A dummy battery is a device, the same size as a real battery, that you use as a "place holder" when you are going to use a device with a battery it's not originally designed for. An example would be using a 2-AA flashlight with a Lithium Ion 3.7 battery instead. As you know, AA batteries are 1.5v, so 2 of them equals about 3 volts. If you put 2 14500 batteries in your flashlight, it would be getting 7.4+ volts and would be really bright (for about a second until you burn it out). By putting in one 14500 and a dummy battery, you have 3.7 volts and the device will probably work fine on one rechargeable cell. I build dummy batteries from "sex bolts", aluminum threaded female bolts. You just need to find the right lengths that, when connected together by a headless screw, are the same length as the battery you wish to replace. Here's a pic of the dummy battery I use in my vaping remote control.
