Auxiliary battery drain is a fairly common issue on some electric vehicles, unfortunately, and the Toyota bZ4X – and likely its Subaru Solterra twin – may be a part of this club.
While having a bZ4X on loan for a review, our friend Kyle Conner from the Out of Spec Reviews YouTube channel was surprised to find the electric SUV's 12-volt battery completely dead less than a day after parking it.
The Toyota bZ4X's remote control key didn't work, so he had to use the physical key to unlock the doors, but the vehicle still wouldn't start because the 12-volt battery didn't have any juice left in it to power the auxiliary systems.
As Kyle puts it, it's unacceptable to see that much drain from a 12-volt battery, especially on a battery-electric vehicle that has a huge high-voltage battery that could charge up the auxiliary battery in no time.
Normally, the Toyota bZ4X should be able to detect 12-volt battery drain and wake up the drive battery so that it charges up the auxiliary battery. That didn't happen, though. It's worth noting that the drive battery had about 15-20 percent state of charge when Kyle parked it the day before, not low enough to kick off battery saving protections – that happens at 8-9 percent SoC.
In the interest of full disclosure, Kyle did admit he had left an OBD reader in the port, but he said it's a very low drag OBD reader that's very unlikely to drain the battery. He said he always keeps it connected to every car he reviews, and it has never drained a 12-volt battery before. However, some of the commenters pointed out that an OBD dongle may keep the car awake, so that's probably worth looking into.
Kyle's opinion is that Toyota needs to release a software update to fix this issue on bZ4X vehicles, especially since it doesn't seem to be an isolated issue. Three other owners of Toyota bZ4X and Subaru Solterra vehicles said they faced the same problem in the Twitter thread Kyle started, and there are many similar complaints in online forums.
Until Toyota comes up with a fix – we've contacted them to ask if they're working on it and we'll update this story when they reply – Kyle showcased a workaround that owners can use to jumpstart the 12-volt battery safely.
It's simple: turn the car on, put it in Neutral (with the parking brake on), buckle the driver's seat belt, and then walk away. This will stop the car from shutting off after an hour, allowing you to fully charge the 12-volt battery – for Kyle, a 4,000-amp jump starter did the trick.