[CRBWS6]

Hello, 1st post after recently acquiring a CPO M340i, 12/2019 build. As a long time E46M3 owner I am a little overwhelmed with the electronic controls.

This vehicle has the AGM trunk main battery and the Li-ion ASS battery under the hood.

So my first issue is a flat battery. Initial thoughts are the AGM is likely dead due to age and unknown charging care by the original owner.

I have had to jump start the car with a portable jump starter twice now.

In the short time I've owned the car I have used a Battery Tender on the jump start terminals a couple of times.

Current status (multi-meter check):

Under-hood jump terminals: 11.8v
Li-ion terminals: 11.8v
AGM terminals: 11.8v

Questions:

1. Is the consistent voltage readings due to a parallel connection of the two batteries and this an average reading?

2. Is it common to have to replace both batteries at the same time, or does the AGM usually go first? I've read that the Li-ion last longer.

3. I have read that warranty/CPO doesn't cover batteries?

My next step was to disconnect the AGM and check voltage directly but I wanted to avoid a full power down until I had more understanding of the system.

Thanks.

[Seaford]

Originally Posted by CRBWS6

Hello, 1st post after recently acquiring a CPO M340i, 12/2019 build. As a long time E46M3 owner I am a little overwhelmed with the electronic controls.

This vehicle has the AGM trunk main battery and the Li-ion ASS battery under the hood.

So my first issue is a flat battery. Initial thoughts are the AGM is likely dead due to age and unknown charging care by the original owner.

I have had to jump start the car with a portable jump starter twice now.

In the short time I've owned the car I have used a Battery Tender on the jump start terminals a couple of times.

Current status (multi-meter check):

Under-hood jump terminals: 11.8v
Li-ion terminals: 11.8v
AGM terminals: 11.8v

Questions:

1. Is the consistent voltage readings due to a parallel connection of the two batteries and this an average reading?

2. Is it common to have to replace both batteries at the same time, or does the AGM usually go first? I've read that the Li-ion last longer.

3. I have read that warranty/CPO doesn't cover batteries?

My next step was to disconnect the AGM and check voltage directly but I wanted to avoid a full power down until I had more understanding of the system.

Thanks.

I am far from expert but 11.8v suggests the grim reaper has paid the 12v battery a visit. Presumably you have tried to revive with your battery conditioner?

If I were you I would pop into a dealer…they can look at the battery(ies) condition on their desktop PC by scanning your key fob.

[HighlandPete]

Originally Posted by Seaford

I am far from expert but 11.8v suggests the grim reaper has paid the 12v battery a visit. Presumably you have tried to revive with your battery conditioner?

If I were you I would pop into a dealer…they can look at the battery(ies) condition on their desktop PC by scanning your key fob.

Agree...

One other thought, is whether the 11.8 Volt reading is with the vehicle locked and electronics asleep, (definitely an issue), or car unlocked and services all active. We may then read something like 11.8 - 12.0 Volts if the battery is only partially charged.

Drop off can be 0.4 - 0.5 volts when unlocked and vehicle is awake, ready to start.

[CRBWS6]

No dash or lighting elements on at the time of my measurements.

The current reading after disconnecting the Battery Tender Jr overnight is 11.1V. Hence, the AGM does appear to be beyond flat (11.6 is fully discharged).

Will a flat AGM battery drain the charge from the Li-ion battery?

The car is garaged & unlocked, but I am often fiddling around with it: with the hood and or trunk open, doors randomly opened/closed, etc. Is there a way to keep the car asleep to avoid future drains of the battery?

Also, from what I am reading, use of a Battery Tender Jr. on the main jumper terminals appears to be a safe practice?

When this battery situation is settled, it appears I need to develop better practices to ensure future drains are limited!

[CRBWS6]

Originally Posted by Seaford

I am far from expert but 11.8v suggests the grim reaper has paid the 12v battery a visit. Presumably you have tried to revive with your battery conditioner?

If I were you I would pop into a dealer…they can look at the battery(ies) condition on their desktop PC by scanning your key fob.

I was able to jump the car and go for a 45 min drive which appeared to charge the battery, however upon shut down the voltage dropped rapidly. The Battery Tender did not help even after a couple of days.

So to clarify - I can stop by a dealer with ONLY the key fob for a check? Interesting.

[Seaford]

Originally Posted by CRBWS6

I was able to jump the car and go for a 45 min drive which appeared to charge the battery, however upon shut down the voltage dropped rapidly. The Battery Tender did not help even after a couple of days.

So to clarify - I can stop by a dealer with ONLY the key fob for a check? Interesting.

Sorry, can’t answer the question.

A good point is made above about how much stuff is ‘on’ even when the engine isn’t running. But I still maintain even from my largely uneducated position that your 12v battery is shot. That in my 330i was replaced under warranty and I was pleasantly surprised at the invoice price for that.

[CRBWS6]

UPDATE: the AGM trunk battery was replaced free under warranty. The Li-ion aux battery under the hood is good.

The voltage reading at the jumper posts is very high now at 13.1V. This appears to be an average reading of the 2 batteries.

I asked the SA my questions above and as expected, he knew less about BMW's than me. The one useful bit of info he provided is that the vehicles are on a charger during service.

I will be more diligent at keeping this car on a Battery Tender to prevent discharging from going in/out of car while in the garage.