Full Answer
Yes, and this is the single cheapest piece of insurance in the entire process.
Here is why it matters. During a flash the car sits with the ignition on and the engine off. Every electrical system is drawing current and the alternator is not replacing any of it, because the engine is not running. Meanwhile the ECU is being reprogrammed, and that is precisely the moment when a voltage drop causes real problems.
The guidance from ATLAS is direct: the vehicle should have ample battery capacity to supply the electrical system for at least 30 minutes with headlights and auxiliary components turned off, and ideally it should be connected to a high-amperage battery charger supplying current to the battery itself.
Note the distinction between a charger and a tender. A trickle tender maintains a charge on a parked car over weeks; it supplies very little current. A proper charger supplies real amperage and can keep up with the car's draw during a flash. For flashing, you want the charger.
What happens if you skip it. Losing power to the ECU mid-flash is one of the two leading causes of an incomplete flash, alongside disconnecting the OBD adapter. On Subaru WRX models from 2015 onward the ECU bootloader will detect the incomplete programming and enter recovery mode, where the car will not run until a valid upload completes. That is recoverable but it is an afternoon you did not plan for.
And for VB owners specifically there is a sharper reason to care: on ATLAS 2024.4 and earlier, recovery is not possible on 2023+ WRX manual ECUs. Running a current version matters, and so does not putting yourself in that situation in the first place.
Practical checklist before any flash: charger connected, headlights and climate control off, nothing charging from the USB ports, doors closed so interior lights are not running, laptop plugged in or fully charged, and nothing on the laptop that might pop up or put it to sleep. Then start the flash and leave it alone.
Last updated 8/20/2026