I was going to launch into a lecture about how battery isolation is a Thing, very simple, etc and the author couldn't possibly be dumb enough to not isolate them with diodes, but then I looked at the schematic and yiiiiiiiikes, they're both just tied together, I think. There's a charge management IC and a MOSFET as part of the USB power input...and that's it.
I was sketched out by BSI not being connected to anything, but it seems it's a fixed resistance battery size/chemistry ID function, not a temperature sensor. That said, I don't see any over/under temperature protection except for some sort of vague temperature limit in the charge IC (which is not thermally coupled to the battery in any way, so only a general "the device is way too hot"), so hopefully that's in the battery - don't go buying any cheapo clones.
I also don't see any fusing, which is a huge no-no. A polyfuse is a twenty cent (ish) part. Again, an official battery would have an internal BMS circuit and prevent overcurrent events, but people are probably going to go for the almost-cheapest battery on Amazon/Ebay. Not to mention counterfeit problems even if you do try to get an OEM battery.
I was sketched out by BSI not being connected to anything, but it seems it's a fixed resistance battery size/chemistry ID function, not a temperature sensor. That said, I don't see any over/under temperature protection except for some sort of vague temperature limit in the charge IC (which is not thermally coupled to the battery in any way, so only a general "the device is way too hot"), so hopefully that's in the battery - don't go buying any cheapo clones.
I also don't see any fusing, which is a huge no-no. A polyfuse is a twenty cent (ish) part. Again, an official battery would have an internal BMS circuit and prevent overcurrent events, but people are probably going to go for the almost-cheapest battery on Amazon/Ebay. Not to mention counterfeit problems even if you do try to get an OEM battery.