My son have a question that is out of my league. I hope there is one of you professionals that can help me answer him. I am putting his question as a quote.

Hi guys.

I have a question for the electrical people among you.

I fly RC helicopters, and need to charge my lipo batteries.
The problem I have is getting my battery charger the power it needs.

It can take input voltages from 12v to 18v.
My bats that I am charging are 25v, so when giving the charger 12v, it needs to ramp it up to 25v. That seems inefficient.
So what I would like is 17V input.

The charger can use 200W, so I need a supply that gives 17V @ 20A.
Can any of you think of an easy and cheap way of doing this?

I know off the shelf applications are available but they are very expensive for a student.

Thanks in advance!
Cheers
Quintin

PS. All voltages mentioned are in DC.