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Battery Box Heater

Battery Box Heater
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This 10 x 24 inch heating pad features a 3M adhesive backing and a 300 ohm etched foil flat heating element in a silicon rubber mat.  We've had it specially designed for heating battery boxes.

The heater features an internal thermocouple that allows heating up to the 25C/77F cutoff.  It will restart when the temperature falls to This thermocouple is rated for 250volts maximum and will fail at voltages above that.  However, it fails ON and can be used on higher voltage systems with an external temperature controller.

This heating element is 300 ohms.  Heating in watts is a function of the voltage and current through the device.

With a 100vdc pack voltage applied this element puts out 33 watts - 100/300=0.333 amps. 0.333 amps X 100vdc = 33 watts
With a 150vdc pack voltage applied this element puts out 75 watts - 150/300=0.5 amps. 0.5 amps x 150vdc = 75 watts
With a 200vdc pack voltage applied this element puts out 133 watts - 200/300=0.666 amps. 0.666 amps x 200 vdc = 133 watts
With a 250 vdc pack voltage applied this element puts out 207 watts - 250/300=0.83 amps. 0.83 amps  x 250vdc = 207 watts
With a 300vdc pack voltage applied this element puts out 300 watts - 300/300=1.0 amps. 1.0 amps x 300vdc = 300 watts
With a 350vdc pack voltage applied this element puts out 408watts - 350/300=1.166 amps. 1.166 amps x 350vdc = 408 watts

If that's getting a bit hot for you, string them two in series at 350vdc and have 204 watts each.

The batteries cannot be charged below 0C or 32F.  And their performance is poor between 32 and 50F.  And so we've designed this to heat up fairly smartly at temperatuers below 75 degrees.  But as soon as the heating pad gets to 75, the thermocouple cuts the low current level and the pad begins to cool.  Once down to about 59F, it will come on again.  In this way cycling between 59 and 77 degrees, gently warming the battery box.

By mounting the pad on the aluminum battery box, it is heating a large thermal mass slowly and in heating "pulses".  This minimizes the dangers of resitive heating and slowly warms and maintains a battery box temperature up to 77 degrees.   In this way, even parked for eight or ten hours in subfreezing temperatures, your battery temperature should be maintained at an ideal temperature for LiFePo4 battery cell operation.

The etched foil is a very flat resistive element, preventing damage when weight is applied.  These can be applied to the inside wall or bottom of your battery box.   Recommend a false metal bottom between the pads and the batteries to help distribute the heat.
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Price: $95.00

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