Maintaining 12 V Battery Charge while Physical Distancing

Being retired and living in easy walking distance to a grocery store, pharmacy, hardware store, Costco, and take-out restaurants, I haven't driven our i3 in several weeks and may not drive for several more weeks. I know from storing our i3 for up to 9 months on 4 occasions that the high-voltage battery pack discharges very little over time (maybe 1% per month), so I'm not concerned about it discharging while our i3 is parked. However, 12 V vampire loads like the Comfort Access radio, the telematics module, the burglar alarm, etc., gradually discharge the low-capacity 12 V battery.

The Owner's Manual recommends leaving an EVSE plugged in during short-term storage. However, the DC-DC converter turns off several minutes after the high-voltage battery pack reaches full charge, so it appears that the 12 V battery won't be charged thereafter. Those who wish to minimize the battery cell degradation rate won't leave their battery packs fully charged for days or weeks. Our i3 has been parked at a 73% indicated charge level without our EVSE being plugged in. I'm not aware of any 12 V battery monitor that would automatically turn on the DC-DC converter to charge the 12 V battery as it discharges. Have any of you who have installed a 12 V system logger detected 12 V battery charging that can't be explained by preconditioning or unlocking or opening the doors, hatch, or frunk?

To maintain the charge of our 12 V battery, I unlock then lock the doors remotely via the BMW Connected app. Of course, doing so with the fob or with Comfort Access would work as well, but our i3 is parked 2 floors down and ~100 yards away from our apartment, so using the BMW Connected app is more convenient. After locking the doors, I waited for the high-voltage system to shut off and thus 12 V battery charging to cease. After 30 minutes, I gave up waiting, so I know that the 12 V battery will be charged for at least 30 minutes after the doors are locked.

Unlocking and then locking the doors weekly should be sufficient to maintain the 12 V battery charge. Of course, this would also work from anywhere with Internet access should one be traveling.

I was told by a BMW Service Manager, that most of the problems they have seen are caused by the 12v battery status. At the end of the 12v battery life is when control units are damaged and problems from that point cascade. His suggestion is just replace the 12v battery at the 3 year mark regardless of its status. It is inexpensive to do and prevents a lot of expensive repairs and insures dependability.

Road Jager

RoadJager said:

I was told by a BMW Service Manager, that most of the problems they have seen are caused by the 12v battery status. At the end of the 12v battery life is when control units are damaged and problems from that point cascade. His suggestion is just replace the 12v battery at the 3 year mark regardless of its status. It is inexpensive to do and prevents a lot of expensive repairs and insures dependability.

Click to expand...

I wonder about that advice. It is certainly self-serving for the dealership to replace a 12 V battery before the end of its life.

The expensive modules are high-voltage devices that don't run on 12 V, so I wonder why they would be damaged by low 12 V system voltage. The various 12 V modules in an i3 are similar to those in BMW's ICE vehicles, so does this Service Manager recommend replacing the 12 V batteries in all BMW vehicles before they fail?

Wonder all you want about this advice. This man is not a 'suit'. He was a 20 year factory certified trained technician. He is not a BS'er. He just stated that the root of most of the I3 electrical issues they see in their shop, seemed to stem from the 12v battery that is in later life. He has no financial issue in telling me this. I am a retired 35 year independent BMW, M/Benz and Mini Cooper shop owner. I am merely stating something we talked about in conversation. Analyze this all you would like, I just put it out there for informational purposes and consideration Assuming it is credible, which I do, it may save some I3 owner from large expenses. IF by replacing an inexpensive 12v battery saves big repairs, I would do it.
Road Jager

I posted this on another "12 v battery thread" on this forum:


So I just called my dealer here in the Chicagoland area and they want :shock: $600.00 to install a battery. Apparently the battery is covered under factory warranty but you need to wait until there is a problem?! Not sure I want to do that!

I have a trusted mechanic I have used for 15 years. I think I'll try him as well. Do you think this would void what little warranty I have left. I have a 2017 i3 with 22,000 miles on it. I have a bit of warranty left.

The car was purchased 2 months ago CPO. Do you think the dealer should cover the battery under the CPO?


Doug - out.

PS Thanks in advance for your thoughts

Under the Magnuson-Moss Warranty Act, you or your non-BMW mechanic can replace the battery with non-BMW parts (don't get creative!) without affecting the warranty status of the vehicle.

But no, BMW isn't on the hook to warranty a non-defective battery. That doesn't mean you can't ask!

You should be able to get it replaced at an independent shop for under $300. I bought an OEM replacement battery from my local East Penn Manufacturing distribution warehouse. East Penn makes the i3 OEM 12v battery (AUX 18 L), which they sell under both the Deka and Remy brand name. The Remy is available mail-order. At the East Penn warehouse here, I think the price was around $123, and my independent BMW mechanic quoted $150 to both install and register the battery. Many owners do it themselves. A little more complicated than the standard ICE battery, but do-able. There are several YouTube videos on how-to. Registration takes an ODB tool and BMW software that supports battery registration, like Bimmerlink. East Penn has distribution warehouses all over - you can do a search for ones local to you on their web site. https://www.eastpennmanufacturing.com/contact/where-to-buy/

And no, getting an indy garage to swap out the battery, or doing it yourself, does not void your warranty. The Magnuson-Moss Warranty Act legally prohibits the car manufacturers from voiding your warranty if you do your own routine maintenance outside the dealership - just save your receipts as proof.

Have you asked your dealer if they cover the battery under the CPO warranty? The fine print in the CPO says no, but I have heard of people with CPO warranties getting the 12v replaced for free by their dealer.

Once again.........Thanks for the great info here. Gonna find out if the dealer will cover the battery under CPO. Who knows, I could get lucky.

My mechanic said he could do it for around $300.00 He also let me know he has a few other i3 clients and they have had great luck with the car.


Doug - out

Update:

Took my i3 to my mechanic yesterday to check / replace the battery. He called me back and said he had checked the battery and it was fine. No reason at the moment to change it out. He recommended I do the change in 6 - 9 months.

Love the guy, he is very honest. So I will wait till around Oct / Nov and do the change before we head into winter.

Doug - out