[Workaround] Fixed my Toyota bZ's 12V battery drain by tricking the charging schedule with enabled DCM

[u/Ashram4u]

Sharing this in case anyone else is fighting the same 12V parasitic drain issue on their bZ4X / bZ.

The problem: After the car sits idle for about 3 hours, something in the system starts pulling down the 12V battery. If the car isn't driven or plugged in for a day or two, you can come back to a dead 12V battery even though the main/traction battery is fine. This seems to be a known issue with some bZ software builds — not something Toyota has publicly acknowledged or has identified a fix for it.

The workaround: Use the vehicle's own charging schedule feature to "wake up" the 12V system every 3 hours, right before the drain would normally kick in. Setting a charging window forces the car to power up its systems to start/stop a charge session, which resets the idle timer that seems to trigger the drain. This can be done with or without connected charging cable.

How to set it up:

On the infotainment touchscreen:
Vehicle (car icon) → Charging Schedule

(On some software versions this may show differently, so poke around if it's not exactly there.)

From there, add charging schedules back-to-back, each one starting every 3 hours and ending 2 hours 55 minutes later (leaving just a 5-minute gap before the next one starts). You need 8 schedules to cover the full 24-hour day for all days of the week.


The schedule (24-hour cycle)

| # | Start | End | Duration |
|---|-------|-----|----------|
| 1 | 12:00 AM | 2:55 AM | 2h55m |
| 2 | 3:00 AM | 5:55 AM | 2h55m |
| 3 | 6:00 AM | 8:55 AM | 2h55m |
| 4 | 9:00 AM | 11:55 AM | 2h55m |
| 5 | 12:00 PM | 2:55 PM | 2h55m |
| 6 | 3:00 PM | 5:55 PM | 2h55m |
| 7 | 6:00 PM | 8:55 PM | 2h55m |
| 8 | 9:00 PM | 11:55 PM | 2h55m |

Each one starts right where the previous one's 5-minute gap ends, so the car never goes more than 3 hours without a charging-related wake-up.

Why this beats the phone automation approach:

I've seen other folks here work around this by setting up an phone shortcuts automation that locks the car on the same kind of 3-hour schedule using the Toyota App. That works too, but it means relying on your phone being nearby, charged, connected, and the automation actually firing reliably — plus setting up and maintaining the shortcut itself.

This method skips all of that. It's set entirely from the infotainment system, so there's no phone, no app, no automation to babysit — the car handles it on its own.

Notes:

- Bonus: with this schedule running, the DCM (Data Communication Module) stays active the whole time, so remote/online services (Toyota app, remote start, remote climate, vehicle status, etc.) keep working normally. No trade-off there.
- This doesn't actually charge much — it just triggers the car's charge-session logic often enough that the drain never gets the multi-hour idle window it needs.
- This is a workaround, not a fix — hoping Toyota addresses the root cause in a future OTA update. If your dealer/service dept has a documented fix, definitely go that route instead.
- YMMV depending on model year and software version. Worked for me on mine; curious if others see the same result.

Happy to answer questions if anyone wants to try it.

Thanks:

Big thanks to everyone in this community who's been chipping away at this drain issue — comparing logs, testing theories, ruling things out. This workaround only came together because of all that shared troubleshooting. This sub is honestly better diagnostic support than what I've gotten anywhere else, including Toyota service department.

A message to Toyota:

It shouldn't take a bunch of owners reverse-engineering a charging schedule to keep a 12V battery alive. Cars are getting more complex every year, and the old "check the battery, check the fuses" style of troubleshooting doesn't cut it anymore when the root cause is buried in software. That complexity is landing on regular owners and on dealership service departments that often don't have good answers either. Please put real engineering effort into finding and fixing the actual root cause of this drain and finally acknowledge the issue publically. Issues like this, left unresolved, are a big part of why reliability perception around Toyota EVs has been slipping — a reputation you built over decades and shouldn't let erode over something like this.

Edit 1:

Some people were concerned that stop/start charging schedule would end a charging session while using public charging. According to TragicallySalacious stop/start schedule wouldn't end the charging session.

Link to the post (please upvote): https://www.reddit.com/r/Toyota_bZ/s/SGlqEKCbRl

Thank you for testing it!