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Dew heaters are powered off 12V Optima Red Top absorbed gas mat (but Blue top would be better...I got a deal on the red top).
Commercial dew heaters are sized so that with full 12V they would be way too hot, and thus require a controller, the idea being that by controlling the power down you preserve your battery (and heat). I figure a car battery could power a 3 watt eyepiece heater about 200 hours, or three such for 66 hours. So I saved the money on the controller (or the time building one) and sized the resistors to provide a nice low level of warmth at full 12V.
Plan to use 10 1/2 watt resistors for the eyepiece heater and 5 for the Telrad....makes the math easy and they sell 'em at Radio Shack in packages of 5 for about a buck.
What I'm using is 560 ohm x 10 for the eyepiece heater and 680 ohm x 5 for the Telrad. There's plenty of info online, but here's the basic math so you don't have to go searching:
Total resistance of resistors in parallel is one resistor / # of resistors
Watts = (volts x volts) / Ohms
Ohms = (volts x volts) /Watts
Battery drain:
Amps = Watts / Volts
560 ohm x 10 for eyepiece heater = 2.57 watts (.2 amps)
680 ohm x 5 for Telrad = 1.05 watt (.08 amps)
so my 50 AH battery oughta last 238 hours. That oughta make it through the night :-)
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