone's actually done that).
Edit- I didn't read the previous post correctly.

Correct, but you don't necessarily need to remove it, of you do make sure you're using a battery tender that can monitor voltage vs. a standard battery charger. This approach is not ideal as it'll charge the battery more than the IBS would.


Correct, damage only occurs if you attach a charger directly the the battery itself if it's in the car. This is because it bypasses the IBS and can over charge the battery. Also if you're not using a voltage stable battery tender the pulses in the current delivery can damage the IBS itself.
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