Longshadow Loc: Audubon, PA, United States

The "charge memory" is definitely true for NiCad batteries, not so much for LiIon batteries. However, I have heard that if the charge in a LiIion goes below a minimum value, they will shut down to prevent any problems. Then they will not charge (charger blinks "bad battery").

TriX Loc: Raleigh, NC

👍👍 You are correct, while NiCads have the memory effect, that’s old news as most modern batteries are Li-ion or NiMh. Li-Ion batteries typically have internal protection that will not allow you to recharge the battery if the voltage drops below a minimum set value - this is to prevent excessively high charging current for a completely flat battery and the attendant temperature rise (which can result in fire). If you neglect to recharge a LI-Ion and allow it to go completely flat, you might as well toss it unless you have the appropriate current limiting power supply, the resources to accurately measure charging current and voltage, and the skill set to know the procedure. If you have all the above, you may be able to “save” the battery, but since most people don’t, recharge Li-Ions when required and don’t let them sit for long periods and gradually discharge without recharging. Some chargers have a mode to recharge badly depleted batteries, but even so, it’s bad practice as even if you save it, the life may be shortened.

Longshadow Loc: Audubon, PA, United States

pmorin Loc: Huntington Beach, Palm Springs

I was assuming the op meant Ni-Mh or Ni-Cd as he was concerned with overheating. Sorry if I misunderstood but he did not say.
After some searching I read that L-Ion recently was found to have a tiny bit of memory effect also.
Probably the first study of its kind and will need some confirmation.
https://phys.org/news/2013-04-memory-effect-lithium-ion-batteries.html

Longshadow Loc: Audubon, PA, United States

I know. The odd were that it was a LiIon battery. BUT, I think my first two phones were NiCad.
But the memory effect in a LiIon is nowhere near as bad as a NiCad. NiCads were notorious for it.

TriX Loc: Raleigh, NC

You really have to be careful about letting Li-ions go under the safety cut-off voltage. We let a specialized device sit for months unused while it slowly went flat. Unfortunately, there were zero sources for a suitable replacement battery, even from Chinese manufacturers, and the manufacturer had a flat $200 fee to replace the battery. I had to disassemble the device and remove the battery, which I placed in a Pyrex dish (in case of fire), attached it to a variable current limiting power supply and while carefully monitoring both voltage and current (and reading up on the charging voltage and current curves for Li-ions), after charging for several hours was able to get the internal voltage to the point that the protection circuit would allow regular charging. It saved the battery, but I have no idea how the deep discharge affected it’s long-term life.