I was trying to charge a lithium ion cell. I used a lithium ion cell charger, that I bought from Walmart. The instruction booklet it came with, says it takes 8 hours to the charge the lithium ion cell. I have been charging the lithium ion cell for 2 days and it still won't reach 4.2 volts. What is wrong with this lithium ion cell? I sometimes measure the voltage. 1 time it was 3.91 volts, then I let it charge for hours, then it was 3.71 volts.