Brake Chopper Faults on an Electric Bus: Diagnose the Switch Before You Replace It

Intro (no heading — first paragraphs as plain paragraphs)

These types of support calls usually follow a similar pattern: a bus breaks down on a long downhill stretch, but runs perfectly fine on flat roads. Drivers report that the brakes feel spongy, the warning light comes on, or that the friction braking system is bearing a braking load it shouldn’t have to handle on its own. Subsequently, fleet management often asks how much it would cost to replace a new “chopper.”

Our first piece of advice is usually the same: don’t rush to replace it.

In the vast majority of “chopper” support requests we receive, the problem isn’t actually with the chopper itself. The issue often lies in improperly set threshold parameters, mismatched resistors and switches, insufficient cooling capacity in the cooling circuit, or incorrect signal wire connections. From the driver’s perspective, these issues all manifest as if the chopper were faulty. However, the solutions vary for each situation, and blindly replacing the chopper may actually exacerbate some of these problems. Therefore, before ordering any parts, first categorize the symptoms and then troubleshoot them one by one in order of “lowest cost to highest cost.”

A brief recap (since clearly distinguishing these two components is essential for understanding the following content): The resistor acts as a heat sink, responsible for converting electrical energy into thermal energy; the chopper acts as a switch, controlling when the resistor is connected to the DC bus and for how long. If your inverter has an integrated chopper output module with sufficient current rating, you may only need the resistor; if the inverter does not have one integrated, the chopper is a separate unit. Our article on “How It Works” has already explained this distinction in detail, so this article will focus on potential faults and how to determine which specific component is malfunctioning.

Four Types of Fault Symptoms, Four Major Categories of Problems

Start by examining the vehicle’s actual behavior, rather than relying solely on subjective guesswork.

The first type of issue is an “overvoltage trip” during downhill driving. Everything functions normally when the vehicle is climbing a hill, but when descending (at which point the battery may be fully charged or the battery pack may be at a low temperature), the drive system triggers a protective trip due to excessively high DC bus voltage. At this point, regenerative braking torque disappears, and the driver can clearly feel a change in pedal feedback. This indicates that a component in the braking circuit is failing to dissipate energy—possibly due to a switch failing to close, an open circuit in the resistive circuit, or a blown fuse in the braking circuit. Alternatively, the activation threshold may be higher than the inverter’s trigger point, meaning the protection mechanism has been triggered before the resistor actually needs to engage. All four of these scenarios will cause the fault indicator light to illuminate, but their solutions differ.

The second type of fault manifests as “heat in areas where it should not be present.” If the resistor remains hot while the vehicle is stationary, or if its temperature rises progressively with each trip along the same route, this typically indicates that a switch has remained closed. If a short circuit occurs in the chopper, the resistor becomes directly connected to the busbar and loses switching control; at this point, the resistor no longer absorbs energy in pulses but is forced to absorb energy continuously. Setting the turn-on threshold below the battery’s float charge voltage can also lead to similar consequences, though the process is more subtle: since the chopper mistakenly believes the busbar voltage is consistently too high, it remains continuously on during normal driving. In either case, the resistor itself is not the “culprit”—it is merely performing the function it is designed to carry out once connected.

The third type of fault is an intermittent fault that comes and goes. For example, a fault may be triggered while driving on Tuesday, yet everything functions normally on Wednesday. Such faults often appear only after adjustments to the driving route, software updates, or system modifications, and do not occur during the initial commissioning phase. Intermittent faults are typically related to the “operating window.” The margin between the battery’s maximum charging voltage and the inverter’s trigger point (i.e., the operating window) is inherently narrow and also varies dynamically: when the battery temperature is low, the acceptable charging current is lower; when the battery is nearly fully charged, its voltage is closer to the upper limit; and longer downhill sections or heavier loads inject more energy into the system, thereby squeezing the operating window—which was previously sufficient. If a fault occurs only after a specific change has been made, that change should be examined before suspecting the hardware itself.

The fourth type of fault relates to thermal management. The system operates normally during the first descent but begins to derate or directly cuts off the braking circuit during the second or third descent. This indicates that the heat input exceeds the system’s heat dissipation capacity. The root cause may be that the duty cycle encountered on the actual driving route exceeds the resistor’s design capacity; it may be that the cooling circuit’s actual heat dissipation performance falls short of design expectations; or it may be that the temperature signal is not being transmitted to the controller in a format it can recognize, causing the system to continuously supply power to an already overheated resistor. When addressing heat-related faults, avoid blind guesswork at all costs. You cannot resolve the issue by simply raising a threshold that has not yet been measured.

Diagnostic Procedure

Please follow this procedure and record your observations for each step before proceeding to the next one. If the fault log lacks trigger conditions, it is virtually useless; if only resistance readings are recorded but the bus voltage data at that time is missing, the situation is even worse, as this can easily mislead one into mistaking it for evidence of a fault.

First, review the fault log and its accompanying trigger conditions. These conditions include: the fault code, the State of Charge (SOC) at the time, the gradient, vehicle speed, and—if recorded by the vehicle—coolant temperature and flow rate. Next, ask the question that can save the most money throughout the entire repair process: Is this a new fault, or has it been present since the vehicle was first put into service? If the fault appeared after a route change, increased load, or software update, the problem likely stems from those changes; if the fault has persisted, it indicates an issue with system selection or parameter settings. We have seen cases where both situations were mistakenly diagnosed as “switch failures”—in the first scenario, the solution often involves simply communication and coordination, rather than replacing parts.

Second, compare the bus voltage at the time of the fault with the thresholds specified in the interface specifications for your model. The chopper’s turn-on voltage must be higher than the battery’s maximum charging voltage and lower than the inverter’s trip (protection) threshold. This voltage range is central to the entire diagnostic process. If the recorded bus voltage is below the trigger threshold, it indicates that the chopper was not triggered at all; the issue lies with the parameter settings or the signal, not the switch itself. If the voltage exceeds both of these thresholds, it indicates that the switch or resistive circuit has completely failed to dissipate the energy. Never directly apply threshold data from manuals for other platforms. The industrial drive equipment commonly found in search results—such as 480V systems—is entirely different from the system you are working with, and the bus architecture also differs. The specific values you need should be found in the interface specifications for the equipment you are using. If you do not have this documentation, please add it to the “Supplier Inquiry List” at the end of this article; never make assumptions.

Third, after electrically isolating the vehicle according to the vehicle manufacturer’s specified procedures, measure the cold-state resistance of the resistor. Compare the measurement results with the nominal value on the nameplate and its allowable tolerance range. Taking our HVH-R1402 model as an example, its cold-state resistance is 2.24 ohms, with an allowable tolerance of ±10%. If the measurement indicates a circuit break (open circuit), this directly explains the cause of the overvoltage trip: since there is no current path, energy cannot be absorbed; the inverter (or drive) will only detect that the circuit is open when actual braking is required. If the measured value differs significantly from the nominal value on the nameplate, this indicates a problem with the resistor itself, which requires inspection. If the measured value meets specifications, a problem with the resistor can be ruled out, and the focus of the troubleshooting can shift back to the switching elements and their settings—although this may seem like a dead end, it actually represents significant progress in the troubleshooting process.

Fourth, when the bus voltage reaches its maximum value, verify that the chopper’s rated current matches the resistor’s specifications. Actual calculations should be performed here, rather than relying solely on the nominal values listed in the product catalog. The power dissipated by the resistor is equal to the square of the bus voltage divided by the resistance value. At 450 V, our 2.24-ohm resistor dissipates approximately 90 kW of power and carries a current of about 201 A when fully conductive; at 850 V, the same resistor dissipates approximately 323 kW of power, with the current reaching about 379 A. Although the resistor and switching elements remain unchanged, the power has more than tripled simply because the voltage doubled. If we account for the resistor’s cold-state tolerance, the current at 850 V could even reach approximately 422 A. If the chopping stage’s rated current is lower than the resistor’s actual current requirement at the maximum bus voltage, then the switching elements are the weak link in the system, and replacing the resistor will not resolve the issue. Conversely, if the chopper’s rated current has sufficient margin, it indicates that the switching elements are functioning normally, and the fault lies elsewhere.

Fifth, inspect the cooling system in the same manner. The unit we are using, supported by a liquid-cooling circuit, can achieve a continuous rated power of 140 kW for 20 minutes; under these operating conditions, the coolant flow rate at the DN32 connection must reach 172 liters per minute. This means that within 20 minutes, the cooling circuit must dissipate 168 megajoules of heat. If the coolant flow rate or temperature at the installation site fails to meet the values anticipated during the selection calculations, the resistor will be forced to derate due to insufficient cooling or may trip under operating conditions where it previously functioned normally; In such cases, a failure that should be attributed to a cooling system (piping) issue may instead be blamed on the chopper. Be sure to verify the actual operating parameters of the cooling circuit, rather than relying solely on the specified values in the design drawings.

Sixth, check the relevant signals—these should be verified as actual electrical signals, rather than merely cross-referenced against labels on the circuit diagram. NTC temperature readings should fluctuate reasonably in response to changes in coolant temperature and braking load. If the readings remain constant or deviate significantly from the coolant temperature, it indicates that the controller is unable to obtain accurate temperature information, and its decisions to derate operation or shut down cannot be trusted. In high-voltage interlock circuits, it is essential to confirm that connectors are fully seated before enabling the chopper. If the interlock circuit consistently fails to close, the braking circuit will be unable to engage—even if the switch itself is not faulty. Failure to properly address any of the above issues during the commissioning phase will result in the type of intermittent failure discussed in this article; for this reason, both signals should be included in the pre-delivery checklist rather than being addressed only during subsequent troubleshooting.

Three Don’ts

Never connect a second resistor in parallel on your own to solve an overheating problem without re-verifying the chopping current. Connecting resistors in parallel changes the resistance across the switch, which in turn alters the current it must carry. Doing so may shift the point of failure from the inexpensive resistor to the expensive chopping transistor. The root cause of overheating lies in excessive energy input and insufficient heat dissipation; therefore, the issue should be addressed by adjusting the duty cycle or cooling method, rather than arbitrarily changing the resistance value based on intuition.

Never raise the turn-on threshold to eliminate false triggering. Raising the threshold to a level close to the inverter’s protection trigger point may provide a few weeks of “stability,” but it creates a hidden hazard: if the vehicle descends a hill with a fully charged battery and a low battery pack temperature, the bus voltage may rise faster than the chopper can respond. If the operating range is indeed too narrow, consult with the inverter supplier to negotiate adjustments to the protection trigger point and charging limit, rather than making arbitrary adjustments on the roadside.

Never bypass (short-circuit) temperature signals or interlocks to keep the vehicle running. Bypassing interlocks disables the sole safety barrier—which would otherwise prevent the chopper from continuing to supply power to the resistor if a high-voltage connector becomes loose. Bypassing temperature signals suppresses the early warning before thermal protection is triggered. Furthermore, if you detect a burning smell or see smoke near the resistor, stop the vehicle immediately and isolate (de-energize) it according to the manufacturer’s specified procedures. Never re-energize the braking circuit to test whether the fault reoccurs. Some tests only need to be performed once in a lifetime.

What Information Should Be Provided to the Supplier

If the above checks still fail to resolve the issue, please return the battery pack (or relevant components) to us, rather than simply providing your conclusions. Based solely on fault codes, we (and any other supplier) cannot accurately diagnose issues related to threshold ranges. Please provide the following information: DC bus voltage range; the controller’s current chopping output configuration and its rated current; cold-state resistance values; thresholds set in the interface specifications; fault codes and the operating conditions under which they occurred; coolant flow rate and temperature at the resistor’s installation location; and the vehicle’s operating parameters during downhill driving. In addition, please be sure to note one point (which is often more important than people realize): What changes have occurred recently? For example: route changes, increased load, software updates, or equipment modifications. If no changes have occurred and the fault persists, please state this clearly as well. This points to a “sizing” issue—one that can be fully resolved through theoretical calculations on paper before anyone even touches the vehicle.

A chopper is a device with a simple structure but a demanding task: when the battery can no longer accept energy, it frequently (thousands of times per second) connects a resistor to the high-voltage bus. Once a failure occurs, people often tend to replace the switch immediately, hoping that this will resolve the issue. Be sure to resist this impulse. First, categorize the fault symptoms and prioritize low-cost inspections; use calculations to determine whether the problem lies with the switch, the resistor, the parameter settings, or the cooling system. In many cases, the problem can be easily resolved without purchasing any new parts.

Frequently Asked Questions (FAQ)

How can you distinguish between a “chopper fault” and an “open-circuit resistor”?

First, check the cold-state resistance value and the bus voltage at the time the fault occurred. An open-circuit resistor will cause an overvoltage fault on its own, but the inverter (or drive) can only detect this issue when braking is required. If the resistance value is within specifications and the bus voltage has exceeded the trigger threshold, but the resistor is not dissipating energy, you should focus on troubleshooting the switching elements, fuses, or enable signals. The key to distinguishing between these two types of faults lies in reviewing the fault logs along with the operating conditions, rather than simply observing the fault indicator lights.

Why does the fault only occur on a specific route?

Because only that route met the conditions that triggered the fault: for example, the battery was fully charged at the start of a long downhill section, the battery pack was at a low temperature, or frequent downhill runs occurred without sufficient recovery time. Faults always occur in conjunction with energy flow. Compare the downhill operating conditions and state of charge (SOC) on that route with those on a normal route, and check whether the duty cycle on that route exceeds the rated limit.

Can the vehicle continue to operate if the braking circuit is derated?

This must be done in accordance with the vehicle manufacturer’s guidelines for this fault. “Derating” means that the energy that would normally be absorbed by the resistive circuit is now handled by the friction braking system. On flat sections, this may have little impact; however, on long downhill sections, it means the braking system loses its margin of safety and must bear the entire braking load on its own. Please consider “derating” as a deadline for completing fault diagnosis, rather than a normal operating condition.

If the inverter already has a braking output function, is an additional chopper required?

If the rated current of this output function is sufficient to drive the resistor at the maximum bus voltage (taking tolerance ranges into account), and the interface specifications clearly define the corresponding threshold range, then no additional chopper is required. If the above conditions are not met, a separate chopper unit must be installed, or the resistor value must be adjusted. Such decisions must be confirmed through written calculations before ordering components.

Why is the resistor heating up when the vehicle is stationary?

This indicates that the resistor is connected to the bus while it should not be. Please investigate the following possibilities: a switching element is stuck in the closed position, the threshold is set below the battery’s float charge voltage, or a circuit fault is causing the circuit to remain continuously energized. Please de-energize the vehicle (or disconnect the relevant circuit) according to the manufacturer’s specified procedures; under no circumstances should you continue to operate the vehicle to observe subsequent developments. If the resistor heats up while the vehicle is stationary, it indicates that the switch has malfunctioned; this is one of the types of faults mentioned in this document where failure to address the issue promptly will cause the situation to worsen.