I used for my 4px a 1450mah life pack, the manual recommend to stop using at 5.8 (2s) or 2.9 per cell, because life batteries after certain point drops insaly fast I think is after 3.0v per cell, the good thing is that you can abuse them a little bit more than a lipo but still not to much.

As for the car I used the same just because my servos are low voltage and the 6.6 are perfect I used the same 1450mah and usually it helps me get through practice,q1, 2 races of 12 min each and then change for the 25 min long one

I usually charge them until 3.39 each cell and had no problems.