Portable Stand alone charger question

['Vette Nerd]

I want to make a portable stand alone charger for my Zcar, using its charging mount and connecting it to this cheap controller. (For one of those $1 wired up micros rcs you might see at the Dollar General.) The controller uses two double A batteries(3.0v) instead of the Zcar's controller three triple A batteries(4.5v), meaning I would have to charge the car for fifteen seconds longer(while holding down the forward and reverse buttons) than as if I used the Zcar's controller. Already figured out where everythin's gonna' go, but my biggest question is if this lack of 1.5volts would be bad for the Zcar's battery?