In short
- The value covers the balance lead, its crimp and the busbar contact, not the inside of the cell.
- What matters is the comparison: one lead clearly above the others points to a connection to check.
- A poor connection makes a healthy cell look weak under load; fix the connection before blaming the cell.
Among the values a JK board reports, one is easy to overlook: a resistance for each cell, often called cell wire resistance. It is not the internal resistance of the cell. It is what the board measures along the path it uses to read that cell: the balance lead, its crimped terminal and the busbar contact at both ends.
Why the board measures it
The active balancer pushes current through the balance leads. If a lead has a poor contact, the voltage the board reads for that cell is off while current flows, and balancing decisions can go wrong. Measuring the path lets the board warn you, and raise a wire resistance alarm if a lead goes beyond its limit.
Reading the values
The absolute number depends on the lead length, the wire gauge and the board. What tells you something is the comparison between cells. On a tidy build the values sit close together. One value clearly above the rest, say half again as high, is the lead to look at. Two neighbours both high often share a busbar that is not tight.
| What you see | Likely cause |
|---|---|
| One value well above the others | Loose busbar nut or poor crimp on that lead |
| Two neighbouring cells high | The busbar between them |
| Values creeping up over months | Nuts loosening with thermal cycles, or corrosion |
| All values high | Long or thin balance leads; often harmless if even |
How a connection fakes a weak cell
A resistance in the current path adds a voltage drop when current flows. While charging, that cell reads higher; while discharging, lower. The cell looks as if it runs ahead at the top and falls behind at the bottom, exactly like a cell with less capacity. Before replacing a cell, compare its voltage at rest with the pack at rest: if the gap mostly disappears without current, the connection is the problem.
Fixing it
- Switch the pack off and isolate it from the inverter.
- Check the torque of the busbar nuts against the cell datasheet, with an insulated tool.
- Clean contact surfaces; a thin layer of contact paste helps on aluminium terminals.
- Re-crimp or replace a suspicious balance lead terminal.
- Switch on and compare the values again after a charge.
Watching it over time
JK BMS Monitor shows the resistance of every lead on the dashboard, marks the one that stands out and adds it to the health check. Combined with the cell drift at the top and bottom of charge, it tells you whether a cell is really weak or just badly connected.
Common questions
Is the JK wire resistance the internal resistance of my cells?
No. It covers the balance lead, its crimp and the busbar contacts the board uses to read each cell. The internal resistance of a cell needs a dedicated tester.
What value is too high?
There is no universal threshold that fits every build. Compare the cells with each other: one lead clearly above the others, or a value that keeps rising over the months, is the one to check.