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| Ratings of Batteries |

| Storage batteries are usually rated in ampere-hours, the rating usually being based on the time period in which the battery will be discharged. Thus, the ampere-hour rating of a battery is equal to the product of the amperes output and the normal time such an output can be supplied. The ampere-hours obtainable from a battery are greater for a long low-rate or intermittent discharge than for a short high-rate discharge because the voltage drops at the higher rates. Moreover, the 12R losses in the battery are greater for higher currents than for smaller currents. |

| Battery capacity is usually based on an 8-hour rate. Greater capacity can be obtained if the discharge rate is made lower, and vice versa. A discharge rate above normal will lessen the capacity. For example, a battery rated 40 ampere-hours at the 8-hour rate would have the following outputs: |

| Amperes × Hours |

| = Ampere-Hours |

| 140 × 1/60 |

| = 2.33 |

| 20 × 1 |

| = 20.0 |

| 5 × 8 |

| = 40.0 |

| 0.8 × 72 |

| = 57.2 |

| To restore a battery to its fully charged condition, it is necessary to return to it as many ampere-hours as have been delivered by it. Since battery efficiencies run from 85 to 90 percentsay 90 percentthen if the battery delivered 20 amperes for 30 minutes, 10 ampere-hours would have been removed from it. The battery would be fully charged again if approximately 11 ampere-hours (10 divided by 90 percent) were returned to it. |

| Operation |

| Specific Gravity |

| The specific gravity of the electrolyte provides a convenient way of determining whether a storage battery is fully charged or, if it is not, the extent to which it is discharged. The test is made with a hydrometer, an instrument for measuring the density of a liquid. By density is meant the weight of a substance compared to an equal volume of water. Thus, a substance with a density of 2.0 is twice as heavy as an equal volume of water, and a substance whose density is 0.5 is half as heavy. Density is also called specific gravity. |

| The Hydrometer |

| When a liquid becomes heavierthat is, denser, like the electrolyte of a storage battery when chargingit becomes more buoyant. A floating object |

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