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My DIY design can go together in less than an hour.
With your terrain, I doubt the pack will out last the car. Hills are murder on the IMA system.
I'd keep the original as a spare.
The grid charge/deep discharge/charge process essentially "refurbishes" the pack with results similar to pulling all the sticks, cycling them and then reinstalling. However, you can do it in a long weekend with a couple hours of bench time vs. taking weeks and many hours of bench time at the stick level.
Personally, I'd keep both packs and swap them out every six months for grid charging.
Many people attempt to piggy back Lithium packs for your stated purpose. You could do it in parallel, but the potential complications in service could be problematic. It would take a lot of know-how to do right - way beyond my expertise. You could splice in all the taps, thermistors and main terminals to put the pack in parallel, but the danger would be if cells fail.
In series, the only issue is the voltage drop of the failed cell. Packs in parallel will transfer current back and forth as needed depending on their relative voltage. If one pack has a failed cell, the other pack will discharge into the pack with the failed cell until their voltages are equal. Given the very low internal resistance of the cells, this current could be pretty high. Not explosively so, but potentially damaging if not actively cooled.
Personally, as much as I like the concept, I wouldn't pursue it.
Steve
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