For what would appear common sense to me, but perhaps not to all, I have all of my computers on UPS battery backups. After getting frustrated when the power flickered one day, I immediately went out and purchased a UPS. Well, shortly after, the power went out for longer than the battery could keep my computer afloat. I needed a better solution!
I wanted my UPS to be able to last for at least 60 minutes in a power outage. I needed more power! My solution: Car batteries.
UPS that is rated at least double what you plan to draw (see step 8 to understand why).
Wire (12 awg or larger; two different colors)
Heat shrink tubing
Car battery with terminals on the top
Adapters to go from the car battery terminals to threaded rod.
Wing nuts the same size as this threaded rod
Wire crimp terminals that will fit over the threaded rod.
Plastic case for your car battery
Inline Fuse holder (radio shack)
30 amp fuse for holder (any auto store)
heat gun or alternative
Step 1: Evaluate Your Needs
I was trying to power two computers (desktop and file server), and two flat panel monitors. My total power consumption was roughly 500 watts peak. (yikes!) Currently I was running on two 300 watt UPS's (NOTE: VA is not equal to WATTS. Find the WATT rating) with one computer and one monitor on each. Even though the two monitors were hooked up to the same computer, I needed to distribute my power load more evenly to get longer battery life out of my petty UPS's.
I discovered the hard way after nearly starting a fire and destroying a UPS that you need one that is rated at at least twice the wattage you are consuming. They can't handle being run for longer than a few minutes at this rating, but the batteries die before it's a problem normally.
So I now knew I needed 500 watts, and I wanted 60 minutes of power.. that means:
P / V = I
500 watts / 120 volts = 4.16 ampere hours (at 120 volts)
UPS batteries are usually 12 volts, but some are wired with two batteries in series. Check yours out first to make sure you won't need two car batteries.
So, assuming 12 volts, that means that, after adjusting for the voltage differences, I need a battery with at least 41.6 ampere hours. (yeah, I know there's inefficiencies in the UPS, but lets keep math easy)
Step 2: Remove Battery From UPS
Unplug the UPS from the wall, and unplug all devices from it.
Remove any screws you fine, and open up the case.
If you are as lucky as I was, the battery will have terminals that you can slide off. If not, just cut the wires as close to the battery as you can.
Once you have removed the battery, you will find something like you see in the picture
NOTE: Pay attention to polarity on the battery, and which wire went to when polarity.
Step 3: Extend Wires on UPS
The wires that are in a UPS are typically not long enough to reach much past where the battery sits. We will need to extend them to reach our car battery.
Cut off the the wire terminals (if any) on the wires from the UPS.
Strip at least 3/8 of an inch of the wire on the UPS
Strip at least 3/8 of an inch of the wire we are extending with.
I used a metal crimp to help me get a great connection, but this is optional.
Solder the wires together. This solder joint needs to be able to handle high current. We will be drawing lots of power through here and if we have a voltage drop, the UPS won't last as long.
After making sure the joint is well soldered, place some heat shrink over it, and seal it up good.
Note: Use colors that make sense to you, and will allow you to remember the polarity
Step 4: Drill Hole for Wires
Next we need to make a place for the wires to leave the UPS and go to the car battery.
I drilled a hole. Use whatever size will fit both of your wires.
Add a strain relief so you can't pull on the joints you made, or on the PC board in the unit. I simply tied a knot in each of the wires.
Next pull the wires through the hole, and carefully put the unit back together.
Note: Remember the polarity!
Step 5: Prepare Inline Fuse Holder
Since this is high current, coming from an extremely high current source (car battery), we need a fuse. and you want it as close to the battery as possible.
First, strip the wire on the fuse holder.
Place heat shrink on the wire.
Take your crimp wire terminal that is sized for the thread on your battery posts, or adapter and crimp it to the wire. Then solder. Nothing is complete until it's soldered. Why solder? It conducts electricity better. The joint won't get hot, and you will have a less drastic voltage drop.
Next shrink the tubing.
On the other side of the fuse holder, strip the wire, place the heat shrink on, strip the hot wire you've recently added to the UPS and solder together. Once completed shrink the tubing.
Step 6: Prepare the Remaining Wire
Next, using the same strategy as connecting to the inline fuse holder, connect the Crimp terminal to the end of your ground wire, Solder, and heat shrink.
Remember: Put the heat shrink tubing on before you put the end on.
When you done you should have something like:
Step 7: Attach to Battery, and Test
Next, attach your battery terminals to the battery, and then your wires to the terminals.
Insert a fuse in the fuse holder.
And turn on your UPS.
It will take a long time to charge the battery, but it will also last for a long time in a power outage. Under this setup mine lasted for around 1.5 hours.
Be sure to put the battery in a plastic case with a lid, as, if something were to go wrong on the battery you would want to contain the acid as much as possible. Also, this will prevent you from dropping something and shorting out the battery.
Step 8: A Word of Caution
I learned this the hard way.. it cost me a UPS, and nearly a fire.
The transformer in these UPS's are cheap. They are not designed to be run at 100% capacity for extended periods of time (such as what you will be capable of using this size battery) When I ran my UPS's at 300 watts for more than 30 minutes, the transformer melted through the case. When I pulled out my infrared thermometer it read nearly 400 degrees F!!
I had to redesign my system. I chose two UPS's that were rated at 600 watts each, but used 24 volts (2 twelve volt batteries in series). Under my new setup, I have over four hours of backup capacity as I have two car batteries.
jari777 made it!