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Originally Posted by cRiPpLe_rOoStEr
Messing with the battery sounds much easier than a CNG conversion, plus IIRC any alternate fuel system needs to be EPA-certified. That's why aftermarket CNG conversions might not be so popular there as they are in less-regulated markets.
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Ya. With CNG the tanks are expensive, take up a lot of room, and need to be changed (according to DOT regulations) every 10 to 15 years. Also to be able to use my home natural gas I'd need a way to compress it to 3,600psi. I've been looking for such a compressor and they are hard to find. Then there's integrating it into the engine control system. I could do a non-certified job and probably get away with it seeing how there's no emissions check where I live. Of course that could change some day.
Making a battery that falls into the same voltage range as the NiMH traction battery shouldn't be that hard. IIRC, NiMH cells range from 1.0V empty to 1.4V full. Three NiMH cells would be identical to a typical Li ion cell, going from 3.0 to 4.2V. The auxiliary battery could be connected in parallel and have some sort of BMS that could disconnect it in some extreme case or when not needed.
The only problem with this idea is that the hybrid system seems to want to keep the traction battery at around 70-80% charged, from what I gather on the dash board. Going up mountains it doesn't let the charge drop below what the dash indicates as around 50%. Driving my Leaf over these same passes I saw the batter SOC increase as much as 10 percent going down these hills. That indicates to me that I'd need to absorb as much as a 2kW of energy going down a typical mountain pass. But if the hybrid system keeps the battery voltage up near 75% SOC, I'd need 8kW of capacity to make it useful. Of course this is all just guessing. I'd need to track actual battery voltage and current before deciding on how an auxiliary battery should be constructed.
Of course it would be even better to have a system like Enginer's that has an inverter between the two batteries. That way you could chose how the energy in the auxiliary battery is to be used.