Voltage on starter???

This is the starter that came with the car, it has no specs on it. It is wired for two batteries inside the box, which of course are not there now, cables are loose. I have one of those 12 volt jump starter batteries, and I hooked it up to the starter and it seemed to spin well.

http://ift.tt/1cfVIiV

I assumed when I got the new batteries I was to connect the cables such that they were wired in parallel, maintaining the 12 volts, kind of like in my RV. But it has recently been pointed out to me this may in fact be a 24 volt stater, requiring the batteries to be wired in series. Comments?

Thanks as usual.....