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Average battery output at 0 degrees F is below 50%. More like 40%. This is why it's so important to have your batts fully charged and in good condition at these temperatures.
Batt warmer's also do wonders as well...
+1. Block heater, plus ZeroStart warmers on both batteries, all wired to one of these: 3-way grounded short extension cord, so still just one plug sticking out the front. Total power draw aprx. 1120 W, plugged into a timer to run 2-3 hrs in the morning (aprx. $0.50 of electricity). For our cabin trips (off the grid), we got one of those cheapo Chinese eBay 3000 W "suitcase" generators.
Shell Rotella-T 5w30 synthetic also helps; easier to pump cold in the crankcase, and I'd bet a nickel it's easier on the HPOP at these temperatures.
