Originally Posted by rustyus

You only have a 60 watt charger.
How much power do you need? Amps x Volts = Watts
Want to charge a 4S pack (16.8 Volts max) at 7 Amps max? Need a charger with at least 118 watts.
7A x 16.8V = 118 Watts

I'm no math wiz, and some people will use battery nominal voltage for figuring wattage.
2C charge rate for your 4S 6500mAh pack? You would need a charger with at least 218 watts.
13Amps x 16.8Volts = 218.4 Watts