I wanted to ask a question before I rush into things. Would it work if I used a 150mah Nimh battery as my first battery and a stock NiCd as my second battery?
Yes, but you will notice that the car will run like a dual cell for a while, then when the 50mah runs down, it will run like a single cell car that's lugging an extra battery. You will also wear out the small one faster, and since you won't be completely discharging the 150mah, it may develop a memory and not hold a charge as long. I would try to match your cells if possible.
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