1. What alkaline means
Standard AA, AAA, C, D and 9-volt batteries are alkaline, and they’re designed to be used once. Their chemistry doesn’t reverse cleanly when charged.
2. What can happen
Gas builds up inside the case while you charge it. The battery can leak corrosive fluid, swell, get hot or, in rare cases, burst.
3. “Rechargeable” means a different type
Batteries marked rechargeable are a different chemistry, such as NiMH or lithium. Only charge a battery that says it can be recharged, and only in a charger made for that type.
4. If you charged one by mistake
Take it out of the device, let it cool and check it for swelling or leaks. Don’t use it again. Tape the ends and recycle it.