Battery-Bank Sizing; Estimating Battery Requirements - Xantrex7 UX - REV A Owner's Manual

Appendix B - Batteries

Battery-Bank Sizing

Estimating Battery Requirements

©2002 Xantrex Technology Inc. All Rights Reserved.
P/N 975-0026-01-01 Rev A 06/2002
The battery-bank's size determines the length of time the inverter can continue
to supply AC output power. The larger the bank, the longer the inverter can run.
An undersized battery bank results in reduced battery life and short inverter run
times.
In general, the battery-bank should be designed so the batteries do not discharge
more than 50% 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 results in permanent damage and reduced battery life.
For off-grid, stand-alone applications, design a battery-bank that can power the
loads for 3–5 days without requiring recharging. To duplicate the conditions on
sunless days or windless periods, the power supplied from other sources
(i.e., solar, wind, hydro, etc.) is not included in this calculation. This is often
referred to as the "number of days of autonomy." If the system is a hybrid, with
daily generator run periods, the battery-bank size can be smaller.
To determine the proper battery-bank size, it is necessary to compute the
number of amp hours that are required between charging cycles. When the
required amp hours are known, size the batteries at twice this amount to ensure
the batteries are not regularly over-discharged.
To compute the amp-hour requirements, the amp-hour ratings of each appliance
powered by the inverter must be added together. Use the figures from the
nameplate label on the appliances, then use the formula WATTS = VOLTS x AMPS.
Then divide the calculated wattage of the load by the system battery voltage to
determine the amperage the load will draw from the batteries.
(AC current) x (AC voltage)/(battery voltage) = DC amps
Watts
Example:
To determine DC amps when AC amps are specified on the label:
Nameplate label specifies 6 amps at 120 Vac.
The system battery voltage is 24 volts DC.
• First determine the wattage by using the formula:
WATTS = VOLTS x AMPS = 120 x 6 =720 watts.
• Then divide the wattage by the system battery voltage to determine the
DC amperage.
720/24 = 30 amps DC
To determine DC amps when watts are specified on the label:
Nameplate label specifies 720 watts.
The system battery voltage is 24 volts DC.
(watts)/(battery voltage) = DC amps
(720/24 = 30 DC amps)
B-3