How to determine voltage rating for incandescent bulbs?


Newly Enlightened
Oct 13, 2021
Virginia, USA
I have some old flashlights that take screw mount bulbs, E10 I think. How does one determine what voltage rating bulb to use? I have a couple that allegedly have their original bulbs in them. The 2-cell ones are rated at 2.5v. The 3-cell ones are rated at 3.5v or 3.8v. This confused me. I couldn't find a way to make the math work. Two cells (alkaline as these were designed to use undoubtedly) would be 3.0v nominally. Three cells would be 4.5v nominally. So there is no rhyme or reason. If you go by number of volts, one is 0.5v lower and one is 0.7/1.0v lower. If you go by percentage, one is 17% lower and the other is about 22% to about 16% lower. No pattern.

So, is there a general rule? How do you chose replacements if all you know is the nominal voltage of the batteries to be used?

And what about choosing LED replacements? Does the voltage rule (if there is one) hold true?

My old European hand held lanterns/flashlights come in two types. One type uses the 3R12 battery (3-cells). The other type uses two "C" cells or two "D" cells. How do I chose replacement incandescent bulbs or LED bulbs? I want to get the right voltage and amperage ratings.

Thanks a million!


Flashlight Enthusiast
Sep 20, 2020
"...that famous Texas part of Hamburg"
I usually assume alkalines to be about 1,2v under load and choose the voltage accordingly. 2x1,2=2,4v. 3x1,2=3,6v. Some bulbs can be safely overdriven (like running a 6v halogen on 2x liions) but it depends on the bulb. If you want to be on the safe side stick to the rating.


Nov 23, 2007
Somerset UK
Agree, assume about 1.2 volts per cell. Ratings varied a bit but not by that much. Physically small cells supplying a large current relative to the size of the cell might be about 1.1 volts per cell. Note that lens end bulbs intended for a pair of AA or AAA cells were sometimes 2.2 volts.

Physically larger cells supplying a small current in relation to the size of the cell might be a slightly higher voltage. Consider for example the once popular 3.8 volt 0.3 amp bulb used with three cells each D size, or with an AD28 battery that contained three F cells.

Lights that used five cells were never very popular but did exist. By convention bulbs for five cells were often called 5.95 volts and not 6 volts. This was to avoid confusion between five cell flashlight bulbs and 6 volt or 6.3 volt pilot lamps or vehicle bulbs.