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4-2 Battery Capacity Rating
4.2.1 Battery Discharge Rate
Deep cycle batteries have their amp-hour rating expressed as “at the x-hour rate”.
The hour rating refers to the time it takes to discharge the batteries. A faster hour rate (10 hour rate) means
more current is withdrawn from the batteries during their discharge period. There is an inevitable amount of
heat associated with the flow of current through a battery and the higher amount of current the greater the
amount of heat will be generated. The heat is energy which is no longer available to the battery to power
loads. a relatively long discharge rate (120 hour rate) will result in a larger number of amp-hours being
available for electrical loads.
4.2.2 Depth of Discharge
The battery bank`s size determines the length of time the inverter can supply AC output power. The larger
the bank, the longer the inverter can run.
In general, the battery bank should be designed so the batteries do not discharge more than 60% of their
capacity on a regular basis. Discharging up to 80% is acceptable on a limited basis, such as a prolonged
utility outage. Totally discharging a battery can reduce its effective life or permanently damage it.
4.2.3 Understanding Amp-Hour Requirements
To estimate the battery bank requirements, you must first calculate the amount of power you will draw from
the batteries during your period of autonomy. This power draw is then translated into Amp-Hours (Ah)
the unit of measure to express deep-cycle battery capacity.
Amp Hours are calculated multiplying the current drawn by the load by the length of time it will operate.
To calculate amps when the power consumption is expressed in watts, use the following equation:
A = P/V
P = Watts ; V = Volts DC ;
For example:
A 60 watt light bulb will draw approximately 5.0 Amps.
5.0 = 60 /12
If the light runs for three hours it will consume (5.0 x 3) or 15 Ah of power.
The length of time a load is operated will affect the power draw. In some cases, an appliance which draws a
large wattage may not consume as many amp hours as a load drawing fewer watts but running for a longer
period of time.
For Example:
A circular saw draws 1500 watts or 125 amps. It takes 5 seconds to complete a cross cut. Twelve such cuts
would take a minute and you would consume 125A x 0.016* hour = 2 Ah. (*0.016 = 1/60 )
Suggestion :
All electrical appliances have labels which state their energy consumption. Look for an amps rating on
motors and a watts rating on other appliances. If the label plate has expressed power consumption in amps,
multiply by volts for the watts required. (watts = volts x amps).
When calculating battery bank size, consider the following:
