Recently I'd acquired an old APC Smart-UPS 750XL from a client. The UPS contained a dead battery which it was refusing to charge - a good sign it's a goner. With the unit out of warranty, and replacement battery packs upwards of $450 (if you want a warranty from APC), the client had elected to remove it and replace it with a new unit of larger capacity, fresh battery and importantly - warranty. This left an opportunity for me to see if I could resurrect the thing. Physically it looked fine, and I was assuming that the only thing wrong with it was that the battery was shagged. While researching, I'd come across this thread by MorTus. It appeared that he'd done an extremely similar thing to what I was intending to do. MorTus made the whole changeover look quite easy. I was prepared to solder if I had to, but it turned out that this wasn't at all necessary. This is just going to be a quick log on how I did it. First, the victim: Pictured here is an APC Smart-UPS 750XL - model number SUA750XLI. Some poking around on APC's site revealed APC's product page for the unit. Reading the manufacturer's page for the device shows that the battery it uses is a RBC7. The batteries used appear to be quite standard - after looking up the battery number your UPS requires, you can often just Google the code, find the battery specifications - and then find a suitable replacement battery to match those specs. Fortunately someone had already done this for me - here's the battery I needed: Jaycar product ID SB2490. Two of these is all you're going to need as far as materials goes. Tools - one each of a flat blade and philips-head screwdriver - and possibly a beverage of choice. If you've got no idea what you're doing, you probably don't want to go poking around with high-voltage/current mains-connected devices. Do this at your own risk and all that. Hey, your UPS could start leaking and explode - who knows. Either way - not my responsibility! It's a good bet that you're going to want to power the thing off, remove connected devices and pull the safety fuse out of the back before proceeding. I did. The Smart-UPS 750XL opens from the front like a drawbridge - so with the front of the unit slightly elevated just pull down from the notches on either side of the front panel. Once it's off, "unhinge" it and place it on top of the unit: Two screws conceal the battery bay. Remove them, and pull this cover down too: And voila, the battery pack. Attached to the bottom of this is a stuck-on flap. Pull on this to withdraw the batteries from the bay. Here, I've compared the original battery (on the right) with two of the Jaycar batteries placed side by side: They're barely different from each other. Moving on - you'll want to disconnect the battery from the unit after having slid the battery out. There's enough slack on the cords that you can fully withdraw the battery from the bay and disconnect it. Just pull the plugs apart - there's no trick to it. Pop the little plastic covers off of the battery - they just protect the terminals from shorting. They're just held on with double-sided tape. The flat-blade screwdriver will come in handy for this. Once the terminals are exposed, use the philips-head screwdriver to remove the cord and fuse from the battery pack: Place each battery side by side so that the terminals all meet in the middle. Each positive terminal should come up next to the opposing battery's negative terminal. If this is the case - reattach the fuse and battery plug as they were set up on the original battery - just make sure you get the polarity correct! And to stop the battery coming apart and breaking the fuse, I used some packing tape to keep it together as a single unit: Then slap the terminal covers back in place, reattach the bit of plastic used to slide the battery out and you've got yourself a replacement battery. Put the whole lot onto the battery tray, reconnect the battery plug and slide it back in: That's pretty much all there is to it. Plug the fuse back into the back, give it mains power and switch it on. The UPS detected the battery OK, did its self-test and the incessant beeping audible before the battery change has now disappeared. Not bad for $150 and a long struggle carrying 25KG of UPS up a large hill to my car.