I use a variety of power converters around the garage but these adjustable buck converters are really handy:
A buck converter works by applying a counter voltage to control the output. This is seamless for the user but limits them to lower output voltage than the input. I like these for jobs such as running a 6 volt bike from a 12 volt battery, or charging an odd voltage battery. They're also handly to combine with a larger or not smart charger to prevent the voltage from going too high and over charging your bike battery.
The little brass screws on the blue boxes allow adjustment of voltage and amps.
This one is the best thing I've bought in years:
I added a shunt type ammeter, voltmeter & jumper leads. The two little blue adjustable potentiometers were replaced by a pair of remote mounted panel potentiometers which are located with the meters in a box on top of the unit.
Now I can adjust voltage to the desired value and then leave the bike "running" with engine off to test lights, grip heaters and such. It's great not to have to keep playing with a battery charger to keep the voltage within limits when a smaller smart charger won't do the job. Also ideal for the 6 volt bike projects and charging batteries. Love the thing!