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10,000 Watt AC Inverter by Wagan Tech
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Using the 500 watts (or 50 amps) for 0 hours example, 50 amps are needed for 0
hours. This provides us with the basic amp-hours (AH) of battery life that is required. Ten
hours at 50 amps equals 500 Amp-hours (AH). This answer is just a starting point because
there are additional factors that determine actual run time. These include:
Cable gauge and length (cable losses)
Charge level of the batteries (between use, chargers have to be able to fully charge
the batteries)
Temperature of the batteries (colder batteries provide fewer amps)
Age and condition of the batteries (older batteries lose AH capacity)
Use of DC appliances
Compliance with turning off unnecessary AC and DC loads.
derATIng The bATTery bAnK
Most lead-acid batteries have a rating expressed in amp-hours (AH). The most common
rating of AH is “at the 20 hour rate”.
NOTE: Despite several Internet explanations, there is no relationship between cold cranking
amps (CCA) and amp-hours (AH).
For example; if a 20AH battery is discharged at a amp rate, is will take 20 hours to
discharge that battery. The terms “charged” and “discharged” relate to actual battery
voltage. This means that the output voltage of a 2 volt battery starts at 3.2 volts (fully
charged) then drops to 0.0 volts (discharged). If a load on the battery causes the battery to
discharge faster than the 20 hour rate, the apparent capacity (AH) of the battery is reduced
(derated). When batteries are under heavy load, derating is a major run time factor. The
following graph can help to determine what a battery bank can deliver under heavy load.
The results are used to estimate how much additional battery capacity is needed to deliver
desired run time.
The left vertical numbers of the curve represents percentage of the battery capacity at the
20 hour rate. In this example, the user needs a one hour run time. If the example battery is
220AH (20 hour rate), and the load is 220 amps that is 00 percent (horizontal number)
of the AH (20 hour rate). Starting at the 00 percent horizontal point and looking up to
the curve the results are that only 56 the percent of the battery capacity is available. This
means that a higher battery capacity is required to get the desired run time of one hour. The
curve also shows that a load of 200 percent of the 20 hour rate yields only 3 percent of
the battery capacity. The installer must carefully plan the capacity of battery bank or the run
time may be seriously affected. To the inexperienced installer, several trial battery capacities
may be required to make sure a large enough battery capacity is available to achieve the
desired run time.
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