Derating The Battery Bank - Wagan ProLine 2007-2 User Manual

ProLine 3,000W Power Inverter by Wagan Tech
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2. Estimate the number of hours the appliances will be in use between battery recharges.
This will vary depending on appliances. For example, a typical home use coffeemaker
draws 500 watts during its brew time of 5 minutes. It maintains the temperature of
the pot, requiring 100 watts. Typical use of a microwave oven is only for a few
minutes. Some longer operating time appliances are lamps, TVs, computers and
refrigerator/freezers.
3. Determine the total watt-hours of energy needed. This is done by multiplying average
power consumption in watts by hours of run time. For example: 1,500 watts for 10
hours = 15,000 watt hours.
4. To get an estimate of the maximum current (in amps) that a battery bank must be
capable of delivering to the inverter, divide the load watts by ten (for 12V battery). For
example a 1,500 watt appliance load will need 150 amps at 12 volts DC.
5. Using the 1,500 watts (or 150 Amps) for 10 hours example as above, then 150 amps
is needed for 10 hours. This provides us with the basic amp-hours (AH) of battery that
is required. Ten hours at 150 amps equals 1500 amp-hours (AH). This answer is just a
beginning because there are additional factors that determine actual run time. These
include:
• AC appliance load and time in use (basic AH)
• 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)
• Compliance with turning off unnecessary AC loads.
• Use of DC appliances and compliance with turning off unnecessary
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 Ampere Hours (AH).
For example; if a 20AH battery is discharged at a 1 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 nominal 12 volt battery starts at 13.2 volts
(fully charged) then drops to 10.6 volts (discharged). If the load on the battery causes
the battery to discharge faster than the 20 hour rate, the capacity (AH) of the battery is
measurably reduced (derated). Derating is a major run time factor. The curve in the following
chart can help to determine what the battery bank can deliver under load. The results are
used to estimate how much additional battery capacity is needed to achieve the desired run
time.
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