It is PowerCore 20100 model A1271, right? Please charge it with a standard 5V wall charger, like an iPad charger, via the supplied Micro USB cable.
Generally speaking, the external battery is normally capable of a 60%-70% charge transfer of its capacity depending on the internal resistance of the USB cable. Approximately 30-40% of the battery capacity is unable to be used because of efficiency problems that affect all electronics devices. These include power lost from circuit heat and voltage conversions from the battery, the charging cable, and your device. Therefore, it is normal that the PowerCore 20100 offers about 3 charges to a 4000mAh phone.
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