Battery Size

Battery Capacity Standards

A number of different standards are used to rate battery energy storage capacity. Automotive and marine starting batteries are normally rated in cranking amps. This is not a relevant rating for continuous loads like an inverter. Deep- cycle batteries use a more suitable rating system, either “amp-hours” (“Ah”) or “reserve capacity” in minutes.

Battery Reserve Capacity Battery reserve capacity is a measure of how long a battery can deliver a certain amount of current—usually 25 amps. For example, a battery with a reserve capacity of 180 minutes can deliver 25 amps for 180 minutes before it is completely discharged.

Amp-hour (Ah) Capacity Amp-hour capacity is a measure of how many amps a battery can deliver for a specified length of time—usually 20 hours. For example, a typical marine or RV battery rated for 100Ah can deliver 5 amps for 20 hours (5 A x 20 hours = 100 Ah).

Actual Battery Capacity Actual battery capacity decreases as discharge current increases. A battery rated at 100Ah which can deliver 5 A for 20 hours may deliver 20 A for only 4 hours, resulting in an actual capacity of 80 Ah. For this reason, it is difficult to compare rated Ah capacity with battery reserve capacity. For example, a battery with a reserve capacity of 180 minutes has the following calculated Ah capacity:

180 minutes ⎟ 60 = 3 hours x 25 A = 75 Ah

However its actual Ah rating will be closer to 100 because it is rated at the discharge current required to get 20 hours of operation (about 5 A).

To calculate the battery capacity you require, read “Estimating Battery Requirements” on page 31 and “Battery Sizing Example” on page 32, and then complete the “Battery Sizing Worksheet” on page 33.

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