How to build a mini UPS (uninterrupted power supply) for the Arduino.
My current draw on my board is ~ 100ma. I needed my Arduino to keep running no matter what, for a high security system, if the "AC mains" were cut or interrupted by a brownout. Warning ... You need to take a bank loan for this application or you can just go out and buy a bulky commercial UPS system. My UPS is about 2"long x 2 1/2" wide x 1 inch thick.
1 Li-Po USB charger from Sparkfun. $17
2 Single cell lithium polymer battery for Li-PO charger with 2 pin JST 2mm connector.
Sparkfun has several single cell capacities of batteries. I chose the 1100 mahr battery. $12
3 2 pin JST 2mm connector cable. $1
4 5 VDC regulated wall wart from Sparkfun. (optional) $6
5 A step-up switching regulator. Dimension Engineering has several models but I chose their AnyVolt Micro.
This adjustable output switching regulator will take an input voltage range of 2.6 V to 14V and step it up to 2.6 to 14 vdc @ 500ma max. $20
Operation: The Li-Po charger takes the USB 5VDC power or external 5-7vdc power and charges the Lithium polymer battery. If both sources are connected, it takes the external power source eg wall wart. There is even an onboard charge status LED indicator. When the charging is done, the Lithium battery provides the power to the system load (Arduino) which is steped-up using the AnyVolt adjustable Micro switching regulator. The output can be adjusted for anywhere between 2.6-14vdc. For fixed 5vdc systems adjust the output to 5vdc. For un-regulated systems adjust for the input required. When the lithium battery reaches a certain voltage level the Li-po charger recharges the battery. This charging system acts like a cellphone or laptop battery charging circuit. If your Arduino uses less than 100 ma you could run on battery power for days.
Warning!!!! DO NOT POWER YOUR ARDUINO BOARD FROM TWO SOURCES OF POWER!You can only use only one power source at a time. USB, +5VDC or external unregulated power can only be used one at a time. Disconnect all other sources of power.
Optional use of the mini UPS - You could add a USB output connector and power/charge your portable USB devices with or without USB or external power. 1.1 amp of juice!
May the power be with you ... 8-) 8-) 8-)