A simple electrical circuit consists of an energy source, such as a battery, supplying energy to a suitably-rated device. This energy source performs within its safe operating limits, while the device behaves as expected. Short circuiting a battery means excessive current follows an unintended path, due to an abnormal connection with little or no impedance.
Short Circuiting a Battery Causes an Abnormal Condition
This condition allows an excessively high current to flow with little resistance. An uncontrolled surge of energy can damage the circuit, and result in overheating, skin burns, fire, and even explosion.
This can be quite dramatic if the circuit is inside a battery cell. This is usually the consequence of a technical fault, or an out-of-specification condition. However, it can also occur if the opposite terminals of two similar batteries accidentally touch. This usually happens during some-or-other incident, but it can also be the result of human carelessness or malice.
Short circuiting a battery deliberately, or accidentally connects the positive and negative battery nodes, forcing them to be the same voltage. The result, as Wikipedia puts it aptly, is a connection with almost no resistance. In such a case, the current is limited only by the resistance of the rest of the circuit.
How a Battery Can Also Cause a Short Circuit
This current is limited only by the resistance of the rest of the circuit. Therefore, it follows, an abnormally high current will flow if a low-resistance device, even electrical wire completes that circuit. Now energy, as we know is potentially heat. This is how a high current flowing through a battery can cause a rapid increase in temperature.
A short circuit fault inside a battery can release a current thousands of times larger in milliseconds. This can irreparably damage all devices in the external circuit. Avoid short circuiting a battery in several ways. Buy decent batteries and devices, and use them wisely. Never allow battery terminals to connect directly, or damage or modify the cells in any way.
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