So you've got an Eco-Charge that you haven't used in a while, or maybe you've drained it and it just won't take a charge. This happened to me recently - left my gear plugged in after a show and it drained my BP-50 to the point of no return. These things cost $80 new if you can get them. Fortunately, the construction is pretty simple - all you need is a replacement battery (the one here is a Werker WKA6-7.2F, $21.99 at the local Batteries Plus) and a screwdriver or other prying tool. This will likely also work for the old GP-90's, but probably NOT for the new EC-90's as they've got more circuitry inside (some of which is designed to prevent deep discharges like this anyway!). But I digress.
The dead Eco-Charge BP-50. Note the little lip on the top part.
Stick your prying tool underneath that lip. Not perpendicular to the battery itself (there's also a groove that separates the body of the battery from the top) but parallel to the face of the battery, between the battery and the top housing. There's a weak adhesive holding it in place; once you get it a little bit loose, just run the prying tool down the length of each side. There's no adhesive on the ends. You should be able to work the top off by rocking it back and forth - it'll take a little bit of force but don't do it too hard or you might yank out the wiring inside the housing.
Now you can see what you're getting for the extra $58.01! The black housing, two 4-pin XLR connectors, a circuit board, and some minimal wiring.
These little spade connectors slide out pretty easily. Just remember which one goes to red and which one goes to black - both of my wires were black, and both of the connector pieces were red!
Refit the spade connectors onto the new battery...
...and push the housing back down over it. I'd suggest rocking it on so that one end goes down first, then the other will follow. It won't snap into place but will be pretty snug even without adhesive.
Congratulations, you've got a brand new Eco-Charge, minus the branding! Toss it back in the nylon case and nobody will be the wiser...