Electric "Heater In a Box" System

If you keep the discharges to less than 50% of capacity, and recharge promptly, several years is possible, although a couple of years is more likely. For this application, where the battery may have to sit at work partially discharged, a "Greensaver" battery might work best. These don't have sulfur in the electrolyte, and supposedly don't get sulfate buildup if they sit partially discharged. I have a broken electric motorscooter with five 38AH Greensavers in it, and may use one of them if the system works.

 

Marine deep discharge batteries are a compromise design: they can withstand deeper discharges than car starting batteries, but not as deep as real deep cycle batteries.

 

Charging and maintaining two lithium cells like that could be problematic. A lithium pack, with appropriate charger, would indeed be better than SLA, but also much more expensive and requiring more skill. I suggest that anyone who wants to go that route buy a pre-built pack, with charger.

 

Yes, I drive a 2013 Leaf SV. The PIP is leased by my housemate and she usually drives that.

 

I did the final "bench test" of the components of the prototype system this evening. The 800 watt inverter, which I hadn't tested with the heaters before, performed fine, powering both heaters smoothly and easily for close to ten minutes. The battery, however, isn't really up to more than testing, as I suspected. It's a roughly ten year old (I get some amazing lifetimes from lead-acid batteries!) 31AH deep cycle Dell battery, IIRC. I figure it has about 10AH of capacity left, now, so when the inverter started to complain (while still powering the heaters) after 8 minutes, I shut it all down. The battery read 12.14 volts immediately afterwards, but recovered on its own to 12.35 volts, which translates to about 45% capacity left. There was obviously some voltage "sag" but it wasn't excessive.

Conclusion, so far: the battery/inverter/ 200 watt x 2 heater setup seems sound. Based on extrapolation only, it looks like a 30AH deep cycle battery (easier to carry) should provide half an hour's heat with this version. A 60AH marine type battery should provide about 50 minutes' worth of heat. Both estimates are based on no more than 50% (marine) and 70% (deep cycle) discharge. I still think it's worth trying a "Greensaver" battery, because of its supposed lack of sulfation when sitting partially discharged (like while the car is at work) and its (also alleged) low internal resistance, which would allow it to provide more amperage with less loss and less heat inside the battery case.

The final test is, of course, with the heater "installed" inside a car. I may try the Prius PHEV first, because of space issues and the greater interest from the PIP folks, or I may use the Leaf if my housemate objects. In either case I should know how viable this idea is within a week or so.