As long as measured voltage (right after charging with some external/universal battery charger) is way belong 4.35V (that should be equal or very close to 4.20V) your Modem card should survive without any damage to it, and proceed to work as expected, as untouched from the fully charged High Voltage battery (LiHv), the one (BPP-L503) that Librem 5 uses to power this Linux phone when inserted into it.
I’ve learned what very important to be understood, and therefore thankful, from this post: