When you keep your EV plugged in, you expect it to be ready to go when it is charged, right? Unfortunately, that is not the case for some owners whose Ford Mustang Mach-Es were built on or before February 3, 2021. As reported by The Verge, the Mach-E’s software does not allow the SUV’s onboard 12-volt battery to be charged while the bigger, primary drive battery pack is being charged. Wait, huh?

Yep—even some EVs have onboard 12-volt batteries for starting and powering accessories when the car is off, just like conventional internal-combustion vehicles do. This means there is a remote possibility that, if that 12-volt battery dies, an electric vehicle cannot be started—again, just like a gas-fed automobile—because that smaller battery handles the circuitry for powering the vehicle up, unlocking the doors, and so on.

Such is the case for the Ford Mustang Mach-E, though the problem is exaggerated by the buggy factory software, which isn’t topping off the 12-volt properly while the SUV is plugged in and charging its larger lithium-ion battery pack. Adding to this inconvenience, an over-the-air software update will not be ready until “later this year,” so affected Mach-Es will need to be taken to an EV-certified Ford dealer for a software fix.

Here is how the relationship between the 12-volt battery and the primary high-voltage lithium-ion battery pack that mainly powers the electric drive motors is supposed to work:

Without an alternator that charges the 12-volt when the engine is running (because, well, no engine), an EV will direct energy from the big battery pack to the 12-volt. The wonderful thing is, this can happen at all times, even when the EV is not running, as long as there is enough juice in the big pack. (The control module would stop this process to prevent the high-voltage reaching too low a state of charge and possibly sustaining damage as a result.) So, while you should drive or start up a gas-fed car at least once or twice a month to keep the 12-volt battery charged, in an EV, the 12-volt can theoretically help itself to energy from the high-voltage pack whenever it needs to.

Affected Mustang Mach-Es, according to the NHTSA service bulletin Ford initiated, the powertrain control module (PCM)’s programming is preventing the 12-volt battery from sipping energy from the big pack while the high-voltage battery itself is sipping energy from the grid. In simpler terms: Even when plugged in and charging up its big battery, the Mach-E isn’t keeping the 12-volt topped off.

If or when the 12-volt battery discharges, it lacks the juice to power up the electronics to turn the car on. Thus, owners are left with Mach-Es in “deep sleep” mode. Before you ask, yes, it is possible to jump-start an EV’s 12-volt battery—just like in your gas-fed ride!—but since the Mach-E’s 12-volt battery is hidden within the front trunk, the process is tedious. You first must jump the electric front trunk (frunk) release just to gain access; then you can jump the 12-volt.

A simple hardware change could have also avoided this outcome—Hyundai, for example, includes a “12V BATT RESET” button on its Ioniq EV hatchback’s dashboard that delivers an on-demand jump start to the 12-volt battery, a handy feature to have in case the onboard computer takes a dive.

In any event, while inconvenient, the Mach-E’s little charging problem belies the automaker’s noble push for data security. It is a good sign that Ford understands the importance of cybersecurity isn’t rushing to deploy the Mach-E’s OTA update feature until it’s ready for prime time, even in the face of a prime use case for those updates.