Looking to mod your Nerf gun with some lithium-ion muscle? Or maybe just use a single, more powerful rechargeable battery in a device than normally runs on a pair of alkaline AAs? Dummy Batteries mimic the cylindrical shell, and conduct electricity like real batteries, so you can use them to fill voids in your compartment, but similar to my ex-girlfriend Karen, they are empty on the inside. They serve as placeholders that provide no additional voltage to the powered device.
AA Dummy Batteries can come in handy when you don't want to over-juice a device that calls for 3 or 6 volts' worth of 1.5V AAs with the same number of higher rated Li-ion or Li-FePo4 batteries. One Amazon reviewer points out that his remote control requires 3 volts, or 2 non-rechargeable AA batteries, to function. His rechargeable Li-ion batteries supply 3 volts apiece, so inserting a Dummy Battery alongside the rechargeable produces a working remote, plus an extra rechargeable to Dummy up on a flashlight or some other device.
Another reviewer pairs his Dummy with a 14500 lithium battery in his portable shaver, which also normally runs on 2 x AA alkalines at 3V total. The lithium battery boosts the voltage to 3.7, and the dude notes, "it runs it on steroids!"