Hackberry-Pi_Zero

With DIY effort, Blackberry keyboard fans can attach a BB Q10 keyboard to modern phones, https://liliputing.com/fairberry-lets-you-attach-a-blackberr... & https://github.com/Dakkaron/Fairberry There's also the HackBerry cyberdeck with Pi Zero 2W and original BB keyboard, https://news.ycombinator.com/item?id=41138701 & https://github.com/ZitaoTech/Hackberry-Pi_Zero

This looks like a pretty cool device! However, I was immediately curious about how the "dual battery" feature works. The IP5306 power-management IC seems to be designed only for a single battery, and as far as I can tell from the schematic[1], the two battery connectors are just directly connected to each other in parallel (across VBAT and GND). This seems really sketchy. If you plug in two batteries that are not at the same state-of-charge, then you're going to get a very large current flowing from the higher-voltage battery to the lower-voltage one, probably significantly exceeding the batteries' rated current limits. At best this wastes a lot of power and generates a lot of heat, and at worst it could be a fire hazard. [1]: https://github.com/ZitaoTech/Hackberry-Pi_Zero/blob/main/Sch...