Joe, a 7.71 reading for a 6-cell pack is too low, it must not have been fully charged.
I would assume you know that you must use the two packs in series to replace a 12v battery? If you run them
in parallel, it won't be enough voltage to drive the starter motor worth a darn.
I'd suggest you look at our TwoLane
, it can charge the two starter box packs at once,
or do both your receiver and xmitter batteries at once.
I would use NiMH batteries, you don't want to deal with NiCads having to be discharged fully before charging them.
There are many cheap NiMH packs available.