Glad to see this thread resurrected
I've wanted to add a fuel burning heater for sometime - Unfortunately, our climate down here just doesn't justify the project
I would definitely want to keep the existing heater, so you can preheat in the garage . . . . and still have heat if your fuel burning heater runs dry. Putting the heaters in parallel by using two Tee fittings in the existing hoses will allow either one to run using it's own coolant pump.
I agree that relocating the big accessory battery would be the logical way to go - All of the plumbing you need to access is right there
I would go with a smaller, motorcycle AGM battery. The OEM accessory battery is 31 AH I think, but it doesn't 'start' anything . . . . other than the car's computer. I think you could replace it with something much smaller and still have it do everything needed
Several six cylinder motorcycles use the UTX24HL AGM battery which is only about 3 1/2 by 6 1/2 by 8 inches and it has 21 AH. I have a couple of these in motorcycles, but I haven't tried physically fitting one into the existing space in the rear, under the motor/inverter cover, which is where I think it would need to go. Possibly by relocating the brake vacuum pump or the coolant tank, the battery could be squeezed in there? There is definitely plenty of room for it behind the drive motor, in front of the rear axle, though putting it there would require welding up a mount and a couple of brackets for it. If the modification became popular enough, someone could make a dozen of the mounts and sell them for $75 or so and make a few bucks
If you decide where you'd like the battery to go, there are literally hundreds of motorcycle AGM's to choose from and it shouldn't be hard to find one to fit the constraints of where you'd like to put it. I would think anything with 15 AH or so should be more than adequate
I'll follow this thread closely - Just wish I could be the one doing the first one!
2012 iMiEV SE Premium, White
2012 iMiEV SE Premium, Raspberry Metallic
2012 iMiEV SE, White
2017 Chevy Volt Premier
2014 Ford Transit Connect XLT SWB wagon