first off.... if you dont even know basic wiring of batteries or know basics of electricy or even just know how your own razor works... you souldnt even be moding it......... im sorry but theses are really basic things.....
but anyway....
battereys on the razor are wired in series....
wich is + to - to + to -
on your pr200 if you follw the wires from the battereys one postive goes to the controller then the negitive of the same battery will lead to the 2nd batteries postive then the negitive to the controller.....
so you can add as many battereys as you want as long if there + to -
on a battery there are volts and amps..
volts = power
amps = life
so to get longer run times you need bigger amp batteries....
example:
now there are 2 12v 7ah batterys
you could get 2 12v 10ah batterys and you run time would be longer...
keep it safe and never mix batterys dates/amps/volts