ipod touch battery behaving strangely?

So my ipod touch 4th gen, i got it used, drains its battery really fast. Ive used ipods before so i know the approximate normal battery life for one. It drains about 5 times as fast as it should. But when i lock it and wake it up again after about 2 minutes, the battery recharges. So if it was at 20 percent and I lock it and wake it up in 2 minutes, it will be at like 60 or 70 percent. Then it drains to 20 again really fast. Could someone tell me what is wrong with my ipod and how i can fix it if possible