Alpha Technologies FXM 650, 1100, 2000 UPS - Manual (Page 87)

85 017-230-B4 Rev B
Appendix A - Peukert Number and Battery Capacity
A.1 Introduction
The Alpha FXM series UPS units run on batteries when the AC utility power fails. In this mode, the user may want
to estimate the remaining time that UPS batteries can supply power to the loads.
The battery run time remaining is calculated based on the Peukert equation. Two critical parameters are required
for the equation:
• Peukert number.
• Peukert capacity.
The Peukert number and Peukert capacity depend on the battery characteristics. This document describes the
procedures to determine the Peukert number, and Peukert capacity for the selected battery.
Once the Peukert’s number and capacity are determined, enter these values in the GUI or web interface. The
FXM will report the remaining battery run time.
The Peukert’s equation and the remaining battery run time are estimates only. The actual run time may vary
based on various parameters like the age and status of the batteries etc.
A.2 Determining the Peukert’s Number and Peukert’s Capacity
1. Obtain the data sheet of the selected battery.
2. Calculate the nominal load current for the application.
Example: If the load is 150 W and the battery string is 24 Vdc, the load current is calculated as 150 W / 24 V
= 6.25 A.
3. Find the current discharge ratings table in the data sheet. From the table, pick two current discharge values
(I1 and I2) that are closest to the calculated load current and look up the two discharge hours (R1 and R2).
4. Use the following formula to calculate Peukert's number and capacity:
Peukert’s number = n =
Log (R
2
/R
1
)
(Log (I
1
) - Log (I
2
))
(1)
Peukert’s capacity = (I
1
) n x R
1
per battery (2)
A.3 Determining Peukert’s Capacity for Series Parallel Combinations
1. For batteries connected in series, the Peukert’s capacity for the battery bank is given by the equation:
Peukert’s capacity
Battery bank = Peukert’s capacity per battery
Ah
(as calculated in equation (2))
2. For batteries connected in parallel, the Peukert’s capacity for the battery bank is given by the equation:
Peukert’s capacity
Battery bank = Peukert’s capacity per battery
x N Ah
(where N = number of batteries in parallel)