How to calculate the capacity of UPS backup battery?
 Author: FENG, Product Manager in INVT Power
 Origin: INVT Power
 Time of issue:20210201
 Views:
(Summary description)UPS backup battery capacity calculation methods, configure your battery more economically, and maximize battery utility.
How to calculate the capacity of UPS backup battery?
 Author: FENG, Product Manager in INVT Power
 Origin: INVT Power
 Time of issue:20210201
 Views:
(Summary description)UPS backup battery capacity calculation methods, configure your battery more economically, and maximize battery utility.
UPS power system is an indispensable part of the data center power supply and distribution system, and the battery is the main component of the UPS system, and its importance is selfevident. Choosing a battery capacity that is too large will result in a waste of investment. There is no cost advantage. The capacity is too small and cannot meet the UPS backup time. The user experience is not good. The battery discharge rate is too large, which seriously affects the battery performance and life. So how to configure UPS backup battery for customers more economically? At present, the algorithms that are common in the industry are roughly divided into the following three types.
I. Constant Power Method
This method is the embodiment of the law of conservation of energy, and the power that the battery can provide must be equal to or slightly greater than the power of the backup load. calculation process as follows:
First, calculate the power Pnc that the backup load power is evenly distributed on each 2V battery cell
Calculated as follows:
Pnc=P*Pf/η/N/n
Pnc=P*Pf/η/N/n
P: UPS nominal capacity is VA
Pf: UPS output power factor 0.9
N: the number of battery cells (the default number of 12V battery cells is 6)
η: The inverter efficiency is 95%
n: The number of 12V batteries connected in series for each group of batteries (default 40 batteries)
Battery EOD point: 1.75V (default 1.75V)
After calculating the Pnc, according to the constant power discharge characteristic table provided by the battery manufacturer, find out the corresponding discharge power at the corresponding discharge end voltage and discharge time, and compare and filter with Pnc to obtain the required battery capacity and corresponding Number of battery packs.
Example
The 3:3 30kVA (HT33030XL or RM030/15X) UPS of INVT Power, fully loaded with 1h reserve, calculate the battery configuration?
First, calculate the power Pnc of the backup load power equally spread on each 2V battery cell ( 30kVA UPS of INVT Power default battery number is 40 )
Pnc =30*1000*0.9/0.95/6/40=118.4 W/Cell
Second, look up the battery constant power discharge characteristic table and screen for comparison.
12V120Ah battery constant power discharge data sheet
Note: There are differences in this table for different battery specifications and battery brands, please refer to the actual situation
According to the battery constant power discharge data sheet, the 12V120AH battery has a 1hour discharge power of 127W/Cell when the final voltage is 1.75V/Cell. Since 127×1=127W/Cell ＞ Pnc=118.4W/Cell, a group of 40 pcs 12V120AH batteries is needed after comparison, which can meet the full load backup 1H requirement of 30K UPS.
II. Constant Current Method
When the UPS battery is discharged, the battery voltage will decrease with the progress of the discharge. Under the condition of a certain backup load, the discharge current will gradually increase, that is, there is no constant current discharge, so it is also called the estimation method. This method can quickly calculate the battery configuration when communicating at the customer's site, and respond to the customer in a timely manner, eliminating the tedious work of checking tables, screening and comparison in the constant power method. The calculation process is as follows:
First, calculate the maximum battery discharge current Imax
Imax= P*Pf/η/Umin
Imax = P*Pf/η/Umin
P: UPS nominal capacity is (VA)
Pf: UPS output power factor 0.9
η: The inverter efficiency is 95%
Umin: Battery pack discharge protection voltage (default EOD is 1.75V/Cell)
Then calculate the required battery capacity C
C = Imax/Kc
Discharge time 
5min 
10min 
15min 
30min 
1h 
1.5h 
2h 
3h 
4h 
5h 
6h 
8h 
10h 
12h 
Discharge rate C 
2.9 
2 
1.66 
0.92 
0.61 
0.5 
0.36 
0.27 
0.19 
0.18 
0.15 
0.12 
0.1 
0.09 
Kc: Refer to the following table for the discharge rate C value corresponding to different discharge times (different battery specifications and battery brands, this table has differences, please refer to the actual)
Example (same as above)
The 3:3 30kVA (HT33030XL or RM030/15X) UPS of INVT Power, fully loaded with 1h reserve, calculate the battery configuration?
First, calculate the maximum battery discharge current Imax (30kVA UPS of INVT Power default battery number is 40)
Imax =30*1000*0.9/0.95/6/1.75/40=67.7 (A)
Then calculate the required battery capacity C
C=67.7/0.61=110.9 (Ah)
(The discharge rate C value corresponding to 1h discharge time is 0.61)
Therefore, choosing a group of 40 pcs 12V120Ah batteries can meet the UPS backup power demand.
Compared with the constant power method to find the discharge power corresponding to different capacity batteries and then compare and screen, the constant current method only needs to remember the discharge rate C value corresponding to a few common backup times, and you can directly calculate the required battery capacity, which is convenient and fast.
III. 51194 Method
This algorithm mainly refers to the "GB 511942016 Communication Power Equipment Installation Engineering Design Specification", which was formerly called the 5040 method (YD/T 50402005). The algorithm is similar to the estimation method, but takes into account the UPS battery in the entire During use, battery status, environmental factors, and battery capacity attenuation, etc., if the battery is running at a higher ambient temperature, the battery capacity can be calculated more accurately. The specific calculation process is as follows:
Calculation formula： ... ... (Table 5.2.4)
In the formula,
Q: Total capacity of battery pack (Ah);
K: Safety factor (1.25);
I: Load current (A);
T: Discharge hours (h);
Η: Discharge capacity coefficient, see Table 5.2.4;
T: The lowest ambient temperature value of the actual battery location. When there is heating equipment in the location, it is considered as 15℃; when there is no heating equipment, it is considered as 5℃;
α: Battery temperature coefficient (1/℃), when the discharge hour rate is ≥10, α=0.006; when 1≤discharge hour rate<10, α=0.008, when the discharge hour rate<1, α= 0.01.
Table 5.2.4 Leadacid battery discharge capacity coefficient (η) table 

Battery discharge time（h） 
0.5 
1 
2 
3 
4 
6 
8 
10 
≥20 

End of discharge voltage（V） 
1.65 
1.70 
1.75 
1.70 
1.75 
1.80 
1.80 
1.80 
1.80 
1.80 
1.80 
1.80 
≥1.85 

Discharge capacity factor 
Acid proof battery 
0.38 
0.35 
0.30 
0.53 
0.50 
0.40 
0.61 
0.75 
0.79 
0.88 
0.94 
1.00 
1.00 
Valve controlled battery 
0.48 
0.45 
0.40 
0.58 
0.55 
0.45 
0.61 
0.75 
0.79 
0.88 
0.94 
1.00 
1.00 
The load current in Table 5.2.4 shall be calculated as follows：
In the formula,
S: UPS rated capacity (kVA);
cosФ: UPS output power factor;
μ: The efficiency of the inverter;
U: The input voltage of the inverter when the battery is discharged (V), the single battery voltage is 1.85V.
Example (same as above)
The 3:3 30kVA (HT33030XL or RM030/15X) UPS of INVT Power, fully loaded for 1h, calculate the battery configuration?
First, calculate the load current I
I=30*1000*0.9/0.95/1.85/6/40=64.0 (A)
Then calculate the total capacity Q of the battery pack
Q=1.25*64.0*1/0.55/[1+0.008*(525)]=173.2 (Ah)
Therefore, a group of 40 pcs 12V200Ah batteries was selected to meet the UPS backup power demand.
According to these three battery calculation methods, we have listed the battery capacity configuration table calculated by three methods for the backup power requirements of different power sections (appendix 1). From this comparison, the constant power method and the constant current method are calculated The battery capacity is basically the same, and the battery capacity calculated by the 51194 method is significantly higher than the first two, which is about 1.31.7 times. This conclusion is for your reference only. Detailed calculations are still needed in specific projects before a conclusion can be drawn.
All the above battery configurations are calculated based on the UPS backup load unchanged. Now more and more industries, especially in the railway, subway, and energy industries, the UPS system backend load will be based on the nature and purpose. Classification. When the mains power fails, the load exits the system according to the degree of importance. This causes the load at the back of the UPS in the battery mode to change, which is a stepped load. The calculation of the battery capacity in this mode is mostly for reference Relevant chapters of the battery capacity ladder calculation method in "DL/T 5044 Technical Specification for Design of DC Power Supply System in Power Engineering".
Appendix 1：
No. 
Backup demand 
Different methods to calculate battery capacity 
Remark 

Constant Power Method/ Ah 
Constant Current Method /Ah 
51194 Method /Ah 

1 
10KW backup time 0.5h 
38 
28 
46 
Default 1*40 12V battery 
2 
10KW backup time 1h 
38 
42 
65 
Default 1*40 12V battery 
3 
10KW backup time 2h 
80 
70 
116 
Default 1*40 12V battery 
4 
10KW backup time 3h 
100 
93 
142 
Default 1*40 12V battery 
5 
10KW backup time 4h 
120 
132 
179 
Default 1*40 12V battery 
6 
20KW backup time 0.5h 
65 
55 
93 
Default 1*40 12V battery 
7 
20KW backup time 1h 
100 
83 
129 
Default 1*40 12V battery 
8 
20KW backup time 2h 
150 
140 
232 
Default 1*40 12V battery 
9 
20KW backup time 3h 
200 
186 
283 
Default 1*40 12V battery 
10 
20KW backup time 4h 
2*40 pcs 120Ah 
264 
358 
Default 1*40 12V battery 
11 
60KW backup time 0.5h 
200 
164 
278 
Default 1*40 12V battery 
12 
60KW backup time 1h 
250 
246 
385 
Default 1*40 12V battery 
13 
100KW backup time 0.5h 
250 
273 
463 
Default 1*40 12V battery 
14 
100KW backup time 1h 
2*40 pcs 200Ah 
410 
641 
Default 1*40 12V battery 
15 
200KW backup time 15min 
2*40 pcs 200Ah 
302 
464 
Default 1*40 12V battery 
16 
500K backup time 15min 
4*40 pcs 200Ah 
755 
1158 
Default 1*40 12V battery 
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