@capriciousduck. Will it boot in that state or is battery dead after showing 48%?
If dead then charge it up fully. You need to do at least a full charge cycle dead to 100% to calibrate battery meter.
@swarfendor437 is right old NiCd betteries had memory which meant full charge voltage reduced over time unless subjected to a full discharge now and again. Not many PC’s will have NiCd batteries these days but some small domestic appliances do.
My Lenovo (Windows) has an Energy Management app which allows you choice of best battery performance or best battery health. I use the latter setting which maintains charge at 60% on a.c. Same app allows you to perform a battery calibration cycle, then returns to chosen otimisation at the end, but not seen anything similar on Linux (yet).