1. Same voltage, bigger case
C and D batteries are 1.5 volts, like the AA. The extra energy comes from their size: a larger case holds more chemistry.
2. How big they are
A C is about 50 mm long and 26 mm wide. A D is about 61.5 mm long and 34 mm wide. The size is usually printed inside the battery compartment.
3. Where they’re used
Large torches, lanterns, radios, older stereos and some toys. They last a long time in devices that only draw power now and then.
4. Alkaline or rechargeable
Rechargeable C and D cells are 1.2 volts, like rechargeable AAs, and suit devices you use every week. For occasional use, check how well a rechargeable holds its charge on the shelf before you choose it.
5. Don’t force the wrong size
A C cell in a D slot will rattle loose and make poor contact, and a loose cell can short out. Use the size printed on the compartment.