How to Calculate the Number of Batteries for UPS Systems

01/21/2025Battery Knowledge news 33810

An Uninterruptible Power Supply (UPS) system is a critical component of modern power protection, providing short-term power backup to ensure the continuous operation of essential devices during power outages. The number of batteries required for a UPS system is directly related to the backup time it can offer. The battery capacity and number needed depend on several key parameters, such as the UPS power rating, backup time, and battery voltage. Let’s break down how to calculate the required number of batteries for a UPS system.

1. Key Factors Affecting UPS Battery Quantity

When calculating the number of batteries needed for a UPS, the following factors play a crucial role:

Load Power
The load power of the UPS determines the battery capacity required. UPS systems typically use apparent power (measured in VA) instead of real power (W) when calculating battery needs.
Termination Voltage (V_low)
This refers to the voltage at which a battery can no longer provide usable power. For a 2V battery, V_low is typically set at 1.7V; for a 12V battery, it’s 10.2V.
Float Voltage (V_float)
The float voltage is the voltage used to keep the battery in standby mode and ensure its longevity. For 2V batteries, V_float is set at 2.3V, and for 12V batteries, it is typically 13.8V.
Battery Capacity Conversion Factor (Kh)
This factor varies with discharge duration. A 10-hour discharge rate has a Kh of 1, a 5-hour rate is 0.9, a 3-hour rate is 0.75, and a 1-hour rate is 0.6.
Battery Current (I)
This is the current required to supply power to the UPS load, directly influencing the required battery capacity and number.
Backup Time (T)
The backup time determines how long the UPS can supply power without AC input. Longer backup times require more batteries.
UPS DC Voltage (V)
UPS systems typically operate at DC voltages ranging from 24V to 384V. The required battery configuration depends on the system’s voltage.

Portable Power Station WPP300-02 — The SunVolt Portable Power Station

2. Basic Calculation Method for UPS Battery Quantity

Using the parameters above, the number of batteries required can be calculated using the following formulas:

Number of Batteries (N):
For a 12V battery system, the number of batteries needed is calculated as:
N=V12VN = \frac{V}{12V}N=12VV
For example, a UPS with a 36V system would require 3 batteries of 12V each.
Battery Current (I):
The current required from the battery is:
I=PtotalVI = \frac{P_{\text{total}}}{V}I=VPtotal
For a 1000VA UPS with 36V, the current would be:
I=100036=28AI = \frac{1000}{36} = 28AI=361000=28A.
Actual Battery Capacity (C):
The required battery capacity is calculated as:
C=I×TKhC = \frac{I \times T}{K_h}C=KhI×T
For example, with 28A of current and 4 hours of backup time at Kh = 0.9, the required battery capacity would be:
C=28×40.9=124AHC = \frac{28 \times 4}{0.9} = 124AHC=0.928×4=124AH.

3. Example of UPS Battery Configuration

For a 1KVA UPS requiring 4 hours of backup time with a 36V system and 100AH batteries, the calculations would be as follows:

Number of Batteries:
N=36V12V=3N = \frac{36V}{12V} = 3N=12V36V=3
Three 12V batteries are required.
Battery Current:
I=100036=28AI = \frac{1000}{36} = 28AI=361000=28A
Battery Capacity:
C=28×40.9=124AHC = \frac{28 \times 4}{0.9} = 124AHC=0.928×4=124AH

Therefore, the UPS system could be configured with 3 x 100AH batteries or 6 x 65AH batteries, depending on the cost and user needs.

4. Conclusion

Calculating the number of batteries for a UPS system is a complex task that involves several parameters, including UPS power, battery voltage, backup time, and battery capacity. By performing careful calculations, one can ensure that the UPS system can provide stable power backup during outages, meeting user requirements effectively.

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