E-Bike Controller Heat Management: Heatsink and Cooling Solutions

Table of Contents

• Why Controller Heat Management Matters
• Sources of Controller Heat
• Understanding Thermal Throttling
• Heatsink Options and Upgrades
• Mounting Location and Airflow
• India-Specific Challenges
• Active Cooling Options
• Frequently Asked Questions

Controller overheating is one of the most common causes of reduced performance and premature failure in Indian e-bike builds. The combination of high ambient temperatures (40-45°C in Indian summers), demanding riding conditions (heavy traffic requiring frequent acceleration, steep climbs), and controllers often mounted in poorly ventilated locations creates a perfect environment for thermal problems. Understanding and managing controller heat is not optional — it is a key part of building a reliable, long-lasting e-bike system.

Why Controller Heat Matters

The BLDC controller’s power stage consists primarily of MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) that switch the motor current. Each MOSFET has an on-resistance (Rds_on) that causes a voltage drop proportional to current flow, and this voltage drop × current = power dissipated as heat. At 30A motor current, a controller with typical MOSFET Rds_on of 3mΩ per MOSFET (six MOSFETs in three H-bridge pairs) dissipates approximately 5-15W just from conduction losses, plus switching losses from the high-frequency PWM switching (typically 8-20 kHz).

Total controller heat generation at full current: typically 10-30W for a quality 30A controller, more for higher-current or lower-quality units with higher Rds_on MOSFETs. This heat must go somewhere — it flows through the MOSFET dies, into the PCB, and then into the aluminum heatsink housing. If the heatsink cannot dissipate heat fast enough, temperature rises until thermal equilibrium is reached or the thermal protection triggers.

MOSFET failure mode at excessive temperature: MOSFETs fail by thermal runaway — as temperature rises, Rds_on increases, causing more heat for the same current, which causes more temperature rise in a positive feedback loop. Without thermal protection, this continues until the MOSFET fails short-circuit (catastrophic), potentially passing full battery voltage to the motor and causing it to run at full speed without throttle control — a dangerous failure mode.

Sources of Controller Heat

Conduction Losses (Load-Dependent)

The primary heat source. Proportional to current squared (I²R). Doubling the current quadruples the heat from conduction losses. High-current operation on hills, during acceleration from stops, and in high-assist PAS modes generates the most heat. This is why a controller that runs fine on flat city streets can overheat on sustained hill climbs.

Switching Losses (Speed-Dependent)

Each time a MOSFET switches on or off (at the PWM frequency), there is a brief period where both drain voltage and drain current are non-zero simultaneously — this overlap region represents power dissipated as heat. At higher motor speeds (higher electrical frequency), more switching events per second means more switching losses. Fast-switching drivers and quality MOSFETs with low gate charge minimize but do not eliminate this.

Freewheeling Losses

The body diodes of the MOSFETs carry current during the dead-time between switching transitions. Body diode forward voltage (typically 0.7-1.2V) × current = additional heat during freewheeling periods. Better controllers minimize dead-time; the best use synchronous rectification (actively switching the complementary MOSFET during freewheeling instead of relying on the body diode).

Understanding Thermal Throttling

Most quality e-bike controllers include a temperature sensor (NTC thermistor) on or near the MOSFET heatsink. When this sensor reads above a threshold (typically 75-85°C for the sensor, corresponding to approximately 90-100°C at the MOSFET junction), the controller begins reducing current proportionally to prevent further temperature rise. This is thermal throttling.

Rider experience during thermal throttling: maximum available power reduces progressively. On a hill, you may notice the motor getting noticeably weaker after 5-10 minutes of climbing — this is often thermal throttle, not battery depletion. The effect reverses as the controller cools: stop at the hill crest, wait 3-5 minutes, and full power returns.

If the controller reaches its hard thermal cutoff (temperature sensor reads above the upper limit, typically 100-110°C at the sensor), the controller shuts off completely to prevent damage. Power returns only after sufficient cooling.

Heatsink Options and Upgrades

Stock Controller Heatsink

Most KT and similar Chinese e-bike controllers come with a thin aluminum shell that serves as the heatsink. The PCB’s MOSFETs are in direct contact with this shell (via thermal pads). The stock heatsink provides adequate cooling for moderate use, but its small thermal mass and surface area limits performance under sustained high-current loads.

Inspect the thermal interface between the MOSFET package and the aluminum shell on your controller. Many budget controllers leave this connection dry or with minimal thermal interface material. Adding a thin layer of quality thermal compound or replacing dried-out thermal pads significantly reduces thermal resistance and can lower steady-state MOSFET temperature by 10-20°C.

External Heatsink Addition

For controllers without adequate built-in heatsinking, attaching an external finned aluminum heatsink to the controller housing provides significant additional thermal mass and surface area. Fasten the heatsink to the controller housing with thermally conductive adhesive (available from electronics stores) or with screws after tapping threads. Apply thermal compound between the controller housing and the new heatsink.

Choose a heatsink with fins oriented to align with the direction of airflow when mounted on the bicycle. For a controller mounted under the seat or on the downtube, fins pointing sideways or down catch wind during forward motion.

Heat Pipe Solutions

For extreme applications (60A+ controllers in hot Indian summer conditions), heat pipes can transport heat from the controller PCB to a remote, larger heatsink mounted in better airflow. Used in laptop cooling and adapted for e-bike applications by some builders. More complex and expensive than simple heatsink additions but effective for very demanding builds.

Mounting Location and Airflow

Controller mounting location is the single highest-impact factor in real-world thermal performance. Best practices:

Under the Down Tube or on the Frame

Mounting the controller on the bicycle frame (attached to the down tube, seat tube, or under the top tube) exposes it directly to airflow during riding. Even at 20 km/h, the forced convection from riding dramatically improves heat dissipation compared to a controller inside a bag. The heatsink fins should face the direction of airflow (forward) or sideways to catch wind.

Avoid Closed Bags and Pouches

The most common mistake in Indian e-bike builds: mounting the controller inside a closed pannier bag, battery enclosure, or plastic box without ventilation. The heat has nowhere to go — the controller rapidly heats the interior air of the enclosure, and heat dissipation stops. Controllers mounted this way routinely overheat on Indian summer days even at moderate power levels.

If the controller must be in an enclosure for rain protection, the enclosure must have ventilation — at minimum, slots oriented to allow natural convection flow (cool air in from bottom, hot air out from top), or better, inlet and outlet vents oriented in the direction of travel for ram-air cooling.

Thermal Isolation from the Battery

Lithium batteries perform worse and degrade faster at elevated temperatures. Do not mount the controller in direct contact with or immediately adjacent to the battery pack without thermal isolation. A layer of closed-cell foam between controller and battery pack reduces heat transfer.

India-Specific Challenges

Summer Heat (April-June)

At 45°C ambient in Delhi, Rajasthan, or interior Maharashtra, a controller dissipating 20W into ambient that is already 45°C starts its thermal equilibrium calculation from a very high baseline. The MOSFET junction temperature needed for 20W dissipation may only allow 40-50°C above ambient (heatsink design dependent), putting junction temperatures near 85-95°C before any additional load. This is why controllers that work fine in winter overheat in Indian summer.

Summer strategy: reduce current limits by 20-30% in April-June relative to winter settings, ensure maximum controller ventilation, and avoid sustained high-power climbing on peak-heat afternoons (2-5 PM).

Monsoon Humidity

High humidity during monsoon reduces convective cooling efficiency (humid air is a slightly better thermal insulator than dry air). More importantly, moisture on heatsink fins reduces their effective area. Keep heatsink fins clean — dust mixed with monsoon moisture forms an insulating mud layer on fins that dramatically reduces cooling.

Inspect and clean heatsink fins every 2-3 months in Indian conditions. A compressed air blast or toothbrush removes dust and mud accumulation.

Stop-and-Go Traffic

Mumbai, Delhi, and Bengaluru traffic involves constant acceleration from stops — the highest-current events in normal e-bike operation. Back-to-back stop-start cycling at traffic lights with a high-assist PAS level generates more heat than sustained moderate-power riding. For city builds, size the controller generously (a 40A controller running at 20A thermal maximum is far cooler than a 30A controller at its limit) and set current limits conservatively.

Active Cooling Options

For very high-power builds (45A+) or controllers permanently mounted in sealed enclosures, passive heatsinking may be insufficient. Active cooling options:

Cooling Fan

A 40mm or 60mm 5V/12V fan mounted to blow air over the heatsink fins can dramatically improve cooling. The fan’s power supply can come from the controller’s 5V accessory output or a small DC-DC converter from the main battery voltage. A simple NTC thermistor circuit or a fan controller IC can make the fan speed proportional to temperature — silent when cool, maximum airflow when hot.

Liquid Cooling

Exotic and uncommon in e-bike applications, but used in some high-power commercial e-bike and scooter controllers. Requires pump, coolant lines, and radiator. Not practical for most DIY Indian builds but noted for completeness.

Larger Controller

The simplest active-cooling equivalent is using a larger controller at a lower current fraction of its rating. A 60A controller running at 25A dissipates roughly the same heat as a 30A controller at the same current, but the larger controller has more heatsink mass and surface area. Its MOSFETs run cooler because they have lower Rds_on (higher-rated MOSFETs typically use larger die sizes) and the thermal mass averages out current spikes better.

Frequently Asked Questions

My controller runs hot to the touch but doesn’t cut out. Is this normal?

“Hot to touch” for a metal object is roughly 50-60°C. For an e-bike controller heatsink, this is normal during heavy use. The heatsink is meant to be hot — that is how it transfers heat away from the MOSFETs. Concern begins when the controller starts thermal throttling (reduced power at sustained high-current use) or cuts out. If neither is happening, the temperatures are within design limits.

Can I use automotive thermal paste (like Arctic Silver) on my controller?

Yes — quality CPU/GPU thermal compound is excellent for e-bike controller thermal interfaces. Arctic Silver, Thermal Grizzly, or similar products all provide significantly better thermal conductivity than the mediocre pads typically used in budget controllers. Apply a thin, even layer to the MOSFET package surface before mating to the heatsink.

How do I know if my controller has a temperature sensor?

Check your controller’s LCD display for a temperature reading — KT LCD8H displays show temperature on one of the data screens. For controllers without display temperature readout, open the controller and look for a small NTC thermistor (a small bead-shaped or disk-shaped component, usually epoxied against the MOSFET heatsink area). If present, the controller has temperature monitoring.