Draw the connections before adding the numbers
Cells in series add voltage while keeping the same ampere-hour capacity. Identical strings in parallel keep the same voltage and add ampere-hours. Energy in kWh is voltage × ampere-hours ÷ 1,000 at the stated nominal conditions.
The BESS studio models each “module” as one complete series string at the target nominal system voltage. Its modules within a rack are parallel. Commercial vendors may use “module” for a lower-voltage block; map their topology explicitly before entering counts.
Audit the default 1,500 V target
With 3.2 V nominal cells and a 1,500 V nominal target, the tool rounds upward: ceil(1,500 ÷ 3.2) = 469 series cells. At 280 Ah, that string has a nominal energy of 469 × 3.2 × 280 ÷ 1,000 = 420.224 kWh. Eight parallel strings give 3,361.792 kWh and 2,240 Ah, while nominal voltage remains 1,500.8 V.
| Cell operating point | 469-cell string | 900–1,500 V PCS window |
|---|---|---|
| 2.80 V minimum | 1,313.20 V | Within lower boundary |
| 3.20 V nominal | 1,500.80 V | Already above maximum |
| 3.65 V maximum | 1,711.85 V | Above maximum |
The 1,500 V label is not an operating-voltage ceiling. A full-charge condition is incompatible with a 1,500 V maximum PCS in this example. Adding parallel racks increases energy and current capability; it does not reduce string voltage.
Calculate a possible series-count interval
Maximum cells = floor(PCS maximum V ÷ cell maximum V)
For these illustrative limits, ceil(900 ÷ 2.8) = 322 and floor(1,500 ÷ 3.65) = 410. A count between 322 and 410 passes this simple static voltage check. For example, 400S gives 1,120–1,460 V and a 1,280 V nominal value.
This interval is only a screening test. It does not account for loaded voltage sag, DC cable drop, tolerance, temperature, balancing margins or a converter whose full-power operating window is narrower than its absolute limits. Supplier-approved series counts and insulation design can further constrain the options.
Repeat the energy and current calculation
Changing from 469S to 400S reduces each 280 Ah string to 358.4 kWh. Eight parallel strings then provide 2,867.2 kWh per rack. If the nominal energy requirement stays the same, the rack count may rise.
The same AC power also requires greater current at lower DC voltage. Recheck battery continuous C-rate, BMS and contactor limits, fuses, cables and busbars at the lowest operating voltage. A passing energy result is never a substitute for those checks.
Read the warning as a design stop
The studio deliberately exposes its default voltage conflict. Resolve the equipment combination before taking the rack count into a proposal. Preserve the input values and the warning in the exported study so another reviewer can reproduce the result.
For a purchasing specification, request the actual cell operating envelope, string diagram, nominal and usable energy bases, full-power PCS DC range, minimum-temperature rating and protection philosophy. The website does not certify that any topology complies with installation or safety standards.
Sources and scope
- U.S. DOE: Energy Storage Grand Challenge Roadmap (2020) ↗
Storage terminology and performance context. The voltage arithmetic above is an independent illustrative equipment check.
Reference links provide technical context; the worked examples and interpretation are specific to this guide. No external source certifies these tools.