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.