> It takes a 3V power supply, the operating current is 40 to 70mA,
> and the output power is 3mW.  However, I don't know how to work
> out (for example) what battery(/batteries) to use

You start with the milliamp-hour (mAh) or amp-hour (Ah) rating
of the battery. AAA batteries are a few 100 mAh, depending on
the type (red, black, alkaline, heavy duty etc), AA could be up to
1000mAh / 1Ah, C and D cells are into the Ah

So, if you have 2 x AAA batteries rated at say 400mAh each, then
you would be able to power your laser at 40mA for (400/40) hours,
oir 10 hours. I believe the rating is from fully charged (~1.65V) to
1.0V so you may see the laser output dropping off before the 10
hours. That's for batteries in series - add the voltage, the current
reserve is the mAh rating. For batteries in parallel the voltage is
the same as for one battery and the current reserve doubles. Two
AAAs in series - 3.3V at 400mAh, two in parallel - 1.65V at 800mAh

Note that different battery types have different terminal voltages
because of the different chemistries used. Alkalines (and most 'dry'
or carbon-zinc) are 1.65V, NiCd are 1.2V, then you have silver,
thionyl chloride, sealed lead, lithium, metal hydride - on and on

Grab a catalogue and/or look through some www battery sites

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