Design guides

From load profile to BOM in five steps

Module sizing is arithmetic, not alchemy. This page walks the five decisions that turn a power profile into a module count — and finishes with a worked example you can re-run against your own rack.

Engineering workbench with a supercapacitor module, calipers and an oscilloscope under cyan task lighting

Before the spreadsheet

Two numbers start everything

Every ride-through spec reduces to a power figure and a time window. The grid needs 1–90 minutes to ramp generation; your load trips in milliseconds. Everything inside that gap is storage you have to engineer — and the smaller the window, the more a capacitor's physics works in your favor.

The five steps below assume an LIC (lithium-ion capacitor) module string, because the millisecond-to-second tier is where LIC wins on energy per size and cycle class.

The five steps

Window, bus, math, balancing, heat

Define the ride-through window

How long must the bus hold, and how deep is the transient? Millisecond events (1–50 ms) are pure LIC territory; seconds-to-minutes bridging stacks LIC modules ahead of a BBU or genset.

Map the bus voltage window

Set V-max at the charger output and V-min at the lowest voltage your load still regulates at. The wider the droop you accept, the more stored energy you can actually sweep out of the capacitors.

Compute capacitance and count

Energy swept per module is ½ C (V²max − V²min). Divide demand by usable module energy — after depth-of-discharge, ESR and conversion losses — and round up.

Choose balancing

Passive or resistive balancing suits strings that float most of their life. Active balancing earns its keep on hard-cycling racks — and keep the standby draw in mind: our boards sleep below 20 µA.

Check the thermal margin

ESR heat at your RMS current sets the temperature rise. Confirm airflow path and cell spacing — forced-air builds open 3 mm air ducts between cells — and verify the −40 °C end if cold starts are in scope.

Worked example

A 120 kW rack riding through 500 ms

GB300-class shelves peak around 120 kW. Say the rack must ride through a 500 ms sag with no BBU behind the modules — here is the whole calculation in six rows.

Step	Value
Rack transient power	120 kW
Ride-through window	500 ms = 0.5 s
Energy demand	120 kW × 0.5 s = 60 kJ
Module swept energy	½ × 166 F × (48.6² − 38²) V² ≈ 76 kJ
Usable after derate (DoD + ESR + conversion ≈ 50%)	≈ 38 kJ per module
Module count	60 ÷ 38 ≈ 1.6 → 2 × RackBridge 48V in parallel

The same table re-runs in any direction: halve the window and a single module carries the load with margin; stretch it to 2 s and the rack wants a battery BBU tier behind the modules — which is exactly how three-tier AI power stacks are built. Drop the final count onto a catalog module such as the RackBridge 48V, or take a non-standard bus to the custom route — samples ship 2–10 pcs with test reports.

Sizing FAQ

Four questions that follow the math

Plan on roughly half. The ½ C (V²max − V²min) sweep between your two bus voltages is what counts, and derates for depth-of-discharge, ESR heat and conversion efficiency stack on top — the worked example above budgets 50% of swept energy as usable.

Charge to the module's 48.6 V class and let the sag floor sit around 38 V — about a 22% droop that most server PSUs and DC-DC stages ride through without dropping regulation. Tighter floors work too; they simply buy less energy per farad.

Resistive or passive balancing is enough for strings that float on standby and discharge rarely. Choose active when the rack cycles deep every shift, when strings run hot, or when module-to-module auto-balancing has to hold a large parallel fleet in step.

Capacitance drops and ESR rises as temperature falls, so size the current for the −40 °C ESR, not the 25 °C datasheet value. LIC chemistry keeps delivering at −40 °C — which is exactly why cold-site pitch and outdoor cabinet projects specify it over batteries that fade below −20 °C.

Check our homework

Run your numbers, then send them.

A five-row table is enough for a 48-hour engineering reply — we will check your window, your derates and your module count, and return a cell-level quote.

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