Loving this little guy right now:
It's a digital DC-DC converter / Power supply -- give it power, from a battery or a fixed DC power supply, and set it to whatever voltage and current combination you want. Up to 60v DC in, up to 120v DC out, up to 15A in or out.
In the photo, I was charging a "nonoperational" 18v lithium cordless power tool pack. It was being powered by a 12v/5a power supply I have a few of. Set the final charging voltage, set the maximum current I wanted to charge at (output watts equal or less than input, of course), and just let it do it's thing.
I didn't trust it's accuracy at first, but after confirming that it was stupidly accurate (identical current and voltage readings as my best DVM), I decided I could trust it. Which is a relief...especially when dealing with lithium cells!
Lithium...my new-to-me hybrid bike has given me new appreciation for it. And I've been learning a fair bit about it. But that's a topic for another time...