Any suggestions how to size an Odyssey battery for a rotary?

The Maxton really, really, really needs a new battery. Like, really.

Based on some of Keith's comments on which batteries he's using for his fleet, I'm thinking about getting an Odyssey as the car doesn't see that much usage, and shouldn't have much of a parasitic drain on the battery either.

The additional wrinkle here is that the battery is in the trunk and the "official" mounting method seems to be to just chuck the battery in the trunk with no hold down or anything. Obviously I'm planning to rectify that and one of the nice features of the Odysseys is that you can find some nice mounts that I should hopefully be able to use to and have a proper stable mount.

I don't really want another regular battery, especially as the current size is a reasonably tight fit and doesn't leave that much space for putting in a proper mount.

I think it's currently runing a size 34 battery - it's not like there is a lot of need for battery power, it's got a carb'd 12A rotary, the usual set of lights, a heater fan and an engine cooling fan. So power demands shouldn't be big, but the battery needs to provide enough power to crank and start the engine.

I'm currently thinking of either a PC925 or a PC1200. Both should physically fit with decent mounts, so I'm mainly trying to decide if bigger is better or not?

Any of the local rotaryheads have any suggestions or opinions?

I use a PC925 to comfortably start a 6.2L V8 with 11:1 compression. Are rotaries particularly hard to turn over? I would think they'd be roughly the same as a six, but they are mysterious creatures. 

Well, this one still has some compression left, so there is that. IME they're not that hard to turn over, but you need a pretty decent cranking speed to get them going. Sounds like the PC925 should do the job, though. The battery cables are pretty long so there might be some voltage drop to account for, but that's why I pretty much immediately ruled out a PC680.

The PC925 battery is in the trunk of said V8 car, so it's probably got cables as long as you can fix in a Maxton :) 

There's really no downside to being too big other than cost, weight and packaging. But I don't think you'd need to go that far.

Odyssey lists PC1200 for first gen RX-7s, I think that would be a safe choice.

Keith Tanner said:

I use a PC925 to comfortably start a 6.2L V8 with 11:1 compression. Are rotaries particularly hard to turn over? I would think they'd be roughly the same as a six, but they are mysterious creatures. 

Compression is directly related to how fast the engine is turning, thanks to all of those flat sealing surfaces with corners, and the gaps between the different hard seals.  There is ZERO compression at no speed, you can rotate the engine by pushing on the alternator fan blades.  Mazda specifies a minimum cranking speed for a valid compression test for this reason.

So: The faster you can spin the engine, the happier a time you will have starting it.

With that in mind, battery size is not something to skimp on.  Nor are the cables.  And it's probably not a bad idea to pull the starter and clean up the commutator with some emery paper, can sometimes get another 50 cranking RPM just from doing that.

I have a Group 35 in the RX-7 with the battery still underhood and a Group 65 in the one with the battery remote mounted, because it's the biggest one with the most reserve capacity I could fit in the battery box.  No slow-crank here, even if the battery is half discharged because the fans were running too long or the belt broke or the car sat for two months.

I use a Deka AGM motorcycle battery in the Abomination. It doesn't take much to start mine as long as you don't flood it. 

Pulling the starter and giving it a good service is probably a good idea anyway.

I'm space limited by the size of the trunk pocket behind the rear wheel as that's the place where the battery needs to go, so I can't increase the size much beyond the current battery. I know a PC1200 will fit, and I think that if it doesn't provide enough oomph in the end, there might be some Lithium batteries available with the same form factor.

The lifted Miata has this one. Pictures explain.....