It's not clear to me where the mistake is. Almost everything uses Li-ion batteries with nominal voltage of 3.7V, so if it's mAh, 3.7V is assumed. else if Wh, im

It's not clear to me where the mistake is. Almost everything uses Li-ion batteries with nominal voltage of 3.7V, so if it's mAh, 3.7V is assumed. else if Wh, implies raw_terminal_voltage != 3.7. mAh ratings normalized for 3.7V are comparable, and Wh ratings are comparable too. It's also probably not wise to connect mAh rated battery to Wh rated ones(if it ever could be). In electrical design, electrical current(A) matters to trace widths and wire thickness selection; voltage(V) matters, separately, in matching supported voltage ranges, and also insulation for components. Total wattage(W) do not tell either of the story. It will concern me if a single strand of 32AWG wire was supposed to carry 10kA, or two studs 1.27mm apart would be energized to 10kVDC, but same for 10W of power at unknown current or voltage... I won't be so sure. mAh is fine. So long you don't mix different techs. Like standard alkaline, NiMH, and Li-ion, in a same bucket. Which you don't anyway.