Why Does CAN Communication Fail in Energy Storage Battery Racks?

Intermittent CAN communication in an energy storage battery rack can trigger BMS alarms, missing battery modules, inverter communication faults, charging interruptions, or complete rack shutdown.

The most effective fix is to review the CAN physical layer as a complete system: twisted-pair wiring, termination, topology, connector pinout, shielding, grounding, routing, and common-mode noise.
In high-power ESS cabinets, a CAN harness that works on a bench can become unstable after installation beside DC busbars, contactors, PCS/inverter cables, and switching power electronics.

When communication failures appear only during charging, discharging, contactor switching, or high-power inverter operation, the BMS software should not automatically receive the blame. The wiring harness and EMC environment should be investigated early.

Why Does CAN Communication Become Unstable Inside an Energy Storage Battery Rack?

If CAN_H and CAN_L are routed poorly beside high-current switching circuits, electrical noise can distort the differential signal and create error frames, lost nodes, or intermittent BMS communication.

Use a controlled twisted pair, correct bus topology and termination, and route the communication harness away from major EMI sources.
Battery racks contain contactors, DC power cables, busbars, DC/DC converters, auxiliary supplies, and PCS/inverter interfaces that can create a much harsher electromagnetic environment than bench testing.

The current ISO reference for high-speed CAN is ISO 11898-2:2026, which specifies high-speed CAN physical medium attachment requirements.^[1]

How Do Shielding and Grounding Cause BMS CAN Bus Problems?

Adding a shield does not automatically solve CAN interference; an incorrectly terminated shield, long drain-wire connection, or uncontrolled ground path can reduce EMC performance and introduce new common-mode problems.

Define the shield and grounding architecture at system level instead of connecting the shield wherever a convenient ground point exists.
The required strategy depends on the transceiver, cable, connector, enclosure, EMC targets, grounding architecture, frequency range, and equipment manufacturer's design.

For high-frequency shielding, the physical connection matters. A shield transition with a long pigtail can have significantly different high-frequency behavior from a low-impedance, short or circumferential connection to the intended chassis or enclosure reference.

Do not apply a universal “ground the CAN shield at one end” or “ground it at both ends” rule to every ESS installation. The correct implementation must follow the equipment and EMC architecture, especially when battery racks, PCS equipment, and control cabinets have different ground potentials.

How Should a BMS CAN Wiring Harness Be Designed and Tested?

Replacing BMS boards repeatedly will not fix a physical-layer problem caused by harness topology, termination, connector pinout, shielding, or power-cable coupling.

Troubleshoot from the physical layer upward.
First verify CAN_H/CAN_L continuity and polarity, termination, connector contacts, bus topology, branch lengths, shield continuity, grounding, and harness routing; then inspect waveforms and error behavior under real charging and discharging conditions.

For a conventional high-speed CAN network using two 120-ohm end terminations, measuring across CAN_H and CAN_L on an unpowered network will typically produce approximately 60 ohms when both end resistors are correctly present in parallel. This is a useful diagnostic check, but the actual network architecture must be confirmed before interpreting the measurement.

The harness should also maintain the CAN pair's twist as close as practical to connector termination points. Long untwisted sections at BMS connectors can weaken the pair's ability to reject coupled noise.

For recurring failures, oscilloscope testing at different rack positions can reveal ringing, excessive common-mode movement, distorted dominant/recessive levels, or noise correlated with contactor and inverter switching. ISO 11898-2:2026 is the current ISO high-speed CAN PMA reference and includes ECU/network design material in its annex structure.^[1]

What Should Engineers Specify When Buying a BMS CAN Wiring Harness?

A purchasing drawing that says only “2-core shielded CAN cable” leaves too many uncontrolled variables and can produce different communication performance between harness suppliers.

Specify the complete CAN harness construction and interface requirements.
Useful drawing inputs include conductor size, characteristic impedance requirement where applicable, twist construction, shielding type and coverage, drain wire, shield termination, connector and terminal series, splice locations, branch lengths, maximum untwist length, grounding method, temperature range, and test requirements.

For an ESS battery-rack harness, engineers should also define CAN, RS485, BMS power, interlock, temperature sensing, current sensing, contactor control, and HV cable routing as part of one wiring architecture. Communication reliability can be affected by how these circuits are packaged together.

FAQ

Why Does My BMS Keep Losing CAN Communication?

Common physical-layer causes include incorrect termination, reversed CAN_H/CAN_L wiring, poor connector contacts, excessive stub length, grounding problems, EMI coupling, or damaged communication cables.

Should CAN Bus Cable Be Shielded?

Not every CAN installation requires shielding, but shielded twisted pair can be useful in electrically noisy battery and power-electronics environments. The shield must be integrated into a defined EMC and grounding strategy.

Should a CAN Shield Be Grounded at One End or Both Ends?

There is no universal answer for every ESS system. The correct termination depends on the equipment grounding architecture, cable construction, EMC requirements, connector design, frequency range, and manufacturer recommendations.

Why Are CAN_H and CAN_L Twisted Together?

Twisting helps both conductors experience similar electromagnetic coupling, supporting the noise-rejection advantages of differential signaling.

What Resistance Should I Measure Between CAN_H and CAN_L?

On a conventional unpowered high-speed CAN bus with two 120-ohm termination resistors, the measured resistance is typically about 60 ohms. Different architectures must be checked against their actual design.

Why Does CAN Communication Fail Only When the Inverter Is Running?

That pattern can indicate electromagnetic coupling, common-mode disturbance, grounding problems, or inadequate separation between communication wiring and switching power circuits.

Can Long CAN Stub Wires Cause BMS Communication Errors?

Yes. Excessive branches can create signal reflections, particularly as data rate and network electrical length increase. Keep the network topology and stub lengths within the transceiver and system design requirements.

Can CAN and High-Voltage Battery Cables Run Together?

They should not be routed together casually. Harness separation, crossing geometry, shielding, grounding, and EMC validation should be reviewed when CAN wiring is near high-current or switching power circuits.