Today I tested … external charging.
The charger that I bought is: https://www.amazon.com.au/Onite-Universal-Battery-Charger-High-Voltage/dp/B005REEWYY
It has universal AC voltage, independently adjustable positive and negative pin position, auto-sensing polarity, and adjustable battery width. So, mostly idiot-proof.
It seemed to work with the battery for the Librem 5, although charging is quite slow.
Like the phone charger itself, it has pins for a US AC socket. A good thing I bought a 2-pack of US-to-Australian AC plug adapters. 