CAN Bus Communication
How the Solterra charger uses CAN bus to communicate with the battery BMS - the protocol, state handshaking, message flow, and what happens when the communication link breaks.
What is CAN Bus?
CAN (Controller Area Network) is a robust, differential serial bus originally designed for automotive applications. It allows multiple devices - charger, BMS, vehicle controller - to share a single 2-wire bus (CAN_H and CAN_L) and communicate using message IDs with built-in error detection.
In the Solterra charger context, CAN enables the charger to receive real-time battery status from the BMS and for the BMS to command the charger with exact voltage and current limits - enabling safe, efficient, closed-loop charging that voltage-only chargers cannot achieve.
1 twist per 25 mm is recommended for most applications.
CAN Bus Twisted Pair Design Guide
Controller Area Network (CAN) bus uses a differential signaling scheme over a two-wire bus: CANH and CANL. The physical layer specification expects these two wires to be twisted together as a twisted pair, because that geometry is fundamental to the bus's noise immunity and signal integrity at higher bit rates.
CAN bus depends on common-mode noise rejection. Any electromagnetic interference from motors, inverters, ignition systems, or nearby harnesses tends to couple equally onto both CANH and CANL when the wires remain close and consistently twisted. The CAN transceiver reads only the differential voltage CANH - CANL, so identical noise on both wires cancels at the receiver.
Without twisting, the two conductors can take slightly different physical paths, pick up unequal noise, and lose the symmetry that differential signaling relies on. Twisting also reduces the loop area between CANH and CANL, which lowers radiated emissions and makes the pair less vulnerable to magnetic-field induction.
| Parameter | Typical Requirement |
|---|---|
| Twist rate | 10-40 twists per metre with a common target around 20-30/m |
| Lay length | 25-100 mm per full twist |
| Consistency | Twist rate must stay uniform along the cable run |
A tighter twist improves noise immunity, but it also increases stiffness and cable cost. For automotive CAN based on ISO 11898-2, harness teams often target roughly 25-33 twists per metre for medium-speed buses, and tighter rates when the harness runs near traction inverters or other high-EMI zones.
CAN bus has a 120 ohm characteristic impedance target. The twist rate, conductor size, insulation thickness, and dielectric material all combine to set that impedance.
- Typical wire gauge: 0.22 mm² to 0.5 mm² (AWG 24 to AWG 20) for most automotive and industrial applications
- Characteristic impedance: off-the-shelf CAN cables are typically pre-characterised to the required 120 ohm target
- Termination: both ends of the bus must use 120 ohm resistors between CANH and CANL to prevent reflections
| Type | Use Case |
|---|---|
| UTP (Unshielded Twisted Pair) | Most automotive and industrial CAN runs in typical EMI environments |
| STP (Shielded Twisted Pair) | Preferred near traction inverters, motor drives, or antenna systems; shield should be grounded at one end only to avoid loop currents |
In EV charging and powertrain harnesses, STP is commonly preferred where the CAN run passes close to power electronics. UTP remains acceptable in lower-noise zones such as BMS-internal harnesses or instrument clusters.
- Stubs must be minimised: keep branches under 30 cm at 500 kbit/s, and under 10 cm for CAN FD at 2 Mbit/s
- Do not untwist at connectors: keep the pair twisted as close to the pins as possible
- Keep the same pair together: CANH and CANL must remain on the same twisted pair from node to node
- Respect bend radius: excessive bending can distort the twist geometry and shift local impedance
| Standard | Scope |
|---|---|
| ISO 11898-2 | CAN high-speed physical layer, including transceiver and cable expectations |
| ISO 11898-3 | CAN low-speed fault-tolerant physical layer |
| SAE J1939-11 | Heavy-vehicle CAN physical layer using shielded twisted pair and 120 ohm impedance |
| IEC 62228 | EMC requirements for CAN transceivers |
| CiA 601 / CiA 603 | CAN FD physical layer recommendations |
The twisted-pair requirement in CAN is not just convention; it is the physical mechanism that makes differential signaling work reliably in noisy electrical environments. The practical priorities are: match 120 ohm impedance, keep a consistent twist rate, minimise stubs, and use shielded cable when the harness runs near inverters or high-current switching stages.
Charger-BMS Communication Flow
When AC power is connected, the charger follows a strict communication sequence before delivering any charge current.
Charger-BMS State Machine
A cleaner state view of the charger logic: listen first, validate CAN and battery voltage, use activation only when needed, then branch clearly into charging, completion, or fault.
View Mermaid Code
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flowchart TD
AC["STATE 0<br/>AC DETECTED<br/>AC input is connected"]
LSN["STATE 1<br/>LISTENING<br/>Passive CAN listen window for 5 seconds"]
HSK["STATE 2A<br/>CAN HANDSHAKE<br/>Read BMS voltage, limits, and charge request"]
VCHK{"Battery voltage<br/>already present?"}
ACT["STATE 2B<br/>ACTIVATION<br/>Pulse Vmax / Ilow for up to 10 seconds"]
CHG["STATE 3<br/>CHARGING<br/>Closed-loop current and voltage control"]
DONE["STATE 4<br/>CHARGE COMPLETE<br/>BMS stop command or current tapers to near zero"]
FLT["STATE F<br/>FAULT MODE<br/>No CAN and no sensed voltage after activation"]
NOTE1["PASSIVE LISTEN WINDOW<br/>No charge output is allowed during the 5 second listen window"]
NOTE2["ACTIVATION RECOVERY PATH<br/>Used only when the BMS is asleep, isolated, or still holding the MOSFET open"]
AC -->|T = 0| LSN
LSN -->|CAN detected within 5 s| HSK
HSK -->|Evaluate pack state| VCHK
VCHK -->|Voltage present| CHG
CHG -->|BMS stop command or current tapers| DONE
LSN -->|No CAN after 5 s| ACT
VCHK -->|0 V / MOSFET open| ACT
ACT -->|Comms recovered and voltage sensed before 10 s| CHG
ACT -->|10 second timeout| FLT
NOTE1 -.-> LSN
NOTE2 -.-> ACT
class AC state-idle
class LSN state-listen
class HSK state-handshake
class VCHK state-decision
class ACT state-activation
class CHG state-charge
class DONE state-complete
class FLT state-fault
class NOTE1,NOTE2 state-note
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linkStyle 10 stroke:#fa913c,stroke-width:1.8px;Immediately upon AC connection, the charger enters a passive listening state. No output is produced. The charger listens on the CAN bus for messages from the BMS. This 5-second window allows the BMS to boot and establish communication before the charger acts.
If the BMS responds within 5 seconds, the charger reads the battery's current voltage, state of charge, and requested charge limits. If the battery voltage is already above the minimum threshold, the charger proceeds directly to normal charging. If battery voltage reads 0V (BMS MOSFETs are open), the charger initiates the Activation procedure.
During charging, the BMS continuously broadcasts charge parameters: max charge voltage, max charge current, and protection flags. The charger adjusts its output in real time to stay within these limits. This enables precise SOC-based charging profiles (CC/CV) and allows the BMS to reduce or stop charging if a cell approaches a limit.
The BMS sends a stop-charge command when target SOC is reached, or the charger self-terminates when output current drops to near-zero at the CV voltage. The relay opens, output ceases, and the green LED lights constant - indicating charge complete and safe power-off.
BMS Message Types
| Direction | Message Purpose | Key Data Fields | Update Rate |
|---|---|---|---|
| BMS -> Charger | Charge Request | Max_V, Max_I, SOC, Status | 1000 ms |
| BMS -> Charger | Protection Flags | OVP, OTP, OCP, CellBalance | 1000 ms |
| Charger -> BMS | Charger Status | V_out, I_out, State, Fault | 1000 ms |
| Charger -> BMS | Heartbeat | Alive counter | 1000 ms |
What Happens When CAN Fails?
CAN Lost During Charging
If the CAN link drops mid-charge, the charger has no updated setpoints from the BMS. Depending on configuration, the charger may continue at the last known setpoints for a brief timeout window, then reduce current to a safe minimum or stop entirely. A fault LED pattern will activate.
CAN Never Established
If 5 seconds pass with no CAN messages, the charger assumes either the battery is deeply discharged (BMS is off) or a wiring fault. It enters Activation Mode - pulsing Vmax/Ilow to try to wake the battery. If no voltage appears within 10 seconds, the charger enters fault mode. See the Activation page.
Common CAN Wiring Issues
- CAN_H and CAN_L swapped (differential reversed)
- Missing 120 ohm termination at one or both ends
- Corroded or intermittent connector pins
- Incorrect baud rate configured
- Ground loop (CAN GND not tied to chassis)
Diagnosing CAN Issues
Measure between CAN_H and CAN_L with the bus active. Expect ~2.5V idle, swinging to ~3.5V (dominant) / ~1.5V (recessive). A flat line at 0V or 5V indicates a short or open fault. Use a USB-CAN adapter (SavvyCAN / ZCANPRO) to verify message traffic.
Tools: SavvyCAN | ZCANPRO | PCAN-ViewContinue the Diagnostics
Keep moving between the support guides below to narrow the issue faster.
Activation / Wakeup Function
See how Solterra chargers wake deeply discharged batteries and what risks apply during activation.
Step-by-Step Troubleshooting
Pick a symptom and follow the correct field checks before escalating to service.
LED Fault Codes
Review Solterra LED fault codes, match blink patterns, and use the interactive simulator and reference table.