Oh yeah, Mylar. Good thinking redpoint5...
But I just remembered, I was sent a large sample of Tyvek Reflex 'metalised' breathable membrane a while back. That is very tough, as lightweight as paper, electrically safe and completely waterproof. It is supposed to reject 88% of radiant heat, and it certainly works well as a sun-shade. I could cover the existing black plastic battery box with that stuff.
I was going to get an Odyssey starter/deep cycle battery, and the PC1220 is the one that will fit in my car, but I don't think there is a metal jacket available for that model. I'm kind of leaning away from spending megabucks on a really good replacement starter battery like the Odyssey when a supercapacitor array might do a better job and cost less.
Thanks for mentioning the glowplugs. As far as I know this car doesn't use the glowplugs at all above -5 degrees C, and I use an engine preheater anyway, so I would very rarely use the glowplugs. I didn't use them at all last winter as it was so mild. But when you need them, you need them!
I believe the glow plugs use 50A for a few seconds. If my DC/DC converters put out 25A, then after using glow plugs for five seconds, I'd need to wait another 5 seconds for the converter to have fully replaced the energy in the capacitors. But that wait may not even be necessary. The great thing about capacitors is that they still dump huge currents out even when their voltage has dropped. Even at 9v or 10v, a capacitor array can still start an engine. Also they are not affected by cold weather to the degree that lead acid batteries are.
Some people have combined a small, lightweight battery (LiFePo4?) with a small supercapacitor array to increase energy reserves. But in my case it might be cheaper and easier to simply add another DC/DC converter module to increase the current going to the capacitor array from the donor batteries in the rear.
An interesting article here about starting diesel locomotives with supercapacitors charged via DC/DC converters...
http://www.kemet.com/Lists/Technical...capacitors.pdf
And yes, using a DC/DC converter means the current is always safely limited despite the voltage being good, so no fat cables are required. With my single 12A converter I'm currently just using the existing wiring which feeds the 12v rear power outlet. That is fused at 30A. It would be an advantage to run a thicker wire, especially if I add a second converter, as I will be losing a few watts of energy in that thin wire, but it's only really necessary if I add a third converter module as I'll then be putting 36A through it.
I think I could just try the capacitor array with the glowplugs one winter morning by deliberately leaving the engine pre-heater switched off, and if the car doesn't start easily then I could add another DC/DC converter module at that point, but somehow I don't think it would be necessary.