Taking the used battery this adaptor draws the energy left in that will no longer be of use for the HF5590 Disposable batteries giving you an immediate power source without the need for solar panels etc 

Charge USB powered devices using standard military batteries: BA-5590/U, BA-5390/U, BB-2590/U, and BB-390/Us. The dual power charger works with most devices like your phone, PDAs, Cameras, Tablets, portable media players and much more.

• The BTP-70791-USB2 can be powered by batteries with an X90 connector.
• Provides 5V at 2.1A and 5V at 1A, a total of 3.1A.
• Draws power evenly from two halves safely on batteries with two sections.
• Internal protection circuitry automatically shuts down in cases of output shorts, excessive current draw, and temperature overload.
• Implements the D+/D- voltage divider functions to be compliant with Apple Mobile Digital Devices, allowing them to draw the full 3.1A of current from the charger.
• The use of original charging cables ensures the best charge time.
• Charges Apple products (iPhone, iPod, iPad, etc..), Android products (Phones, Tablets, etc…) and most USB chargeable devices.