I have a laptop that has an application installed that allows a "gauge reset" for the battery, to keep it accurate.
This isn't always available on a given platform. To do a manual gauge reset for a Li-Ion Battery, you need to understand how it works.
There is a controller in the phone itself that tells you about the battery status. This controller is expecting the same type of information. It can be reset in pretty much all li-ion batteries the same way, by depleting the battery to an extremely low level (go below 10 percent), then charging it to full. Do this 3 or 4 times, and it should reset and readings should become more accurate.
In addition to resetting the gauge, it will reset the circuitry within the battery to actual limits and it will have a more efficient charge/discharge cycle.
I recommend doing a battery depletion and recharge once per month, as it will prolong the life of the battery.
CAUTION: Never completely discharge a li-ion battery! Doing so will prevent it from being recharged! The battery must have a little bit of a charge for the charging circuitry in the device to recognize it and charge it. Even if you're using an external charger, that external charger needs to recognize the battery, which it can't do if it has zero charge. Never allow the battery to become 100% depleted.