New battery won't charge

My old battery wasn't holding a charge very long so I bought a new one. The new battery charged to 51 percent after being plugged in overnight. Every following full discharge and recharge brings the battery to less and less capacity (now at 14 percent). I returned it for a replacement, and it did the same thing. The old battery still charges but I noticed that the light on the power adapter is always green, with either battery. I zapped the PRAM, PMU, and NVRAM but nothing changes. I also respaired permissions. Is it a voodoo curse or am I missing something?