Yes, i have..
On a minimoto with Big Block motor at 72v and a chinese 2000w controller - at normal speed setting its all well and 40kph top speed but att max setting (3-way switch) everything goes CRAZY! Throttle is On or off above 40kph and the power is 3 times higher. Extremely awkward!
I...
The most easy way is to look for a AC adapter with 4.2v output. They work with dc if its not a transformator inside. I have it on my bike - 48 to 8.4v.