"manufacturers of Li‑ion cells are very strict on the correct setting because Li-ion cannot accept overcharge (...) Most cells charge to 4.20V/cell with a tolerance of +/–50mV/cell. Higher voltages could increase the capacity, but the resulting cell oxidation would reduce service life. More important is the safety concern if charging beyond 4.20V/cell. ... mnohem víc tady http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries