Raspberry Pi 400 Design Change Puts an End to Battery Power GPIO Add-On Boards

A design shift to prevent back-powering means that the Raspberry Pi 400 is the first model not able to be powered via its GPIO header.

Engineer Patrick Van Oosterwijck is looking to become the first to develop an uninterruptible power supply (UPS) specifically for the new Raspberry Pi 400 single-board computer, but has run into a few hiccups along the way — including a design change from the Raspberry Pi 4 family that makes powering the device from a battery somewhat more of a challenge than in its predecessors.

"Physically this will be a cartridge that plugs into the GPIO port on the back of the Pi 400, with a duplicate GPIO header coming out the other side so it is still available for other uses," Van Oosterwijck wrote of his original plan for the project, building upon an early Raspberry Pi UPS. "External power will be connected to the existing power input of the Pi 400, so this will only be dealing with 5V. Not dealing with higher voltage input and things like solar will simplify this a lot. The system will boot when external power is connected to the Pi 400, and if external power is removed, a clean shutdown will be triggered after which power will be removed. Or the user may have the option to keep running from battery power."

Unfortunately, it turns out that designing a UPS for the Raspberry Pi 400 isn't as easy as for earlier models of the popular single-board computer range — and not because it's encased in a plastic keyboard shell. "I found out today that the Pi 400 seems to have a 1A load switch on the 5V pins going to the GPIOs," Van Oosterwijck explains.

"It pretty much means that 'running the Pi 400 from a battery' is pretty much out. Working as a UPS may still be salvageable. Only testing will tell. This topology may need to morph back into the original 'simple UPS for a normal RPi' if it doesn't work for the Pi 400. The Pi 400 may need a different approach, involving only the USB Type-C power input."

The design change in the GPIO header was confirmed on the Raspberry Pi forum, with principal engineer James Adams explaining that "powering via GPIO is not possible on the Pi 400 due to stuff in there to stop nasty back powering issues happening." As a result, any battery power projects designed without the need to physically modify the Raspberry Pi 400's PCB will have to connect to its USB Type-C power input rather than the GPIO header.

Van Oosterwijck has not yet confirmed whether he plans to retool his project back to concentrate on the other models in the Raspberry Pi 4 family or scrap it and start again with a design focused exclusively on the Raspberry Pi 400. Further updates will be posted to the project's page.

Gareth Halfacree
Freelance journalist, technical author, hacker, tinkerer, erstwhile sysadmin. For hire: freelance@halfacree.co.uk.
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