Busbar and battery interconnect sizing basics
The short cables and bars between your batteries and busbars carry serious current - the full bank current in a series string, and each battery's share of it in a parallel bank - but they get a fraction of the attention. Undersized interconnects heat up, drop voltage, and quietly unbalance a parallel bank.
The principle is the same as any conductor: size for the full current the circuit can carry with margin, keep runs short, and keep resistance low and equal.
How it works
The busbar and any series interconnect must be rated for total system current with the same safety-factor thinking as the main run. In a parallel bank each battery's lead nominally carries its share of that total, but most builders size those leads for the full current anyway: the margin covers uneven sharing, and the moment one battery's BMS trips, its siblings carry more. Copper cross-section, clean terminations, and proper torque are most of the job.
Symmetry is the subtle part: in a parallel bank, each battery's path to the load should have equal resistance, or the battery with the shortest path works hardest and ages fastest. Diagonal takeoff (positive from one end of the bank, negative from the other) or a proper busbar with equal-length leads keeps current sharing even.
Common questions
What is the difference between a busbar and interconnect cables?
A busbar is a rigid common tie point rated for the summed current; interconnects are the cables from each battery to it. The busbar always sees the full system current. A series interconnect does too, while a parallel lead nominally carries one battery's share - and is usually sized for the full current regardless, as margin.
Why do parallel batteries need equal-length cables?
Because current divides by resistance. Unequal paths mean unequal sharing: one battery over-cycles while its siblings coast, and the bank's effective capacity and lifespan shrink.
How do I know if my interconnects are undersized?
Warm cables or lugs under heavy load, voltage differences across parallel batteries while charging or discharging, and one battery hitting cutoffs before the others. All are resistance symptoms.
Crimped or soldered lugs?
Properly crimped, with the right die and adhesive heat-shrink. Solder wicks up stranded cable and creates a stiff stress point at exactly the spot that vibrates. Crimping is also far easier to do consistently well.
The builder's wire and fuse guidance covers each segment of your build, computed for the components and distances you actually choose.
Open the builderThese guides are educational and qualitative on purpose - the numeric answers depend on your exact components, and the builder computes them with every formula shown. Always verify a final design with a licensed professional.