This is a good use for older UPS units, they often get replaced due to age, power needs, maybe a problem, or changing a battery is not economically justifiable.
The one i made was from a unit from the mid 1990s, and the battery was totally destroyed. It wasn't worth fixing that so i got it. Sometimes a surge, or other issues happen you may be able to fix it by changing some bad parts, but not always.
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Step 1: Materials, Tools
Wire 18-8 AWG The larger the better, preferably cord type with 2 or 3 conductors in one cable
Plug to fit car power outlet, or battery clamps, terminals if you want direct connection.
Crimp on spade terminals for breaker, battery input leads, although soldering works too
Something to power the inverter Car, Boat, spare battery, etc.
Screwdrivers flat/ philips should do. Remove battery, Opening, closing the unit, making plug
Soldering iron To make solders that may be needed. Some of the stuff you can use terminals, splices, etc. The 12V plug if used will probably need soldering though.
wire stripper, cutter
Pliers for removing grommets, possibly spade terminals.
Voltmeter for calibration, etc if you do that. Else a small load like a lamp, etc just to verify it works
Step 2: Remove the Battery
In most cases these units have a Sealed lead acid battery in them. Most units have some sort of access for the battery. Smaller units often use a flap on the bottom that is like a door and removal of a screw or two opens the door. Some larger ones you pop the front bezel off, and the battery slides out.
Disconnect the battery sometimes they use simple spade terminals right on the battery these may be latching spade terminals, so you may have to press a release. However some units especially larger ones may use some other type of plug.
I have found sometimes this step is tricky because the battery has leaked, is bloated, corroded. Sometimes the batteries actually crack and leak(ed) acid. If it looks like may have leaked, white crystals, crack you may want to use plastic gloves to keep acid, etc. off you. Sometimes if its really bad you may want to go to the next step and remove the cover so you can get it out.
Once it's out you can close the access door again, as most units we won't need this again.
Please recycle the old battery many stores like Home Depot, etc have some place you can recycle them for free.
Step 3: Remove the Cover(s)
This typically involves a removing a few screws on the bottom and maybe the back. Most units seem to have 4 screws on the bottom to remove. Then slide the cover back some because usually it slides under the front bezel.
These are usually sheet metal of some form, so the edges could be sharp.
Step 4: Electrical Modifications
Assuming the electronics are still okay on the unit continue by making these modifications.
1. Remove the AC cord usually these are a plastic clip type thing squeezing, prying usually remove. or if you prefer, and have a lighter plug, but maybe wires too small. cut the line plug off solder on the 12v plug. While this the line cord will work it's better to use larger wire. Lower voltage and DC means higher amp load= larger wires.
2. Take the wire you chose and run it through the original cord hole. Make a knot to stop it from pulling out, out something on the cord or metal to stop damage to the wire.
3. Disconnect the original wires from the circuit breaker. Connect one end of the new power cord to the circuit breaker on the back with a terminal or solder.
4. Connect the battery leads to the new input. Connect one lead to the other terminal of the circuit breaker if needed splice in more wire using terminals or soldering. The other wire connect to the remaining battery lead with terminals, or soldering. (Wire-nuts could be used to join wires also.) OBSERVE POLARITY when doing this. Typically US car the pin on the plug is + (red) and the shell is -(black). If unsure do an ohm test from pin to end of cord if connects then this is your + the other has to be -
5. Modify the front switch so the unit runs when plugged in. (You can use another switch with on/off also). Many of these with mechanical switches use 3 wires. a comm, test, silence. You need to determine what wire is what. Comm is generally center, the other 2 check using ohm meter and pressing switch when you find terminal that connects when in test, power on mode. (A UPS with a button may work too just solder button so is always on.) Connect the comm and the test/power-on lead you identified together.
Step 5: Test/set Adjustments
I recommend testing it just to verify it works, and the output voltage, frequency are correct. Sometimes they use potentiometer to set these and they can be off. Just be careful not to touch the circuits when its plugged in as parts operate at 120V.
Step 6: Put Back Together, Enjoy
Put the covers back on, put all the screws back into the unit that hold the covers on.
Enjoy your new inverter use it to run laptops, etc. Avoid running things with motors off this, and observe the ratings of the UPS, generally 75% the output rating is recommended. Unless you used rather large wire and a plug or large wire and battery clips i would reduce that rating to 50-60% the load.