A couple other points on the odyssey battery.
1) Probably everyone knows this but just in case: A std aviation battery will sulphate if you leave it on the shelf for a year. this happened to me so at that point I had a choice of buying an expensive de-sulphating charger or buying odyssey batteries which don't have this problem. I bought the batteries.
2) the pc680 is called the pc680 because it provides 680 amps for 5 seconds. Similarly the pc625 provides only 625 amps for 5 seconds. However, in every other cranking metric: "cranking", "cold cranking", "hot cranking" the pc625 outperforms the pc680. It also has more total storage capacity. On my airplane, the cranking situation I'm most worried about is in the air, trying to get the prop out of feather. (k. we just had the debate about props and their handling so we don't need to do that again). I decided cold cranking amps is what I care about.
details are available here: