Are 2C or 2AA bulbs the same as 2D bulbs?

Are 2C or 2AA bulbs the same as 2D bulbs? AA, C and, D batteries are all 1.5 volts so shouldn't they use the same bulb?

Typically not.

Vacuum bulbs were 0.27A for 2AA (PR4), 0.3A (PR6) or 0.5A (PR2) for C, and 0.5A (PR2) or 0.7A (PR31) for D

Krypton bulbs are 0.47A for AA (KPR104), 0.52A for C (KPR101), 0.7A for D (KPR102).

The reason for the spread is partly runtime and partly the lower internal resistance of the bigger cells.

Put a low-current bulb in with big cells, and you sacrifice brightness for runtime.

Put a high-current bulb with small cells, and you achieve nothing except reduced runtime. The extra current will drag the cell voltage down so the extra brightness is not achieved.

I used to replace the standard krypton bulb in AA and C cell lights with a PR4 for great run time. The PR4 voltage is too low and the thin filament burns out too quick. The PR6 works better.

On 2D lights, I put in a .49A Xenon for good light with runtime.

also 2AA bulbs are usually rated at a lower voltage than 2C or 2D ones.

If you switch to NIMH cells it doesn't really matter, pick a bulb with the runtime you want. (lower amps = less brightness, more runtime, vice versa)

For alkaline cells, they tend to have more specific bulbs for specific applications due to the less than desirable performance of those cells.

Eric