Polarity – the settings of the charges found at the terminals of the battery – which terminal is positive and which terminal is negative.
The positive terminal is usually marked with a ‘+’ symbol and/or the color red while the negative terminal is usually marked with a ‘-‘ symbol and/or the color black.
When a battery is connected to a device or circuit, electrons flow from the negative terminal to the positive terminal via the device. For some appliances (such as a simple light bulb), this has no importance. The light bulb will work no matter which way it is connected. For other devices connecting batteries with incorrect polarity can cause them damage.
Getting polarity right is critical when using multiple batteries connected in series or parallel. If connected in series and the polarity of one battery is incorrect (you cycle the energy the wrong way) with the batteries connected correctly trying to charge the incorrect one. With no system to limit the situation the correctly connected batteries will discharge rapidly causing them to overheat and possible explode while the incorrectly connected one will fight the charge also causing overheating.
See also: Reverse Polarity.
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