There is a power shortage in where I live, so I manage to make a power back-up from some component I've already got and an old car battery.
This is how to make back up power from old car battery.
Old car battery still can store electricity so when my car needs a new battery I keep the old one, until the new battery getting old and need replacement. Of course the old battery is not as good as new one, so we have to charge it periodically.
This way it will make the use of the battery about 2 year long..... or until you have to replace your car battery with the new one...