New Zaurus battery not working :[

So I found a Sharp Zaurus in my basement and I was like "omg, so cool, I wanna make it work!!!" Turns out it was "charging" for like 8+ years (haha), so the battery couldn't hold any charge. After some searching, I found and bought a battery that seemed to have all the same requirements as the old one. However, when I put it in, the battery light on the Zaurus just flashes rapidly, and it doesn't turn on.

The battery itself is fully charged, and the Zaurus does turn on (the bad battery does still work, I just need to keep it charging the whole time).

Any tips? Thank youuu!!!!

:3