Is this battery a direct replacement for the part number, or is that what your measuring? I've just tested the 9v (3 3v lithium batteries) in my smoke detector came out to 8.95v, under a load, that is. If it's already unplugged and you're measuring that, great. Now you just have to jump temp power (usually there's 2 solder pads near the battery), and replace it. I usually look up the part number and get a direct replacement just to do a professional job, but that depends on how comfortable you are with soldering/desoldering. The easiest would be get the 2 AA holders and solder them to the pads, then cut the old battery once you're sure you got voltage. If you haven't unplugged it yet I'd strongly suggest checking the board ID, and taking the top off the receiver and looking at the battery. You may be able to probe through the plastic vents on the decoder (don't probe through the metal vents you'll ground it and kill it). If it is, you can find a way to get 2 small 'kynar' (30ga wire) through the phone jack and solder them through the vents, to either the solder pads or somewhere on the board that circuit goes to, while it's still live. Then you can pull your decoder unit out and do the final replacement. Remember to wear safety glasses, and though I've worked on stuff live before, it is dangerous. Those lithium batteries can blow up in your face like a fire cracker when the new current rushes in, or if you heat them too much with the soldering iron. Also if you're working on it live, make sure you know what your doing as you can shock yourself, and possibly die if you get shocked the wrong way. Certainly don't do it alone, at least have someone there if your heart stops (worst case scenario), all electricians are trained this way if they work on live stuff.
