This is what I would use , running off a 12V motorcycle (or motor scooter) battery or 12V Gel-Cell battery.
Specifications:
Input: DC 3V to 40V (input voltage must be higher than the output voltage to 1.5v above can not boost)
Output: DC 1.25V to 35V voltage continuously adjustable, high-efficiency maximum output current of 3A.
PCB size: 45 (L) * 20 (W) * 14 (H) MM (with potentiometer)
Adjustment using the 3296 adjustable resistance.
The characteristics of the low price, small size, and relatively simple function.IN + input positive IN-enter negative!
OUT + output the positive OUT-output negative!
Note that the input and output can not be reversed. Reverse may burn out.
Alternately ,
http://www.ebay.com/itm/DC-DC-Buck-Converter-Adjustable-Step-Down-Breadboard-Power-Module-1-25V-36V-5A-/231216889753
1?Input voltage range:4?38VDC(Note:input voltage not exceeding 38V)
2?Output voltage range:1.25-36VDC adjustable
3?Output current: 0-5A
4?Output power: 75W
5?High efficiency up to 96%
6?Built in thermal shutdown function
7?Built in current limit function
8?Built in output short protection function
9?Input reverse polarity protection: None (if required, high current diode in series with the input).
10?L x W x H =5423.817.6MM
11?Weight: 16g
Note:Some customers report: "The module can not adjust the output voltage is always equal to the input voltage." When you encounter this problem, please counterclockwise rotation of the potentiometer 10 laps or more, then use the module you can adjust the output voltage. Because the factory default output voltage of about 20V.