1. Voltage: 1.2 volts, not 1.5
A NiMH AA gives about 1.2 volts, against 1.5 for alkaline. Most devices run happily on it, but the lower voltage can make some show a low-battery warning sooner.
2. Some devices are fussy
Smoke alarms, some clocks and certain toys check the voltage closely. Read the manual before you use rechargeables in anything safety-related, such as a smoke alarm.
3. Low self-discharge types hold their charge
Standard rechargeables lose charge within weeks of sitting on a shelf. Low self-discharge (LSD) NiMH cells keep most of their charge for months, so they suit remotes and torches you use only now and then.
4. Use a proper charger
Choose a charger that switches off on its own when the cells are full. Don’t leave cells on a basic charger for days, and never charge a cell that is damaged, hot or swollen.
5. Don’t mix cells
Don’t mix old and new cells, or rechargeable and alkaline cells, in the same device. Cells at different charge levels can leak or wear out the device.