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Some of the jump packs have air compressor, light and invertors built in, mines not as heavy as a lead battery and useful for other things.
If it has an invertor built in you can use a cheap clock timer to turn it on and off. Don't need a battery charger either. You need to verify the watt draw of block heater to be sure the invertor has the capacity you need. You could always rig up an external invertor and use the 12v cable to power it instead of the 12v power outlet.
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