I have a few backup battery for my mobiles phones. Can I use one of them as a UPS?
+--------------+
5V | | +--------+
from ---> 5V backup | 5V | |
wall | battery -------> Pi |
| | | |
+--------------+ +--------+
A backup battery like those on Alibaba .
I'm afraid it's one of those 'It all depends' answers. There are some battery packs that can do this... many of them can't. Alex Eames over at RasPi.tv has found one that does work even though it's not necessarily recommended:
http://raspi.tv/2013/testing-rs-5200-mah-usb-lithium-battery-pack-as-a-ups
There is a project coming out that gives your Pi UPS via a LiPo battery mounted onboard a board which itself is mounted on top of your Pi's GPIO pins. I can't find the link right now but I'm down as a beta tester.
It is possible to safely charge Lithium batteries. Your cell phone is probably well-engineered and you can safely leave it plugged in at home unattended.
It is possible to save a few pennies on the charging circuit in ways that make it less safe. It is also possible to save pennies without endangering anything. Which did the manufacturer do?
If the charger and power adaptor that come with it are not tested by Underwriters Laboratory or other Nationally Recognized Testing Laboratory, then it is likely that the manufacturer was interested in saving a few pennies.
Note that the FCC mark is not a safety testing mark, it just means that it won't blot out TV reception on your block. The CE mark is not a safety testing mark--it is a statement that it meets certain standards, but it is entirely self-certified in most cases.
First, I'd double check that the battery is actually 5 V, close to it won't cut it and I can't think of any phone battery off the top of my head that is 5 V. Here's how I'd do it, assuming I had a Nokia 3310. Warning, involves hacking!
Basically, all Nokia phones charge off of 5--5.7V, and the batteries are 3.6 V Ni-Mh or Li-Ion batteries. First step hack a high current (at least 1 A) USB charger to also plug into the Nokia. This provides power of the Pi and to the charging circuitry of the phone. The positive battery terminal on the phone then need to be soldered to a 3.3 V Zener diode. The diode then connects to the 3.3V, and a second Zener connects the 3.3V to the 5V. When power cuts out, the Pi should keep going off the phone battery for a little bit, but all the peripherals would stop (including ethernet!) due to a lack of 5 V. Not an ideal solution but a clever little hack I think. More on running on 3.3 V.
It might be easier (if less fun) to go for a more ready solution. The MoPi project is billing itself as a UPS, although it currently doesn't provide any battery charging capabilities. It does have an enormous uptime though.
Lithium batteries are prone to overheating and burning along with your house, if charged improperly or handled improperly or, sometimes, just for no reason. Google "lithium battery fire" and try something else, like lead-acid, these are much heavier, but much, much safer.
Personally I would not even dream leaving lithium battery charging or discharging at home unattended.
