The IEEE 485 section method correctly sizes battery capacity for a four-period duty cycle — but the resulting cell count doesn't actually keep the string above its minimum discharge voltage.
| Duty cycle | 5 A for 15 min, then 35 A for 10 min, then 15 A for 75 min, then 45 A for 10 min |
| Correction factors | Temperature 1.0, aging 1.25, design margin 1.1 |
| System DC voltage | 125 V nominal |
| Cell voltages | Nominal 2.17 V, equalize 2.33 V, end-of-discharge 1.75 V |
| Acceptance window | 135–105 V for a 58-cell string |
For each section, every load step (the change in current when a new period begins) is multiplied by the capacity-rating factor (Kt) for the time from the start of that step to the end of the section, then the steps are summed: F = Σ (Ip − Ip−1) × Kt(time from start of step p to end of section).
| Section | Current | Cumulative time | Kt (full duration) | Required capacity |
|---|---|---|---|---|
| 1 | 5 A | 15 min | 1.75 | 8.75 A |
| 2 | 35 A | 25 min | 2.15 | 54.25 A |
| 3 | 15 A | 100 min | 4.10 | 62.75 A |
| 4 | 45 A | 110 min | 4.35 | 110.0 A |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Required battery capacity | n/a (this is the sizing result) | 151.3 Ah | ✓ PASS |
| String voltage at equalize charge | ≤ 140 V | 135.14 V | ✓ PASS |
| String voltage at end of discharge | ≥ 105 V | 101.5 V | ✗ FAIL |
Key insight: Battery capacity sizing (how many amp-hours) and cell-count/voltage-window checking (how many cells, and does the string stay in an acceptable voltage range through the full discharge) are two genuinely separate calculations that can each pass or fail independently — a battery with plenty of capacity can still be the wrong string configuration if its end-of-discharge voltage falls outside what the connected DC equipment can tolerate.
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Open Battery & DC System Sizing calculator →Adding one more cell (59 instead of 58) raises vEodString to 59 x 1.75 = 103.25 V — still short. The real fix here is usually a battery/cell chemistry with a higher end-of-discharge voltage point, or accepting a load-dependent cutoff earlier in the discharge (raising vEodCell), or revisiting the connected equipment's actual minimum operating voltage, since simply adding cells one at a time doesn't close a 3.5 V gap quickly without also pushing the equalize-charge voltage toward its own 140 V ceiling.
The section method reflects that later sections have to be supported by whatever capacity remains after earlier sections have already drawn on the battery — so Section 4's 110.0 A requirement already accounts for the earlier load steps (each weighted by the Kt for how long it has been drawing by the end of Section 4: 5 A × Kt(110 min) + 30 A × Kt(95 min) − 20 A × Kt(85 min) + 30 A × Kt(10 min) = 21.75 + 119.25 − 74.5 + 43.5), rather than needing the sections added to one another separately. Simply summing all four sections' raw currents would badly oversize the battery.