Fry's Electronics in Seattle (Renton?) and Batteries Plus have little rechargeable gel batteries, in 12V and 6V. Mine is 4"x2x1 3/4, weighs less than a pound and will run e-caller plus a big TOA external speaker all day on full volume raspy low tones, which drain batteries faster than high tones. Batteries Plus near South Center had a gel battery half the size of this one, about 2x2x2, which I wish I had bought.
The gel battery shown blow fits in a shirt pocket and makes for an extra step to plug into the e-caller on a stand. Even when running the minimal e-caller with no external speaker nor external battery, I take the gel along in case I run down the internal AA batteries.
I have a connecter pigtail so I can run the e-caller off of the external gel battery, shown below. It plugs into the recharging port of the e-caller. Online similar batteries run from about $6 to $30, in various sizes and modes. I think I paid $16 for mine at Fry's, can't remember.
Maybe ask the call maker about external batteries, though most makers want us to buy their proprietary items. You might take your e-caller into Batteries Plus or a Radio Shack to fit power draw and connector plugs.
I have a recharging set up in my vehicle but have never used it on the gel battery, even on three day calling trips.
