Stationary battery by duty cycle
Size a stationary battery and charger to a duty cycle.
Try a job 4
Each job fills in example values. Replace them with your values.
- Size a substation battery by duty cycleThe Ah and charger of a 125 V battery for a trip and three hours of load.
- Size a 48 V radio site batteryThe Ah and charger for four hours of radio load and a transmit burst.
- Size a 24 V control panel batteryThe Ah for two hours of PLC load and a valve close at the end.
- Find the charger for a faster rechargeThe charger current to put a 110 V battery back in 8 h, not 12.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Nominal battery voltage
- 110 V
- Cells in series
- 55
- Lowest voltage the loads accept
- 96 V
- Periods, one after another
- 3 rows
- Rate factor
- 1
- Temperature factor
- 1
- Design margin
- 1.1
- Ageing factor
- 1.25
- Load on the charger while recharging
- 10 A
- Recharge time
- 12 h
- Recharge factor
- 1.1
Capacity by Ah
112.1Ah
- Ampere-hours of the duty
- 81.5Ah
- Charger current
- 17.5A
- End voltage per cell
- 1.75V
- Highest current in the duty
- 70A
- Energy at the nominal voltage
- 12.33kWh
This result is a design aid. Verify it against the current standard.
Calculation steps
- Period 1: 70 A for 1 min1.17Ah
- Period 2: 10 A for 478 min79.67Ah
- Period 3: 40 A for 1 min0.67Ah
- Ampere-hours of the duty81.5Ah
- Duty length480 min
- Rate factor, from the maker's table1x
- Temperature factor1x
- Design margin1.1x
- Ageing factor1.25x
- Capacity by AhResult112.1Ah
- End voltage per cell
96 V ÷ 55 cells = 1.745 V
- Charger current17.5A
- Energy at the nominal voltage12.33kWh
About this calculator
How this is calculated
A stationary battery carries a site's d.c. loads when the charger loses its supply: protection relays, breaker trip and close coils, emergency lights, radios, controls. Those loads change through the outage. The duty cycle lists them in order, as a current for a number of minutes.
The tool adds the ampere-hours of each period: the current times the minutes over sixty. That is the charge the duty takes out of the battery.
A battery gives less than its rated capacity when it discharges faster than its rated hours, or to a higher end voltage. So the tool multiplies the duty by a rate factor you read from the maker's table. It then applies a temperature factor for a cold battery room, a design margin for growth and unknowns, and an ageing factor so the battery still does the duty near the end of its life. The product is the capacity by ampere-hours. When a later period draws more than the one before it, that sum is too small, and the tool warns.
The end voltage per cell is the lowest voltage your loads accept, at the battery terminals, over the number of cells in series. Read the maker's capacity at that end voltage.
The charger must carry the standing load and put back what the duty took, with a margin for charge losses, in the recharge time you set. The tool adds the continuous load to the duty's ampere-hours times the recharge factor over the hours.
AmpSize does not hold IEEE 485 or any cell's capacity factors. With the maker's Kt on every period, the tool works the last section, each change in current times its Kt, and that figure governs. Every factor is an assumption you set, and the result says so.
| Step | Table or clause | What it decides |
|---|---|---|
| Duty | Σ I × t ÷ sixty | The ampere-hours the periods take out |
| Capacity | Duty × rate × temperature × margin × ageing | The battery capacity by ampere-hours |
| Sections | Σ ΔI × Kt × temperature × margin × ageing | The capacity with the maker's Kt, which governs |
| End voltage | Lowest voltage ÷ cells | The end voltage per cell to read the maker's table at |
| Charger | Continuous load + duty × recharge factor ÷ hours | The charger current |
Worked examples
- A 110 V substation battery for a trip, eight hours of standing load and a close needs 112.1 Ah. The duty takes 81.5 Ah, with a charger of 17.5 A to recharge in 12 hours.
- A 125 V battery for a trip and three hours of lights and controls needs 159.2 Ah. The duty takes 91.5 Ah, with a charger of 25.1 A.
- A 48 V radio site with a transmit burst needs 49.7 Ah. The duty takes 36.17 Ah, with a charger of 12 A. The burst draws more than the radio load, so the tool warns that the sum understates the battery.
- A 24 V control panel with a valve close at the end needs 11.7 Ah. The duty takes 8.5 Ah, with a charger of 5.2 A. The valve close draws more than the load before it, so the tool warns that the sum understates the battery.
Limits
Without Kt values, the tool adds ampere-hours. A battery's capacity falls at high rates, so a heavy load of a minute or two can need far more than its ampere-hours show. The rate factor carries that, and only the maker's table gives it.
It does not pick a cell, a chemistry or a float voltage, and it does not check ventilation or the battery room. Follow the maker's data for those.
See also: Battery, Fire alarm battery and DC cable size.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
How do you size a battery for a duty cycle?
List what the battery must carry, period by period: the current and the minutes. Add the ampere-hours. Multiply by a rate factor from the maker’s table, a temperature factor, a design margin and an ageing factor. When a later period draws more than the one before it, the sum is too small: use the maker’s Kt and the section method.
Why does a short heavy load need more than its ampere-hours?
A battery gives fewer ampere-hours at a high rate than at its rated rate. A one-minute trip load can need far more capacity than its share of the duty. The rate factor from the maker’s table at your end voltage allows for it.
What end voltage per cell should I use?
Divide the lowest voltage your loads accept, at the battery, by the number of cells. Read the maker’s capacity at that end voltage. A higher end voltage gives less capacity from the same cell.
How big should the charger be?
The charger carries the standing load and puts back the ampere-hours taken out, plus a little more, in the recharge time. The tool adds the continuous load to the duty’s ampere-hours times the recharge factor over the hours.
Does the tool follow IEEE 485?
AmpSize does not hold IEEE 485 or any cell’s Kt values. Enter the maker’s Kt on every period and the tool works the last section of the method. Without them, it adds ampere-hours, applies your factors and warns when a later period draws more.
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