I wonder if you were perhaps starting out with less than a full charge. I know you had it on charge for 8 hours, but if the phone was ON while charging it may not actually get a FULL (100% of actual battery capacity) charge. There is a situation that occurs when charging the phone while powered on where the phone's power consumption while charging "fools" the charging circuitry into thinking the battery has reached a 100% capacity charge even when it's only 85% or so.
Also, while on the charger, it will stop charging the battery to prevent overcharge, but since the phone is still drawing power, the actual State Of Charge (SOC) can diminish. The charging circuitry will not resume charging until the battery drops by 10%, and if it started at only 85%, that means it will drop by another 8.5% from there, or to a low of 77% before trying to replenish back to the 85% it started at.
To be sure you have reached a true 100% capacity charge, power the phone off BEFORE plugging it in. Then plug the power cord in, and the phone will boot into Charge-only mode. A large battery icon will appear and there will be a simulated liquid level displayed. The screen will timeout after about 10-15 seconds. Allow it to charge in that state over night. To check the charge level, quickly press and release the power button and wait 1-3 seconds until the display lights up. The large battery will indicate the actual SOC. Once it's at 100%, you can remove it and power up the phone.
Now you can enjoy a FULL charge and experience the actual battery life.
