Mains power to battaries

[system]

Hello
I tried to google it today, but I had trouble even to express my query in simple terms

I have device (Airport express) powered by mains (internal wall-wart outputting 1.21A for 5V and 0.7A for 3.3V).
I'd like to add backup battery power to it - so when

• device is connected to the mains it is powered by wall-wart & battery is charged up
• device is disconnected from the mains power then battery immediately kicks in supplying power to the device

I'd like Arduino to control the charging process & report battery status via led bar.

I don't expect to find ready solution, main steps, building blocks etc?

Image is from here http://uavbob.wordpress.com/2009/03/25/airport-express-power-supply-repair/

[johnwasser]

How about using this $12 battery shield to power the Arduino and using 5v and 3.3v from the Arduino to power the AirPort Express?

http://www.cutedigi.com/product_info.php?products_id=4353

Then you could use any USB charger to connect it to AC and keep it charged.

If the built-in lights are sufficient charge information you could eliminate the Arduino and use a 3.3v regulator.

[weirdo557]

it doesnt look like that shield satisfies the power requirements.the 3.3v pin can only supply 50ma and im sure the 5v pin cant supply anywhere near an amp.

something like this
http://cgi.ebay.com/HP-COMPAQ-207066-001-DC-power-converter-340watt-/130503971214?pt=COMP_EN_Networking_Components&hash=item1e62a4918e

coupled with a 12 volt battery would give you plenty of power. cheap little supply provides 45 amps at 3.3 volts and 20 amps at 5.0 volts. way overkill but its cheap and regulated to computer standards. couple that with a resistive divider to read the battery voltage (pretty easy to approximate a lead acid battery charge by its voltage) and an led bar to show the status. a 12 volt wall wart can provide charging voltage (they tend to go over 12 volts, so as long as you're not using the max power it can put out, the wall wart will trickle charge the battery).