[Volasko]

I'm troubleshooting the following errors that I believe are all associated with the drivetrain fault that will not go away:

21A50A: Starting system, pinion starter: timeout occurred several times in succession
218219: Additional battery: aging detected
21A511: Starting system, pinion starter: timeout

I believe the aux battery error is causing issues engaging the starter for the ICE engine when at low charge and in EV mode. There seems to be a delay when the ICE switches on to fill the TQ demand from the driver. It feels like I hit a soft speed limit until the ICE kicks on and pulls me away. Given the Aux battery handles the base car systems while in EV, it makes sense that these errors could be associated. I've opened the trunk liners to gain access to both batteries and when the car is completely off, I measured both batteries with my multimeter and read ~13.6V on both. When I plug the car in both jump to 14.5v, so I assume everything is connected.

What I'm curious to know:
1) Do I need to disconnect the HV battery and isolate both 12v batteries from their connectors to get a real reading off each battery? Given the HV tenders the AUX batteries I would assume they're all interconnected.
Update: Disabling the HV safety switch, and disconnecting the batteries gave me different readings vs. taking measurements with everything wired.

2) Is there a way to isolate the HV battery before messing with any connectors? Is it just a matter of disconnecting the green clip near the aux battery? Is this recommended before messing with any 12v system?
Update: Yes the HV battery should be disabled with the green safety clip. The clip has a sequence to enable/disable which can be difficult watch the sequence here
3) Do aux batteries need registration after replacing? Can this be done with BimmerLink? It only seems like it defaults to the main 12V?
Update: Yes new batteries, main and aux need to be registered as a new battery. If you change any of battery spec details vs. the old battery you need to code these parameters in with coding. BimmerLink does NOT support aux battery registration

Additional Info:
- I've cleared the error codes multiple times and these 3 always come back
- I'm at 188k kms, HV battery is in good health and reports 84% health
- I have the 90Ah 900CCA AGM Varta main 12v and 60Ah 660CCA Aux battery

Steps to replace 12v batteries safetly on these hybrid cars was found in a shop manual I found online. To summarize here:

1 - Disable the HV battery with the green safety switch. Make sure before you disconnect this switch the car is disconnected from any charging cables and the car is fully off. Do no lock the car, but instead hold the stereo on/off button for 5-10 seconds to shutdown the car. Wait at least 10-20 seconds for all systems to shutdown.
2 - Starting with the aux battery, disconnect negative lead first, then positive.
3 - Disconnect the main battery, negative first then positive.
To reinstall repeat steps in opposite order. You will get some extra error codes that may need clearing after fully disabling all 12v systems.

[srybicki]

If they are originally I would just replace both. Had to do both on my 2018 530 in October. My had pretty much same issues
I would not waste time on diagnosis until replacment and clearing all codes. I bet nothing comes back. It's not cheap but they are past their life expectancy.

[Volasko]

Originally Posted by srybicki

If they are originally I would just replace both. Had to do both on my 2018 530 in October. My had pretty much same issues
I would not waste time on diagnosis until replacment and clearing all codes. I bet nothing comes back. It's not cheap but they are past their life expectancy.

Did you do this at the dealer? I can do the work myself, but registering the Aux battery with Bimmerlink doesn't list the aux battery.

[srybicki]

I had an euro shop outside of Orlando do it.

[Volasko]

Originally Posted by srybicki

If they are originally I would just replace both. Had to do both on my 2018 530 in October. My had pretty much same issues
I would not waste time on diagnosis until replacment and clearing all codes. I bet nothing comes back. It's not cheap but they are past their life expectancy.

The crazy thing is I've yet to see anyone actually diagnose this issue to see if the battery is actually bad. I'm all for fixing issues, but I need to prove to myself the issue actually the battery before blindly tossing cash at something while it may have usable life left.

[srybicki]

Take the battery out and have it tested. Small deviation in voltage will make the whole system act crazy.

[Volasko]

The aux battery was replaced Dec 2022 and once isolated from the electrical connectors read a 13.64V (this seems HIGH for a completely isolated battery). My main 12v which looks to be original was reading 12.64V fully isolated. The aux voltage of 13.64v FULLY disconnected seems super high. The main 12v battery seemed within spec, even though it is nearly 9years old. So now two questions I'm left with here:

- Even though the error codes show the aux battery is aging (its technically not, the main 12v is oldest) should I just swap the main 12v and see what happens?
- The readings on the aux battery seems very high. Given the age and voltage reading it seems like this battery is more than healthy or would a failed battery management module cause the high battery reading?

I left the car unplugged from the PHEV charger, and remeasured the batteries. The main 12v (H8) battery reads 12.19v and the aux battery reads 12.99v. I suspect the main 12v is gone (typical resting health voltages should be > 12.4v), and the error message doesn't know how to differentiate between the two? When I plug the car in, both batteries look to be tendered from the PHEV cable as both read 14.69v once plugged in. Given my car spends all night charging 99% of its life, if the main 12v was reading low and given its age, it must be replaced.

Lets see how much the CAD dealers are going to hose me for a new one? Update: The dealer H8 battery prices seem to be reasonable at $350ish CAD, which are actually cheaper than most aftermarket options.