I bought four new 12v 12ah SLA batteries and four Covert 12v-to-6v battery cables. When I connected each cable to a battery (not yet connected to camera), two of the converter boxes smoked (and a third made a pop). Batteries showed a full charge of about 13.15 volts. I popped open the box on one that smoked and it is fried. What gives?
This certainly doesn't make sense. You hooked up the Covert cables, with no load, and they fried.
It sounds to me like the polarity was reversed; there's several ways this could happen:
1. It's unlikely, but never the less a possibility, that you hooked them up to the battery incorrectly (put positive to negative and vice versa).
2. The cables were manufactured incorrectly.
3. Your 12V batteries were charged with the polarity reversed, which will make the red + terminal be negative and the black - terminal be positive. You can test for the correct battery polarity by hooking up a voltmeter to the 12v battery; it'll read a negative voltage (-13.5V) if it's been charged backwards.
I would contact Covert.
I used quite a few of the Covert 12v-to-6v cables and have never had any issues.