I just doubled the house battery capacity on my 1984 C36 (Mk. I), and when the dust cleared, my separate engine starting battery was sitting on the cabin sole. Oops! Anyway, the story is this. My first season with the boat, 2001, I installed two Group 24 75AH deep discharge trolling batteries from Wal-Mart, plus a huge marine starting battery (like a Group 27). I chose Wal-Mart based on my analysis to the information in the online Deep Cycle Battery FAQ - there are only a few manufacturers, you should buy the freshest batts you can, and buy them by weight. I was right, those house batts were great! The starting batt was way oversized. I'm now convinced you could get by, on a Universal M25, with a motorcycle or garden tractor sized battery. After 5 years of faithful service, and abuse, they gave up. I got two Group 24 Seavolt batts at West Marine, on sale. Same dimensions and rated capacity. They were never, ever as good as the originals, and are inadequate for our needs while cruising of 24 hour refrigeration, DC lights at night, anchor light, and daytime use of autopilot, VHF, sailing instruments and GPS. In addition, we've added more load, including a notebook PC (5 Amps!) and more. I found that two 24's fit in the forward starboard seat locker, where the starting batt was. (The primary house batteries are in teh aft starboard seat locker). Used the existing #4 wires, added some new #4 jumpers, and got two more Wal-Mart 24's, off the palette. So, the two SeaVolts and the two Wal-MArts equal a theoretical 300 AH, in two banks. All of this is fed and managed by a 125A Hamilton-Ferris Powermax alternator, Xantrex 3 stage charging regulator, and 100A 3 channel Pathmaker. So, now, where should I mount the starting battery? I temporarily connected the engine to bank two. I though that the space behind the engine, just port of midships, and just aft of the water-lift muffler would be a good spot; I'd glass-in a plywood floor, make hardwood cleats and clamps fixed with threaded rod. Any issues with that location? Any better ideas? Any recommendations for a smaller-than-24 sized engine battery, like a lawn and garden tractor battery? Thanks, jv