Everybody wants a battery with zero internal resistance. But in real life, we get real batteries. Suppose we are stuck with a particular battery with an internal resistance *r*. We are interested in how the efficiency of our circuit is affected by the value of our external resistance *R*.

The efficiency of the circuit can be written as

So clearly, the larger the external resistance, the higher the efficiency. As the internal resistance becomes a smaller fraction of the total resistance, the power wasted in the internal resistance also becomes a smaller fraction of the total power. Take special notice that when , the efficiency is exactly 50%!

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**Video Explanation **

How Efficiency Varies with External Resistance

**Concept Test **

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