To decompress the spring all you need to do is shoot the gun 2 or 3 times in semi mode.
When firing in automatic mode the spring may be in a half compressed stage (ready to shoot another BB, just needs the spring to be let go) when you're done for the day. By decompressing your spring you will extend the life of the spring and should not see a decrease in FPS (sometimes if a spring is left compressed for storage it will lose FPS, you should have learnt why in Gr 11 physics).
A tip, apparently a lot of people leave RC cars after a little while (when they realize it's not for them and have dropped hundreds or thousands into RC cars) so sourcing a used charger from a RC forum or other such place might be the cheapest. Otherwise local RC shops should have what you need. The chargers that we use are the same. As far as the batteries go that's kind of where it ends since the voltages and configurations are different as Coach has already said above (you'll need to fit your battery pack in a PEQ or have it rear wired to a full stock instead of how they do it in RC cars).
EDIT: Batteries can easily be found from cheapbatterypacks.com
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE