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How to calculate the capacity of UPS backup battery?
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How to calculate the capacity of UPS backup battery?

  • Author: FENG, Product Manager in INVT Power
  • Origin: INVT Power
  • Time of issue:2021-02-01
  • 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:2021-02-01
  • Views:

(Summary description)UPS backup battery capacity calculation methods, configure your battery more economically, and maximize battery utility.

Information

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 self-evident. 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 1-hour 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 51194-2016 Communication Power Equipment Installation Engineering Design Specification", which was formerly called the 5040 method (YD/T 5040-2005). 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  Lead-acid battery discharge capacity coefficient (η) table

Battery discharge timeh

0.5

1

2

3

4

6

8

10

≥20

End of discharge voltageV

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*(5-25)]=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 1.3-1.7 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 back-end 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
Unless otherwise stated

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
Unless otherwise stated

15

200KW backup time 15min

2*40 pcs 200Ah

302

464

Default 1*40 12V battery
Unless otherwise stated

16

500K backup time 15min

4*40 pcs 200Ah

755

1158

Default 1*40 12V battery
Unless otherwise stated

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