The Heat Pump Defrost cycle is probably the most complex operation in the heat pump, so we won’t get into it here.
There are several defrost systems: Time -Temperature; Time- Refer Pressure; Time - Air Pressure - Temperature; Time -Air Pressure - Reefer Pressure…
Something starts the cycle and something else stops the process.
Electric heat does not melt the ice. Heaters are downstream to the evap coil.
Heaters prevent a cold blast of air when in defrost. The heat to melt outdoor ice comes from the heat pump’s previous cycle.
There is less water, because there is less water vapor in cold air than when it is 40F outside. But it will always drip. As for ice build-up, if ice makes it up to the bottom coil, the unit will stay in defrost longer and melt this ice. It is more of an energy loss problem (not to raise the unit above the ice) than the ice on the coil. Contact with the hot outdoor coil and hot water running off the coil will melt more than it will freeze (unless the defrost cycle is not correct).
So if you see ice build-up into the OD Coil, there is a defrost problem.
If you have a low refrigerant charge, sensor out of calibration etc. this type of thing will result.
If you see stripes of ice on the OD Coil and they do not go away after a defrost, you have a low refrigerant charge (Very Low).