I couldn\'t quickly find the typical current ratings of different battery sizes. It will depend not only on the size but also the type (e.g. standard or alkaline) and even a bit on the brand. I think a good rule of thumb is to assume that the current which can be supplied before the battery voltage starts to drop appreciably is proportional to the cross-sectional area of the battery. Two C\'s more or less being comparable to a D sounds about right, but you can start by measuring them to guess a little better. Of course, you\'ll be able to see from your LEDs if you don\'t have enough batteries in parallel.
I guess you\'re trying to figure out cost effectiveness- whether you can do better with fewer Ds or more Cs. You probably want to get a nice little cheap digital multimeter so you can track the current-voltage relations for the different batteries. How long the battery will last is also important. If batteries of a given type have pretty much the same net energy density, you might just see which are cheapest per ounce.
Mike W.
Another consideration is the internal resistance of the battery. An alkaline battery has an internal resistance of 0.2 to 0.3 Ohms. Your current requirement, 240 x 30 mA = 7.2 Amp will cause a voltage drop of over a Volt, enough to seriously dim the LED\'s. NiCad\'s have only about 0.075 Ohms internal resistance and should perform better. Check out the Wikipedia site: http://en.wikipedia.org/wiki/Internal_resistance
LeeH
(published on 05/16/2013)
