The picture shows some sample UPSs and an example of the gel cell from one of them. The UPSs come in various capacities and, although you can boost the capacity, the output power is fixed. When you start out, make sure that the UPS you're going to modify will provide the volt-amps and power that you need. Also note that the volt-amp rating is higher than the power rating. The difference is because AC powered devices have a power factor. Check online for more info about this. Another, similar Instructable, also warns against trying to max out the capacity of your UPS because some use transformers that will run continuously at the rated output. It really depends upon the quality of the UPS, but plan to run at not more than about 75% of rated output capacity.
Another thing to consider is whether or not your scrap UPS has AVR or Automatic Voltage Regulation. You'll want this if you can find it.
Step 1: The Guts
When you open the UPS box, you'll usually find either one or two gel cell lead-acid batteries. You're going to replace it / them.
What you'll need:
1 scrap UPS - but it needs to be a WORKING UPS!
4 feet of #10 copper wire (2 feet of red and 2 feet of black if you can)
You may need to adjust this length, but keep it as short as you can tolerate.
4 spade lugs (two mating pair
2 ring lugs for single battery or 4 if you have two in the UPS
1 or 2 marine, deep-discharge batteries (85 - 120 AH capacity)
A drill with bit set
Crimper for the lugs
Sandpaper or a small file to smooth out holes in plastic
Some notes on the parts: look at the pricing of the batteries and get the capacity that gives you lowest cost per amp-hour. Sometimes the 110-140 AH batteries are only a little more than the 70-90 AH ones. Check warehouse clubs for good pricing. Sometimes you can reclaim the core charge with an old motorcycle or car battery you have lying around. Make sure that what you get is a deep-discharge type battery. Some batteries are labeled as marine, but not deep-discharge.
Make sure that your battery has adapter studs that clamp to the lead posts. Then, get ring lugs from your hardware store, electronics store, or online store that will fit over the threaded studs and also accept #10 wire.
You can either get two pair of mating spade lug connectors, or get one set that are the mating connectors to the ones that come from the UPS. In the picture, these are the black and red wires that connect to the batteries.
Note the blue wire. The batteries are in series in this UPS, so it requires 24 volts (black terminal connected to red terminal). Do not try to power it with one battery if it comes with two! Get two more ring lugs if you need two batteries.
Step 2: Tap into the UPS
Drill two holes in the side of the case. The holes only need to be large enough for the wire to pass through. You don't want the connectors to be able to sneak outside the case, to be safe. Make sure there are no sharp spurs or edges around the holes that might cut or damage the insulation. Thread the wires from the inside out, with the connection inside the case as shown.
As noted in the parts list, you'll want to keep the wires short to minimize voltage drop, but long enough that you can place the UPS and batteries comfortably near each other and the device you're powering (computer, stereo, etc.). Lay everything out, including battery box(es) if desired, and clip the wires to an appropriate length.
Step 3: Connect the Battery / Batteries
If you have two batteries, make sure to cut a short piece of wire before you add the ring terminals to the other end of the wires you connected into the UPS wires in the previous step. Make sure that this jumper wire is not too short if you are putting the batteries in containers (see end of article).
Note: You might have to ream out the inside of the ring terminals if you could not find terminals with the right inside diameter.
Once you know that the ring terminals will fit, crimp them to the red and black wire ends. If you have two batteries, make the jumper with the other two ring terminals and the small piece of wire you cut off.
Connect the red wire to the positive terminal of one battery. Connect the short jumper between that battery's negative terminal and the positive terminal of the other battery. Then connect the black wire to the negative terminal of the second battery. If you do not connect them this way, you will not get any power into the UPS.
Step 4: Use it
You should now be able to plug in the UPS and go.
Some additional thoughts and enhancements:
1. You are using lead-acid batteries. In the photo, I've shown a plastic layer under the batteries. Even though marine batteries are typically sealed, you do not want to take the chance of a leak. Spend $8 - $20 and get a plastic battery box for each battery you use. It will protect the batteries ... and your floor.
2. In the first step I claimed this would improve the capacity, massively. By how much you ask? The largest gel cells I've found in UPSs are 20AH. Others are more like 7 AH. Using an 85 AH marine battery to replace a 20 AH gel cell, you should expect to get at least four times the runtime. HOWEVER, do not plan to run the batteries down by more than 50% or you will severely shorten their life. Figure more than a doubling of capacity, but less than 4 times. If you use a 115 AH battery, figure at least 3 times the run time and a much longer battery life.
3. I have not had any problems with the UPSs recognizing the new batteries or keeping them charged. If you have the UPS monitoring software, use it and try some tests. See how long your new rig will run before hitting the 50% charge level.
Expect the total cost to be about $80 for a single battery setup with 85 AH battery and battery case. Double battery setup with 115 AH batteries should be about $160, max. These are using battery prices in summer 2009. Battery prices spiked briefly last year, but have come back down.