Most rechargeable batteries are not killed by use. They are killed by heat, by being left flat, or by being left full in a hot place for months.
What actually wears a battery out
Two things age a cell: cycles and time. Every full charge and discharge uses up a little of its capacity, and the cell also degrades slowly even when idle.
Heat accelerates both. A laptop battery kept at 40 degrees loses capacity far faster than the same battery kept at 20 degrees. That single fact explains most real-world differences in battery life.
Lithium-ion: the 20 to 80 percent habit
Lithium cells are happiest in the middle of their range. Sitting at 100 percent for weeks, or at 0 percent for days, is what causes permanent damage.
In daily use, just charge what you need. For a device you store, leave it around half charged and top it up every few months. Avoid charging below freezing, and use a charger designed for the cell rather than a generic supply.
Fast charging is not automatically harmful, but it generates heat. A well-designed device manages this. A cheap unbranded charger may not.
NiMH: charge fully, store charged
NiMH cells prefer a full charge and a full discharge now and then. Leaving them flat for months can permanently reduce capacity, so charge them before putting them away.
Low-self-discharge types hold their charge for up to a year. Standard cells may be half flat within weeks, which is fine if you have a charger and annoying if you do not.
The memory effect is mostly a myth
True memory effect was a real but narrow problem in old nickel-cadmium cells used in very repetitive patterns. It is not a practical issue for modern NiMH or lithium-ion.
What people call memory effect is usually voltage sag or a device's battery gauge misreading capacity. Running the device flat once and charging fully will often reset the estimate.
Chargers, cables and matching
A smart charger that charges cells individually is better than one that pairs them, because it will not overcharge the stronger cell while waiting for the weaker. Charge cells of the same type, age and capacity together.
For USB devices, the charger does not push current into the battery directly; the device manages it. The cable still matters, because a poor cable wastes energy as heat and can throttle charging.
When to replace
Replace a rechargeable when it no longer holds a useful share of its original runtime, when it gets notably hot during charging, or when the casing is damaged. For lithium packs, swelling means stop using it immediately and dispose of it properly.
The short version
- Heat is the biggest cause of premature failure.
- Store lithium-ion around half charged, NiMH fully charged.
- Memory effect is not a real concern for modern cells.
- A swollen lithium pack is a disposal job, not a repair job.