My wife's several year old Therm-ic max+ battery backs are rapidly losing their capacity, so in the spirit of curiousity, incureable DIY'er and a bit cheap to boot, I decided to dig into them to see if they are really worth the money they charge for them. Sort answer is, not really, especially if the batteries themselves are the only thing that needs to be replaced. There is a bit of clever (but not exotic) circuitry to allow for multiple voltage levels to be applied to the heating elements, but the batteries themselves are plain old nickel-metal-hydride AA rechargeables.
Some Dremel-surgery and a bit of prying gets the case open. It's pretty much just glued at the four corners and you get 4 NiMH AA cells soldered together in series (4.8V total). A quick google search gets you replacement flat-top AA with or without solder tabs for about a buck apiece, and of higher capacity than the Therm-ic OEM ones. I got 2000mAh ones vs the Therm-ic 1400mAh. I saw up to 2900mAh ones which would more than double the capacity of the original ones!
Once I get them, it is fairly easy to solder them into a 4-battery pack, solder on the switch assembly and put it back in the case. sealing the case will be the hardest part, but I am hoping to do it with some shrink tubing. Next will be trying to reverse engineer the switch assemble. I am betting that I can make up a battery pack for less than $40.