Strange problem with new battery

Maybe someone here can help with this, I've been looking everywhere for an answer. I have an older Compaq presario 2100. The battery that came with it had pretty well lost its original capacity (was down to about 20 mins battery life) so I bought a replacement battery. The batteries look identical on the outisde, same numbers, and the my computer is listed as one of the appropriate computers for the battery. I put the new battery in, allowed it to discharge and fully charge, then calibrated it via XP's calibration tool. It now says that it is 100% charged, but upon removing the AC from the laptop, it instantly shuts off and will not power up. (Much as it would if there were NO battery in at all). The old battery will still power up the computer and run like normal, albeit for short time frames. What could the problem be? BTW, the new battery has a gauge on the side of it that shows its charge state, and it says it is at 100% charge as well. Any help is appreciated. Thanks!