Originally Posted by hellosailor
As long as the deep cycle battery can safely supply the "impulse current" or surge current needed by your starter motor (which can be 1.5-2.5 kilowatts easily) without overheating and warping the plates or internals, it is perfectly safe.
I had a discussion about this with JCI (possibly the largest US battery maker) some time ago, for a starting battery that had to sit for long periods and would be better served by using a deep cycle. Their answer was that if the deep cycle had enough capacity--it was actually the better way to go.
So, depending on how much power your start draws, it can be perfectly good to do it this way. On a 41? Dunno, you'd need to find out how much power that is.
That's kind of what I was thinking. DYNOs are great batteries with an awesome reputation and I know first hand because I've abused some of them before in the most horrid ways. So I figured that would make a better starter than a regular starter with thin plates, as long as it was up to the "impulse current" though I didn't know the term.
So how do I go about finding how much impulse current my starter draws, or for that matter, how much the dyno can handle? Starting batteries give cranking amps, deep cycle amp hours, and the usually never give you the other number though that would be helpful wouldn't it?