Battery Sizing - Trace Engineering DR Series Owner's Manual

Trace Engineering DR Series Owner's Manual - Version 3.2 - 9/7/98 - Page 17

Battery 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 regularly discharged to more than 50% of their capacity. 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 back up applications often have very small batteries. The minimum battery capacity
recommended is 200 amp-hours @ 12vdc and 100 amp-hours @ 24 vdc.
Estimating Battery Requirements
In order to determine the proper battery bank size, it is necessary to compute the number of amp-
hours that will be used between charging cycles. When the required amp-hours are known, size the
batteries at approximately twice this amount. Doubling the expected amp-hour usage ensures that the
batteries will not be overly discharged and extends battery life. To compute total amp-hours usage, the
amp-hour requirements of each appliance that is to be used is 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
= DC amps
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.