DROK Waterproof DC Buck Converter Voltage Regulator 8-22V to 3-15V 5V 12V 3A Adjustable Output Power Supply Transformer Step-Down Volt Module Board for Car Auto Vehicle Motor Regulated SR

There's always a way! See how many batteries they use in series, if AA/AAA then they're 1.5v x however many in a row. Or take a multimeter to each end of the batteries to see their combined voltage. Then pick up one of these and just wire it to the battery, after the fuse for good practice, and set it to the same voltage that they use.