*Question E7D09:
How is battery operating time calculated?*  

*Correct Answer is  C: Capacity in amp-hours divided by average current*  

### AI Response: (may contain errors)  

To calculate battery operating time, you use the formula:

$$
\text{Operating Time (hours)} = \frac{\text{Capacity (Ah)}}{\text{Average Current (A)}}
$$

where:
- **Capacity (Ah)** is the battery's capacity in amp-hours.
- **Average Current (A)** is the average current drawn from the battery in amperes.

### Explanation:

1. **Correct Answer (C): Capacity in amp-hours divided by average current**
   - This formula gives you the operating time of a battery by dividing its total capacity by the rate at which it is being discharged. It tells you how long the battery can supply power at a given current.

2. **Why the other choices are incorrect:**
   - **Choice A: Average current divided by capacity in amp-hours**
     - This would give you a value with units of inverse hours, which does not represent time. It is the reciprocal of the correct formula.
   - **Choice B: Average current divided by internal resistance**
     - This calculation is unrelated to operating time. It would give a value with units of current per ohm, which is not meaningful for determining how long a battery will last.
   - **Choice D: Internal resistance divided by average current**
     - This would give a value with units of resistance per current, which is also not related to battery operating time.

### Key Points:
- The correct formula directly relates the battery's capacity to the current draw, providing a straightforward way to estimate how long the battery will last under a constant load.
- Always ensure units are consistent (amp-hours for capacity and amperes for current) to get the operating time in hours.