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25 Watt-hour Battery Pack for Charging Devices through USB Port
By: Dave Johnson, July 21, 2012

My wife often forgets to plug her phone into its charger.  Later, the phone runs out of juice.  I thought it would be nice to have a compact battery pack which could conveniently fit in her purse and be switched on to charge not only her phone but any device which draws power from a USB port.

There are some new AA size lithium ion batteries available which might work for this application.  They claim to have a capacity of 2.2 Amp hours.   Four such cells held in a box would yield a 14v pack with a 2 Amp hour rating.  That is 28 watt-hours of energy which is much higher than four alkaline or NiMH cells.  An efficient 5v 1 Amp DC to DC converter could be mounted inside another small plastic box and glued to the side of the battery box, together with a USB socket and perhaps a DC power jack.  When needed, my wife could then connect the pack to her phone, switch on the charger and charge her phone’s battery.  The power jack could be connected to a custom DC battery charger, able to charge the 4 AA cells.  At a standard 4.2v per cell voltage, the charger would need to apply 16.8v to the pack and charge the pack at a 200ma or C/10 rate.  The pack would then be fully charged over night.
With 2 Amp-hour cells and an 85% converter efficiency, the total energy available from the 4 cell pack is about 23 watt-hours.  This is roughly 4 hours of full 5 volt 1 Amp power.  That should be enough to charge a number of different devices.  It would be an interesting experiment to put together such a device and see how it works.

3.6v AA Lithium Ion Battery   4 AA pack with USB and on/off Switch   5v 1 Amp DC to DC Converter

Please send comments to me


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