TECHNICAL INFORMATION
Copyright Trace Engineering Company, Inc.
5916 - 195th Street N.E.
Arlington, WA 98223
Telephone: 360/435-8826
Fax: 360/435-2229
www.traceengineering.com
PS Series Inverter/Charger
Part No. 3597
Rev. D: November 23, 1999
Page
97
BATTERY BANK SIZING
Batteries are the inverter's fuel tank. The larger the batteries, the longer the inverter can operate before
recharging is necessary. An undersized battery bank results in reduced battery life and disappointing
system performance.
Batteries should not be discharged more than 50% of their capacity on a regular basis. Under extreme
conditions (such as a severe storm or a long utility outage), cycling to a discharge level of 80% is
acceptable. Totally discharging a battery may result in permanent damage and reduced life.
For stand-alone applications, it is common to size a battery to provide between 3 and 5 days worth of
storage before the battery requires recharging. The power contribution from other charging sources is not
included in this calculation to duplicate the conditions present during a cloudy or windless period. This is
often referred to as the “number of days of autonomy”. If the system is a hybrid system with daily
generator run periods, then the battery size may be smaller. During cloudy periods, the generator would
be expected to run longer.
Utility connected applications often have very small batteries. If the system does not provide utility back-
up function, the minimum battery capacity recommended is 320 amp-hours @ 12 vdc, 160 amp-hours @
24 vdc, and 80 amp-hours @ 48 vdc. If utility back up is required, larger batteries will be needed.
ESTIMATING BATTERY REQUIREMENTS
In order to determine the proper battery bank size, it is necessary to compute the number of amphours
that will be used between charging cycles. When the required amphours are known, size the batteries at
approximately twice this amount. Doubling the expected amphour usage ensures that the batteries will
not be overly discharged and extends battery life. To compute total amphours usage, the amphour
requirements of each appliance that is to be used are determined and then added together.
You can compute your battery requirements using the nameplate rating of your appliances. The critical
formula is WATTS = VOLTS X AMPS. Divide the wattage of your load by the battery voltage to determine
the amperage the load will draw from the batteries.
If the AC current is known, then the battery amperage will be:
(AC current) X (AC voltage)
(battery voltage)
Multiply the amperage by the number of hours the load will operate and you have, reasonably enough,
amp-hours.
Motors are normally marked with their running current rather than their starting current. Starting current
may be 3 to 6 times running current. Manufacturer literature may provide more accurate information
compared to the motor nameplate. If large motors will be started, increase the battery size to allow for the
high demand start-ups require.
Follow this procedure for each item you want to use with the inverter. Add the resulting amp hour
requirements for each load to arrive at a total requirement. The minimum properly sized battery bank will
be approximately double this amount. This will allow the battery to be cycled only 50% on a regular basis.
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