CyberTech High Capacity Replacement Battery for Nokia Lumia 521 for T-Mobile, Metro PCS, 520 AT&T Compatible with Nokia Lumia 521, 520 All batteries use Grade A Cells and have been tested before shipment This replacement battery has an integrated microchip that prevents overcharging and lengthens battery life 1250 mAh Extra High Capacity, extra-long stand-by times, Light Weight, High Capacity & Low Impedance Sold exclusive by CyberTech with one year warrenty