Size the battery to inverter cable

The calculator picks the smallest cross-section that satisfies two conditions at once: the continuous voltage-drop limit (typically 2 % per manufacturer guidance) and the cut-off check under surge load. On a short 1.5 m run the percentage limit is easily met while the cut-off threshold is still violated — which is why a plain percentage calculator is not enough here.

This is the number one failure in self-built systems: during a compressor start the surge current flows, and the drop in the battery cable pushes the voltage at the inverter input below its cut-off — the battery is not the problem. The calculator checks V_at_inverter = V_batt,min − 2 · ρ(T) · L · I_surge / A against the cut-off plus a safety margin. It uses the minimum battery voltage, because surge loads occur precisely at low state of charge.

No — the battery's internal resistance and the additional sag it causes are not modelled, nor are contact resistances at terminals and isolators or the dynamic behaviour in the first milliseconds. The cable's ampacity is a separate check, which the result explicitly flags.

Because current is highest there: the same 3,000 W peak load draws half again as much current at 10.0 V cutoff as at 14.4 V charge end. And exactly at an empty battery comes the worst moment — at night, the fridge compressor starts, the voltage sags. Calculating with nominal voltage sizes the cable for the fair-weather case.

The classic symptom of a too-thin battery cable: at inrush, so much voltage drops across the cable that the inverter momentarily sees less than its shutdown threshold — it disconnects although the battery has plenty of energy. The voltage profile in the chart shows exactly this path; the headroom tile says how much air remains. Remedy: next cross-section or shorter runs, not more battery.

Yes — every terminal, shunt and fuse adds milliohms that cost volts at peak current. The calculator models the cable drop; for the joints the busbar rule applies: assemble cleanly, torque to spec, retorque after the first month. With tight headroom the joints are often the difference between “runs” and “disconnects”.

Yes — since 5 August 2026 via the “check an existing cable” switch. The page then tests your cross-section against both criteria: the permissible continuous-load voltage drop and the cutoff-threshold margin at peak current. In the reference case 16 mm² fails (drop above limit, thin margin to the cutoff), 35 mm² passes cleanly. Unchecked, the page proposes the required size as before — 25 mm².

Almost always because of the battery cable, not the battery: on compressor start the inverter draws 294 A in the reference case — with too thin a cable the voltage at the inverter input sags below the cutoff threshold although the battery terminals still hold enough. Exactly this check is the page's core: voltage at the threshold plus required margin, computed at peak current.

Because peak load strikes exactly when the battery is weak — and current is highest at low voltage: 3,000 W at a 12.0 V minimum is 294 A; computed at nominal voltage it would be only 267 A. Sizing at nominal buys the cable one size too small, discovered on the first winter evening.

No — ampacity is the second, independent check, and at 92.6 A continuous it is often the stricter one. The page says so explicitly as its own note; the ampacity proof with installation method and correction factors lives in the ampacity calculator. Both must pass, plus fusing (DC calculator) and a short, protected run.

The distance between the voltage at the inverter at peak current and its undervoltage cutoff. The 0.5 V default is deliberately tight — contact resistances at terminals and fuse holders add in practice and are not included here. Crimped ring lugs and clean junctions preserve the margin; corroded clamps eat it.