DIY 5v Uninterruptible Power Supply ( UPS )

Hello, I am planning to create an arduino-based surveillance system consisting of motion detectors and the like. Given the importance of such system to be working even if the power goes down I am very interested in creating some sort of DIY UPS to make sure it is always on.

I have searched on the Internet but did not get very satisfactory results so I was wondering what parts and what procedure should be followed to create one from scratch. I was planning to use some 2450 mah AA batteries for this purpose.

Any ideas on what to do? I understand that there should be some sort of check to make sure batteries do not overcharge and that the power supply should automatically switch to batteries if power goes down but I can not come up with any sort of circuit.

I also feel that it might be helpful to build a battery charger and then use that to build the UPS, or am I wrong?

Any help would be highly appreciated !

Perhaps you want something like this: http://www.liquidware.com/shop/show/MBP/Lithium+Backpack

UPDATE, July 2021: The liquidware.com domain and @liquidware Twitter account seem to have been sold off. This link is no longer valid and, although the "Lithium Backpack" and newer (2009) "Lithium Backpack MEGA" are described as "Open Source", I can find no schematics or board layouts for them.

I think the best bet is to try and find a USB power brick that can be charged at the same time it produces 5V over the USB cable to the Arduino. Be warned that many USB bricks will shut off if the load is very low, like an Arduino in sleep mode.

Yeah... possibly... but that costs $47. Seems quite a lot.

Assuming I already have rechargeable AA batteries, diodes, resistors, leds, wires and ac/dc converters I prefer to go the DIY route.

Thanks for posting that, though !

Just a concept, sort of a block diagram. I use a very similar circuit but with alkaline cells and no charging circuit; it’s worked well for me. For input I use a 5V regulated wall wart. The diodes are 1N5818 or similar. For the regulator I use an MCP1640, which is limited to about 300mA output. For the circuit to work, two conditions must be met: (1) The battery voltage must be less than than the wall wart voltage, and (B) The input voltage to the boost regulator must be less than the output voltage. A LiPo battery with an appropriate charging circuit should work as well.

ups.png

Some Arduino derivatives contain (part of) what you need : Olimex has a board with included LiPo charger and connector. Panstamp Battery board hosts one AA cell and 3V3 boost (and even wireless battery voltage status reports). FYI, I have one that works since about 2 weeks on a 1300mAh LIMH. Battery slowly decreasing. No integrated charger there though.

Both solutions are open source hardware, i.e. .pdf are available at no cost.

Personally, I'd recommend going 3V3 instead of 5V. It's not 100% linear, but it does save energy.

quote an improvement of the circuit to eliminate forward voltage drop of schottky, when on battery power.

reverse_protection.gif

I use this and have no problem: http://ali.pub/2lilft