This has to do with the way lithium ion batteries can be charged. They can handle being fast-charged up to about 80-90% but then the charger has to trickle charge the rest of the battery so it doesn't overcharge. Lithium ion batteries can handle a lot of power being dumped into them at once, but it's very bad to overcharge them.The majority of the time I let the battery of my iPhone 4 either run down completely or to a max of 10%, then charge it to 100%. My phone is only a couple months old but I use it a lot. For the past week I've had to charge it only once a day. Tonight after it read 92% I noticed it got very slow. I didn't time it but it seemed to take forever to get from 92% to 97%. There it stayed for another long time. Usually while the phone is charging I'm using it -- on the Internet or watching Netflix, for example. That doesn't slow the charge time at all -- sometimes I'm amazed at how quickly it charges. Tonight when it seemed to have frozen at 97% I decided to watch Dexter hoping it would take my mind off my quirky phone. Lo and behold after a few minutes it read 100%.
This doesn't make any sense to me but do IP batteries charge faster if they're being used during the charge?