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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.