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From what I've read, the block heater is rated at 1000W
Typical inverters run 85% efficient, so to run 1000W heater load, it's drawing nearly 1200W from the battery. Inverters will shut off when battery voltage drops to about 10.6V which is the state just above what's considered dead. 1200W @ 10.6V = 113amps from the battery.
Standard lead acid batteries are designed for high current, intermittent use only like starting. If deeply discharged like running block heaters, they'll tend to sulfate and prematurely die irreversibly
Running a block heater is better served by using deep cycle batteries. The Optima yellow top batteries that fit our trucks are rated for ~50AH each. I'm reading one should not discharge down below 20% of capacity. I've also read starting takes 2-5% of battery charge, so my guess is a pair of these batteries will run a heater for about 40-45minutes max and maybe have enough juice left to run the glow plugs and starter to turn the engine.
Then the issue is running the engine long enough to charge them back up.
I'm wondering if it's feasible to simply run 12V via relay into the block heater instead. It's just a simple resistive element, so instead of getting 120VAC, it runs off 12VDC. It'll draw approx 1/10 the current and take longer to heat up of course.
'05 F150 FX4 SCrew
'10 F350 Lariat FX4 CC SRW LWB
Last edited by aquamans350; 11-10-2011 at 02:01 PM.