My KX3 no longer has internal batteries. I think it will stay that way, given the reason it no longer has them. Safer that way. While they could be charged without opening the case…
My back-up is a QMX (mid-band), but so far I’ve never had to use it.
I have an FTX-1 but I doubt I’ll ever seperate it from the Optima, so the battery it came with will likely never see use. The battery alone weighs 400 grams, which is more than one of my 3x18650 packs plus my QMX, and only 70 grams less than my 2 metres transverter (for the KX3) and an extra battery pack.