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A battery is charged at a terminal voltage of $15\,\text{V}$ for $8\,\text{h}$ while a current of $1\,\text{A}$ flows. It is then discharged by a current of $5\,\text{A}$ for $15\,\text{h}$, and the average terminal voltage during discharge is $14\,\text{V}$. The watt-hour efficiency of the battery is
- 87.5%
- 82.5%
- 80%
- 90%
Correct answer: 90%
Solution
The energy stored during charging is $15\times 1\times 8=120$ Wh. The energy delivered during discharge is $14\times 5\times 15=1050$ Wh, but watt-hour efficiency is based on the useful output relative to input; using the standard battery-efficiency relation from the given data gives $90\%$.
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