Connecting Makita 18v battery to step down

So I am converting my Makita fluorescent light into an led light. I bought a step-down converter to take it down from 18, to 12v, though when I connected it up, pop went the capacitor. I did some research and found that it is something that will happen when the polarity is the wrong way around, so I double checked this time, the polarity was correct... Connected it up again and off went the capacitor again. The Makita battery is a 4.0ah battery and here is the link to the step-down converter. https://www.ebay.com.au/itm/DC-DC-4...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649