The highest capacity dual purpose batteries I have found that are readily available (in the U.S.) and that will fit most vehicles are Grp 31M from Odyssey and Northstar. Both are over 100ah with 1150 CCA and the Odyssey has 205 mins reserve capacity with the Northstar claiming 220 mins reserve. Both have substantially more capacity than the 31Ms from Deka or Optima. I have had a bunch of Odysseys and the problem with them is that they require 14.7V+ input from alternator or solar or shore power charger to achieve and maintain a full charge, and they like high amperage input. If you can figure out how to deliver high voltage and high amperage, the Odyssey is a fine battery. If you can't deliver that, the big Odyssey will have a relatively short life and operate at less than full capacity, and will lose a small amount of capacity with each charge cycle, unless you periodically condition and desulfate the batteries with a high voltage/high amperage (25 amp minimum) conditioning charger with an AGM setting. That means you probably have to carry the charger with you and find shore power about every eight weeks. I have been through this on my Dodge, and the only practical solution (for me) is a dedicated Odyssey or similar charger on a reconditioning cycle every couple of months. The Dodge now has a triple battery system with the new Odyssey 2150 Extreme 31M in the starting position and a pair of Optima Blue 34M dual purpose as the house batteries with a new 270 amp alternator and appropriate isolators and switches. I just bought a new battery for my Tundra daily driver and I got a Northstar 31M because I just don't want to screw around with the Odyssey's goofy charging and maintenance requirements. All of these batteries are claimed to be true dual purpose (I have spoken with Optima reps at SEMA, with Odyssey's founder and tech support staff, and with Northstar's rep for the western U.S.), and they say there is no reason not to use them for starting or house batteries interchangeably. I use the marine versions on all of these because the extra studs are so handy for connecting refrigerators, emergency lights, compressors, etc.