Why is the ageing factor 1.25?

Lead acid batteries are replaced when they reach 80 percent of rated capacity. Dividing 1 by 0.8 gives 1.25, so a battery sized with this factor still carries the full duty on its last day of service.

Can lithium be used in a substation?

It can, but most operators in the region still specify OPzS, OPzV or nickel cadmium for protection DC because of long float life, proven behaviour and simple charging. Lithium suits sites with daily cycling or severe space limits, with its own battery management and sizing rules.

Which standards apply?

IEEE 485 for lead acid sizing, IEEE 1115 for nickel cadmium, IEC 60896 for stationary lead acid cells, and IEC 62485-2 for battery room safety and ventilation. The operator's own technical specification takes precedence where it is stricter.

How often should a substation battery be tested?

A capacity discharge test at commissioning, then typically every one to three years, more often once capacity falls below 90 percent. Impedance or conductance measurements between tests find weak cells early.

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Sizing a substation battery bank

7 min readLast reviewed: September 2026

Short answer

A substation battery is sized from its duty cycle, not from a rule of thumb. List every load and how long it runs during the backup period, convert the profile into ampere hours, then multiply by a temperature correction, a design margin of about 1.10 and an ageing factor of 1.25, as set out in IEEE 485. The result is the minimum capacity; the next standard cell size above it is selected and checked against the one minute trip load.

Protection relays, breaker trip coils, SCADA and communications all depend on the DC battery when the AC supply is lost. If it is undersized, the substation cannot clear a fault at the moment it matters. This is the method EXBATT follows when specifying stationary banks for transmission and distribution sites.

Step 1: define the duty cycle

The duty cycle is the current the battery must deliver, period by period, from the moment the chargers lose supply until the end of the backup time. A typical substation profile has three parts: a short high current at the start (breaker trips and relay operation), a long continuous standing load (relays, SCADA, RTU, telecom, indication lighting), and a short high current at the end (breaker closing when supply returns). Backup times of 4 to 10 hours are common; the operator's grid code or internal standard sets the figure.

Worked example: 110 V DC, 8 hour backup
Period	Load	Current	Duration	Ampere hours
1	Standing load plus breaker trips	170 A	1 min	2.8 Ah
2	Standing load	20 A	478 min	159.3 Ah
3	Standing load plus breaker closing	100 A	1 min	1.7 Ah
Total			8 h	163.8 Ah

Step 2: cell count and end voltage

The DC equipment sets a voltage window, typically 90 to 110 percent of nominal, so 99 V to 121 V on a 110 V system. The maximum voltage divided by the float voltage per cell gives the cell count: 121 V divided by 2.23 V per cell gives 54 lead acid cells. The minimum voltage divided by the cell count gives the end of discharge voltage: 99 V divided by 54 gives 1.83 V per cell. That end voltage is the column used when reading the manufacturer's discharge tables.

Step 3: apply the three correction factors

Temperature: capacity falls in a cold room. IEEE 485 gives a factor of about 1.19 at 10 degrees Celsius, and 1.00 at 25 degrees. Unheated substation buildings in Kosova and Albania regularly sit at or below 10 degrees in winter. Design margin: 1.10 to 1.15 covers load growth and uncertainty in the load list. Ageing: 1.25, because a lead acid battery is considered at end of life at 80 percent of rated capacity, and it must still carry the full duty at that point. In the example: 163.8 Ah times 1.19 times 1.10 times 1.25 gives 268 Ah minimum.

Step 4: select the cell and check the peak

Round up to the next standard size, here a 300 Ah cell at the 8 hour rate. Then check the first minute: the manufacturer's table must show that the selected cell can deliver 170 A for one minute to 1.83 V per cell, with the same corrections applied. The full IEEE 485 method does this for every period using the manufacturer's capacity factors; the ampere hour estimate above is the check that the result is in the right range. Chemistry follows from the site: OPzS for maximum life where the room is ventilated, OPzV where maintenance access is limited, nickel cadmium where the room is unheated and very cold.

Step 5: size the charger and the room

The rectifier must supply the standing load and recharge the battery within the operator's required time, commonly 8 to 12 hours to 90 percent. For the example: 20 A standing load plus roughly 300 Ah times 1.1 divided by 10 hours, about 33 A, so a charger of at least 55 A, often duplicated for redundancy. Ventilation for vented cells is calculated to IEC 62485-2 from the charge current and cell count. EXBATT then verifies the installed bank with a capacity discharge test at commissioning, which gives the baseline for every later test.

Frequently asked

Why is the ageing factor 1.25?: Lead acid batteries are replaced when they reach 80 percent of rated capacity. Dividing 1 by 0.8 gives 1.25, so a battery sized with this factor still carries the full duty on its last day of service.
Can lithium be used in a substation?: It can, but most operators in the region still specify OPzS, OPzV or nickel cadmium for protection DC because of long float life, proven behaviour and simple charging. Lithium suits sites with daily cycling or severe space limits, with its own battery management and sizing rules.
Which standards apply?: IEEE 485 for lead acid sizing, IEEE 1115 for nickel cadmium, IEC 60896 for stationary lead acid cells, and IEC 62485-2 for battery room safety and ventilation. The operator's own technical specification takes precedence where it is stricter.
How often should a substation battery be tested?: A capacity discharge test at commissioning, then typically every one to three years, more often once capacity falls below 90 percent. Impedance or conductance measurements between tests find weak cells early.