1. What it looks like
A small cylinder about 10 mm wide and 28.5 mm long. It’s printed A23, but it’s also sold as 23A, MN21 or V23GA. All of these names mean the same battery.
2. Where it’s used
Wireless doorbells, car key remotes, some garage remotes and some alarm sensors. The code is usually printed inside the battery compartment.
3. It’s not a AAA
A thin AAA gives 1.5 volts. It won’t power a device built for 12 volts, and a 12-volt cell in a device built for 1.5 volts can damage it.
4. Buy fresh and keep spares apart
Check the date on the pack and buy from a shop that sells quickly. Keep spares in their packaging, away from coins and keys, so they can’t short out.