1. Where the myth comes from
Older nickel-cadmium (NiCd) cells could “remember” a partial charge and lose capacity if they were recharged after only a little use. That is the memory effect, and it’s why the advice was given.
2. Modern NiMH cells don’t need it
NiMH cells have very little memory effect. Topping them up when they’re low is fine, and running them flat every time can shorten their life.
3. Lithium-ion is different
Lithium-ion cells, such as the ones in phones and rechargeable coin cells, don’t like being run to empty. Charging them regularly and avoiding long spells at 0% keeps them healthier.
4. Store them partly charged
If you’re putting lithium-ion cells away for months, charge them to about half, not full, and check them every few months. NiMH cells can be stored charged, but they still lose charge slowly.