As long as you are not stopping and starting charge frequently, or power is spiking, it shouldn’t be too much of an issue. Discharge is of course quite often stop start in a flashlight! Personally I think it is more sensible to stop and restart a charge if you have to leave the house, than leaving charging unattended whilst you are out. I usually try and plan my charging around times when I can keep an eye on things and charge without interruption.
It’s fine to stop and restart charging a Li-Ion cell at any stage. The CC / CV charging profile they use is very tolerant of that.
NiMH cells are more awkward.
You do not want to interrupt a NiMH charger that’s based on a timer, because the timer will start from the beginning again and overcharge your cells.
It’s also best not to interrupt a smart NiMH charger that uses -dV/dt termination, because you may interrupt it just before the NiMH cells generate the -dV/dt signal, which means the charger will miss termination and overcharge your cells. It’s fine if the cells still have a while to go before they’re charged, but not a good idea if they’re nearly finished. The problem with that is that it can be hard to judge whether you’ll get away with it or not.
On the other hand, if your NiMH charger uses simple voltage termination, you should be fine, because the charger will stop when the cell hits that voltage, whether it was interrupted along the way or not.
These considerations are a large part of the reason that Li-Ion based systems are preferred to NiMH based ones when it comes to solar charging, because solar power can be very intermittent.
Hey mate it should be okay if you dont do it often. Lithium cells need to be finished a certain way at the end of a charge cycle. Basically you need to lower the current slowly while ending the charge. Not doing this will add wear and tear to the cell.
There can be a bordering case if you turn charging on/off many times each day, because you will be charging a little bit on a “full” cell. I do not know if it leads to any extra degradation of the cell.
A few chargers get around that with hysteresis, i.e. as long as the voltage is above a specific limit (like 4.1V or 3.9V) they will not resume charging, but most chargers will resume charging.