[G30M]

so i only just discovered the M550i has 2 batteries, one in the trunk one under the hood

i plug the car to a CTEK battery tender, which battery am I charging? will the battery charger charge both batteries?

also what happens if i decide to only use one battery?

[430Scud]

The two batteries in the M550iX are isolated from each other by the Intelligent Battery Sensor. You can safely charge them either individually or simultaneously.

To charge the main battery in the trunk, you can connect directly in the trunk, or use the jump start terminals in the engine compartment to attach clamps, or use a pig tail adapter.

To charge the auxiliary battery in the engine compartment you can connect up directly or use a pig tail adapter.

If you wish to charge both batteries at the same time, AND your battery maintainer is approved to charge multiple batteries, you can do what I did. I installed a pig tail adapter to the aux battery and a pig tail adapter to the jump start terminals in the engine compartment. Then I joined both pig tail adapters with a Battery Minder multiple battery connector: https://www.batterystuff.com/battery...or-210-ay.html
BatteryMINDer SmarTECHnology® Multiple Battery Connector - 210-AY

I run the Y adapter cable connector out by the passenger (right side) windshield wiper. This way I do not have to constantly open and close the hood to access the battery connectors to charge both. One thing I do, is place a yellow microfiber cloth on the drivers side of the windshield to remind me not to drive off with the charger connected.

Then I charge both batteries with one of two approved battery tenders I own that can charge multiple batteries:

https://www.batterystuff.com/battery...r-128cec2.html
BatteryMINDer 128CEC2 | BatterMINDer 12v 2/4/8 Amp Charger Maintainer Desulfator & Lithium Charger Maintainer


https://www.batterystuff.com/battery...-2365-lcd.html
Battery Saver 6v 12v 50 Watt (4.17A) Maintainer, Pulse Cleaner & Tester - 2365-LCD

Both are approved to charge two batteries at once and I have been doing this for a couple years now without issue. I mostly use the Battery Minder.

[430Scud]

I did some testing during the pandemic in 2020 when my car sat a lot and tested both batteries out to about one month. I noted the main battery will lose charge at a pretty quick rate, where the aux battery did not.

I like the idea of maintaining both batteries, but the main battery in the trunk is the one that loses the charge fastest. I would be concerned if the car would start much beyond 5 or 6 weeks of sitting.

I load tested both batteries recently and the main battery, rated at 950 CCA was testing at just over 1100 CCA, and the aux battery rated at 660 CCA is testing at 683 CCA.
These are the original factory BMW batteries been in my car since May 2018 when it was produced, so 6 years old.

[G30M]

Thanks, I've charged both. Glad to know I haven't overly neglected my aux battery by only charging the trunk one so these time.

Thanks guys amazing.