Comprehensive Guide: How to Size Busbars for EV Battery Packs
Every battery pack, from a handheld tool battery to a 400-volt EV traction pack, connects its cells with conductive strips called busbars. These simple-looking copper or nickel bars carry the full pack current, and their cross-section decides how much the pack heats up, how much voltage the system loses, and how long the pack lasts. This guide walks through the complete busbar sizing process with the actual electrical and thermal formulas that the Battery Pack Busbar & Heat-Dissipation Modeler applies.
The starting point is the pack configuration, expressed as S for series and P for parallel. A 14S8P pack has fourteen cells in series per step and eight cells in parallel per step, for 112 cells total. The series steps raise the voltage: fourteen nominal 3.6 V cells produce a 50.4 V pack bus. The parallel count raises the current capacity: at 25 A per cell, eight parallel cells supply 200 A to the bus.
Current is distributed asymmetrically through the pack, and that asymmetry is the heart of busbar sizing. A cell in the middle of a parallel group carries roughly its share of the current, but the series busbar that connects one parallel group to the next must carry the sum of every cell in the group. For a 14S8P pack that means the series busbar carries 200 A while each individual cell carries only 25 A. Designers who size the inter-cell nickel strips from the cell current but size the series busbar from the same number are building a guaranteed hot spot.
The required cross-section comes from current density, the standard design rule for conductors. Copper in a battery busbar is commonly sized at 3 to 5 A per square millimeter for continuous current, while nickel, which resists corrosion and welds well to cell terminals but conducts far worse, is sized at roughly 2 A per square millimeter. At 200 A and 4 A/mm², the busbar needs 50 mm² of copper. A 10 mm wide, 5 mm thick bar provides that; a 10 mm by 1.5 mm bar does not.
Voltage drop follows from Ohm's law and the material resistivity. Copper has a resistivity of about 1.72×10⁻⁸ Ω·m, so a 300 mm long busbar with 15 mm² of cross-section has a resistance near 0.34 milliohms. At 200 A that drops 68 mV, which is about 0.13 percent of a 50.4 V bus. That sounds trivial, but multiply it across every busbar, joint, fuse, and contactor in the pack and the cumulative drop eats into the delivered voltage and power.
Heat dissipation is the second half of the story. The power lost in the busbar is I²R, the current squared times the resistance. Using the same example, 200 A squared is 40,000, and times 0.34 milliohms gives 13.6 watts per bar. The pack then has to shed that heat through the busbar surface to the air and the pack housing. How much the bar heats up depends on the convection coefficient, roughly 10 W/m²·K for a horizontal bar in still air, and on the exposed surface area.
The temperature rise equation ties it together. The rise in degrees is the power loss divided by the product of the convection coefficient and the cooling surface area. A busbar with 0.0069 m² of exposed surface losing 13.6 W at 10 W/m²·K rises about 20 degrees Celsius above ambient. The modeler computes the surface from the bar perimeter, two times width plus thickness, times the length, which is a more honest representation than a flat-plate assumption.
Temperature feeds back into resistance. Copper's resistivity climbs by roughly 0.39 percent per degree Celsius, so a busbar that rises 20 degrees becomes about 8 percent more resistive. The modeler applies this temperature coefficient to report the hot resistance, which is the number that matters for sustained-load operation. On a hot pack, a design that was marginal at room temperature becomes measurably lossier, and the temperature rise compounds.
Practical design therefore iterates through the whole loop. Pick a material, enter the width, thickness, and length, and check that the achieved current density stays under the material limit, that the voltage drop is acceptable to the system, and that the temperature rise keeps the pack cavity within its thermal budget. If any check fails, widen or thicken the bar, shorten the current path, or improve cooling. The Battery Pack Busbar & Heat-Dissipation Modeler turns this loop into instant feedback.
A good busbar design is a balance, not a maximum. Oversized copper is expensive and heavy, undersized bars overheat and waste voltage, and the joints, welds, and plating at each end often contribute more resistance than the bar itself. The formula chain, I²R loss, convection cooling, and temperature derating, is the same whether you are sizing a nickel tab in a 5 amp power-tool pack or a silver-plated copper bar in a 200 amp traction pack.
Run your own pack through the modeler and compare a few configurations. Watch what happens to temperature rise when you halve the thickness, and what happens to voltage drop when you double the length. That hands-on feel for how each variable moves the result is the fastest path to confident busbar sizing on any future pack.