That might be a bit high voltage for you 55w HID ballast. I have a couple typical cheap 55 watters. They run fine on 3S 18650's and even better on 4S. Haven't dared go beyond that though.
That rc pack is a 6s pack with no low voltage protection. It you have space for it you can get low voltage alarms that plug into the balance tap ( or main leads ).Radio Control Planes, Drones, Cars, FPV, Quadcopters and more - Hobbyking_123085_Batteries_Accessories-Battery_Voltage_Indicators.html
I think the ballast has input voltage rated at 12-24V, need to check again. This is why I think 22.2V should be fine?
ImA4Wheelr, do you use protected battery carrier like below for your 55W spotlights?
pinkpanda3310, thanks for the link. Is the low voltage alarm connected in series with the batterypack?
Thanks for the input guys.
Fully charged 6s is 25.2v of course voltage sag may save the ballast from popping.