1. Check the label first
Look inside the battery compartment or on the torch body for the size and voltage. The wrong cell can make a torch dim, flicker or not switch on at all.
2. AA and AAA
The easiest choice. Alkaline AAs suit occasional use, while lithium AA cells last longer and cope better with the cold.
3. CR123A and 18650
Some high-output torches take a CR123A or a rechargeable 18650 cell. Use only the cell type the maker specifies, and buy protected cells for 18650s.
4. Empty it when stored
If a torch goes unused for months, take the batteries out. An old cell left in a torch can leak and damage the switch and contacts.