Button batteries are the small circle batteries that tend to be used in watches and other small electronics. If you haven't used them before, you might not be sure how to insert them into your device. As soon as you understand the polarity of button cells, learning to insert them is simple, and you can even stack batteries if necessary for your device. Button batteries vary in size, but this advice works for all of them.
The most important step when you're inserting a button battery is getting the polarity right, but this is a simple thing to ascertain. The batteries have one flat side, which is always the positive side, and a rounded side, which is the negative side. The positive side is indicated with a "+" anyway, so you could work this out by inspecting the battery, but the consistency of the design means that you don't need to do this.
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Inspecting Your Device
Look for the battery compartment on your device and look inside it. The correct polarity for the battery is indicated somewhere in the compartment, either on the interior of the device or the opposite side of the battery cover. Typically, the positive side is marked with a plus sign (+), but the negative side may marked with a minus sign (-) instead. In some cases, there may be a diagram indicating the correct way to insert the battery.
Inserting the Battery
After you know the correct orientation for the battery and identify the positive and negative sides of the cell, inserting the battery is a straightforward process. Usually, the compartment is circular and plastic with metal contacts on the inside. You should be able to push the battery directly into the space, although in some cases, it's easier to insert one edge first, with the battery oriented diagonally, and then swing the battery down into the rest of the compartment.
Provided you have the polarity correct, the battery doesn't have to be spun at a particular angle to work. All that matters is that the positive and negative sides are pointing the right way.
Button batteries have a limited voltage compared to ordinary cylindrical batteries, generally either 1.5 or 3 V. In some applications, you need to stack multiple batteries to increase the voltage difference between the two terminals. This is easy to achieve: All you need to do is align the negative end of one cell with the positive end of the next one. In many cases, devices that require stacked batteries have removable inserts you can use to arrange the batteries or a tube in which you can insert one battery at a time.
You should only stack batteries if the device or project specifically requires it. Stacking batteries increases the voltage. If you're working on a home project that doesn't have sufficient resistance built in, stacking batteries unnecessarily could lead to a high current that damages components.