Computer UPS Hack / Extended Capacity UPS Build

There is a room on my terrace that I use for my hobby projects and fun research. When the room was build a year back I had given a provision for a separate inverter connection for the room, isolated from the rest of the house. I hadn't fixed one as the need never arose. Of late there has been quite a few intermittent power outages and I decided that it was time to fix one. As I had a functional spare inverter lying around, it was the first choice. So I decided to get myself a new battery and hook this up.The power outages normally last from 10 min to 1 hour.

Shift to the UPS:

I have a few electronic test equipment which are sensitive to the power fluctuation, and so is the computer in the room. So I decided that the regular inverter was not a great idea. The next option was a UPS. Considering the load in the room a 600VA power source would suffice and I decided to buy one. A day passed and something struck me, it occurred to me that the computer UPS that I used in the room for my PC was a 600VA UPS. The internal battery was however a 7 AH 12V battery which drove a 300 watt computer for approximately 15 min. So if I bought a computer UPS and swapped the battery with a higher capacity external battery it would be perfect for my use.

Charging and stuff:

The AH reading on any battery stands for the 'Ampere Hour'. If a battery is rated 12V 7AH it would mean that if you draw 7 ampere from the battery it would last for one hour until it is discharged. Like wise if you draw 3.5 ampere it would last 2 hours and so on. To charge a battery efficiently you have to give it 10% of its current rating. So since we have 7 AH on our 12 volt battery the charger will be designed to charge it at .7 ampere [ 700 milli amps]. The important point to note here is that, if we connect a lower capacity battery [less that 7AH] the battery will heat up while charging and will eventually die faster than its life span. If we however connect a higher capacity batter all that would happen is that it will take proportionality longer to charge.

List of items required:

• A 600VA computer UPS 
• 12V DC computer fan 
• Drill for drilling vent holes
• 2 x High gauge wires and terminals
• 26 AH SMF Battery 

On with the build:

When we use a higher capacity battery the UPS will run longer and so it is a good idea to fit a cooling fan. Since we use an external battery we remove the internal battery from the UPS ; this provides us ample space for the fan. I drilled some holes under the UPS and placed the fan in such a way that the air gets sucked in from below and goes out through the side and top of the company cut vent holes.The 12V fan was connected in parallel to the battery and is always on and continuously cools the ups. It draws very little power and barely adds to the load. 

The battery I selected was an Exide 26 AH sealed maintenance free [ SMF] battery that needs no attending to what so ever. This would give us approximately 4 times the backup of a computer UPS. The computer draws 300~400 watts of power and lasts 15 minutes on the built in 7AH battery and we have replaced it with a 26 Ah battery. A regular25 watt cfl light consumes  25 watts of power and a fan consumes about 50 watts if it is very efficient. So assuming that the computer has been shut down, [ I have a second ups connected to the computer which gives me ample time to shut down without taking any load from the extended UPS] the average load in the room would be around 100 watts if there are 2 x 25 watt cfl and the fan running.

This would mean that I should easily get 1~2 hours on the new setup which is more than enough. For extending the wire from the UPS, wires of the same gauge should be used and connected securely or there is chance for loose contact.  

I have had the system running for over a week now and so far it has worked well and switches perfectly. Since it is a UPS the electronic devices don't reset when the power goes off and comes back. 

Cost of the project:

600 VA Wipro UPS - Rs.1,500 [ Bought it from a friends shop and he took back the inbuilt battery and gave me the UPS for Rs.1000]

26 AH Exide Battery - Rs.2,300

Wires and miscellaneous - Rs.200

Total cost of the project: 1000+2300+200 =  Rs.3,500

The final outcome

p.s: The extra length of wire is for future upgrade.