Lithium Ion Battery? Or AGM … or both?

[rajmisc]

I noticed that there is a Li-ion battery under the hood on the passenger's side. (2021 750i … See pic). Dealer says the battery is AGM under the trunk and doesn't know why the Li-ion is under the hood.

If I use a trickle charger meant for AGM batteries, does it also try to charge the Li-ion?

[B-HQC]

The battery under the hood is an auxiliary battery for the headunit, telematics, etc. Not related to the engine / ignition and it's not on the same circuit.

[rajmisc]

Originally Posted by B-HQC

The battery under the hood is an auxiliary battery for the headunit, telematics, etc. Not related to the engine / ignition and it's not on the same circuit.

Awesome! Thanks for the info..

[Qsilver7]

BMW technical information about the auxiliary batteries. The Aux battery or batteries are used to power additional options and a PCU (power control unit) facilitates keeping the aux battery charged so that it is not left up to the car's main battery.

[Atavus]

Two batteries. Like having a Mercedes again.

[rajmisc]

Fantastic information ... thanks @[Qsilver7]! Nice to read about the design considerations that go into a widely thought 'simple battery stuff'!

[Qsilver7]

Fantastic information ... thanks Qsilver7!

[750LiBeemer]

Originally Posted by Qsilver7

So when placing a battery tender on the aux battery under the hood, you're not really charging the main battery in the trunk?

[Qsilver7]

Originally Posted by 750LiBeemer

So when placing a battery tender on the aux battery under the hood, {you're} not really charging the main battery in the trunk?

Correct. If using a battery tender to keep your main/starter battery charged...you should hook your connections to the dedicated positive & negative jump start locations under the hood. Remember...the aux battery circuit has a battery isolation switch.