Alta document for keeping bike away from thermal limiting
A Guide to Thermal Limiting
Alta’s vehicles continuously monitor the temperature of all critical components inside the battery pack, charger, motor, and motor controller. In case any of these components are in danger of exceeding their safe operating temperatures, the vehicle’s control system will gradually restrict maximum output power to ensure that the system remains safe. Maximum output power will be restored as temperatures come back down. These “soft” limits are intended to allow a rider to continue using the vehicle safely without risking damage or a sudden loss of power due to overheating.
Limiting begins when battery cell temperatures reach 150F/65C, or when the motor reaches 265F/130C; these are the two most commonly experienced limits. Different types of riding will cause different heating rates in these components. Generally, riding at high speed and moderate torque (like highway cruise) will lead to aggressive heating in the motor and cause it to limit first while riding at moderate speeds and high torque (such as on a motocross track) will cause the battery to heat more quickly and become the limiting component.
Several factors can help keep these components from hitting their thermal limits. These include:
Staying in maps 1-3, or generally using less throttle while riding
Keeping the surfaces of the battery, motor area and chassis clean. Caked on dirt or mud will act as insulation and make it harder for the vehicle to shed heat.
Taking a “cool down lap” or riding at low throttle before bringing the bike to a stop.
Cooling the battery before starting a ride, or externally cooling between rides will help. This can be done with water or cold air (Alta does not recommend using anything else). Alta recommends not cooling the pack below 50F 10C, as reduced power may result.
Direct sunlight has a major warming effect, utilizing shade to keep your bike cool is recommended.
Battery Thermal Limiting
The Thermal Limiting scheme uses a control loop to limit battery power output for temperatures 150*F and above. Temperatures are monitored at many different points in the battery pack and throughout the high voltage system. When the system is limiting, the user will see reduced "top end" power (peak battery power vs. peak torque). The system will continue to reduce power at an increasing rate if the battery temp continues to rise, especially if the rider keeps using the bike under full load at speed.
Thermal Limiting is managed with the following inputs:
Discharge rate: The continuous average discharge rate is the dominant factor and will define the slope of temp rise over time. Map 4, the Redshift’s overclocked” map, uses around 80% more power than map 2 in a high discharge scenario and has a correspondingly steeper temp rise slope; it can take only minutes for the battery to go from ~80*F to 150*F in high output scenarios. The largest influence in temperature rise and therefore power limiting is the continuous average power being sourced from the pack. Trail riding in Map 4 rarely results in power limiting. While this style of riding will have peak power spikes, the average continuous power is fairly low and so are the heating effects. In our testing at 80*F a pro rider rode for 20 minutes in map 2 before reaching the thermal limit of his motorcycle; Alta believes this is acceptable for amateur MX where race times are usually 15 minutes).
Static Cooling rate: The rate at which a battery cools from peak temp can vary widely based on external conditions. With air moving over the pack and at a low discharge rate, the pack will maintain a temperature 10-20* over ambient (think trail riding). When the bike is at rest and starting at a high temp (just back from riding mx) it will take up to 2 hours to cool to ambient.
Ambient: If a battery pack starts off at 105*F, the battery pack’s temperature can only increase by 45*F degrees before it reaches its thermal limit. Starting with a pack that is already above ambient further reduces the time before limiting happens.
Charging: Charging creates some heat in the pack. If the expectation is to ride hard directly after charging there will be some amount of reduced thermal overhead. If the bike came in hot and was put on the charger the cooling rate will be reduced because of the heat being generated by charging - instead of 2 hrs to cool it may be 3 hrs.
External Cooling: Water, mist, and especially airflow over the battery case will dramatically improve cooling. With a strong fan the cooling time is reduced to ~1/2 hr, even less with a fan, mist, and evaporation on the case surface.
For some use-cases the battery temp will need to be managed with a combination of moving air cooling and evaporative cooling. Alta can’t recommend spraying the battery with anything other than water.
The biggest factor in reducing the effects of thermal limiting are using map 1-3 instead of 4. The next largest factor is by utilizing a riding style with more momentum and less “point and shoot”. Any time dirt or sand is being pumped (roosted), heat is being generated at higher rates. Often this roosting is an indication of too much power being put to the ground relative to the conditions. In the case of the Redshift, riding style might need to be adjusted in order to get maximum range prior to Thermal Limiting.
Motor/Inverter Thermal Limiting
The Motor and Inverter are closely linked when referencing Thermal Limiting. The inverter is what is pulling the energy out of the battery and sending it to the motor when there is a request for torque from the rider. Both of these components are Liquid Cooled with the same cooling circuit on the Alta Redshift. Liquid Cooling doesn’t have as much as an effect with an Electric Motor as it does with an Internal Combustion Engine (ICE) due to the fact that the maximum temperatures aren’t nearly as high with the Electric Motor. The differential in temperature between ambient air and the thermal threshold isn’t great enough for an ambient air heat exchanger to be as effective as it is with an ICE.
The motor of the Redshift is most efficient at providing massive thrust at low RPM in short bursts under high loads. Due to the way the motor is constructed this efficiency is lessened when it is utilized for medium loads at high rpm. This means that after a few minutes at steady-state 70+mph speeds the motor will reach its Thermal Limit. When this happens, the Redshift will sense the Thermal Limit and reduce the output of the motor.
A Guide to Thermal Limiting
Alta’s vehicles continuously monitor the temperature of all critical components inside the battery pack, charger, motor, and motor controller. In case any of these components are in danger of exceeding their safe operating temperatures, the vehicle’s control system will gradually restrict maximum output power to ensure that the system remains safe. Maximum output power will be restored as temperatures come back down. These “soft” limits are intended to allow a rider to continue using the vehicle safely without risking damage or a sudden loss of power due to overheating.
Limiting begins when battery cell temperatures reach 150F/65C, or when the motor reaches 265F/130C; these are the two most commonly experienced limits. Different types of riding will cause different heating rates in these components. Generally, riding at high speed and moderate torque (like highway cruise) will lead to aggressive heating in the motor and cause it to limit first while riding at moderate speeds and high torque (such as on a motocross track) will cause the battery to heat more quickly and become the limiting component.
Several factors can help keep these components from hitting their thermal limits. These include:
Staying in maps 1-3, or generally using less throttle while riding
Keeping the surfaces of the battery, motor area and chassis clean. Caked on dirt or mud will act as insulation and make it harder for the vehicle to shed heat.
Taking a “cool down lap” or riding at low throttle before bringing the bike to a stop.
Cooling the battery before starting a ride, or externally cooling between rides will help. This can be done with water or cold air (Alta does not recommend using anything else). Alta recommends not cooling the pack below 50F 10C, as reduced power may result.
Direct sunlight has a major warming effect, utilizing shade to keep your bike cool is recommended.
Battery Thermal Limiting
The Thermal Limiting scheme uses a control loop to limit battery power output for temperatures 150*F and above. Temperatures are monitored at many different points in the battery pack and throughout the high voltage system. When the system is limiting, the user will see reduced "top end" power (peak battery power vs. peak torque). The system will continue to reduce power at an increasing rate if the battery temp continues to rise, especially if the rider keeps using the bike under full load at speed.
Thermal Limiting is managed with the following inputs:
Discharge rate: The continuous average discharge rate is the dominant factor and will define the slope of temp rise over time. Map 4, the Redshift’s overclocked” map, uses around 80% more power than map 2 in a high discharge scenario and has a correspondingly steeper temp rise slope; it can take only minutes for the battery to go from ~80*F to 150*F in high output scenarios. The largest influence in temperature rise and therefore power limiting is the continuous average power being sourced from the pack. Trail riding in Map 4 rarely results in power limiting. While this style of riding will have peak power spikes, the average continuous power is fairly low and so are the heating effects. In our testing at 80*F a pro rider rode for 20 minutes in map 2 before reaching the thermal limit of his motorcycle; Alta believes this is acceptable for amateur MX where race times are usually 15 minutes).
Static Cooling rate: The rate at which a battery cools from peak temp can vary widely based on external conditions. With air moving over the pack and at a low discharge rate, the pack will maintain a temperature 10-20* over ambient (think trail riding). When the bike is at rest and starting at a high temp (just back from riding mx) it will take up to 2 hours to cool to ambient.
Ambient: If a battery pack starts off at 105*F, the battery pack’s temperature can only increase by 45*F degrees before it reaches its thermal limit. Starting with a pack that is already above ambient further reduces the time before limiting happens.
Charging: Charging creates some heat in the pack. If the expectation is to ride hard directly after charging there will be some amount of reduced thermal overhead. If the bike came in hot and was put on the charger the cooling rate will be reduced because of the heat being generated by charging - instead of 2 hrs to cool it may be 3 hrs.
External Cooling: Water, mist, and especially airflow over the battery case will dramatically improve cooling. With a strong fan the cooling time is reduced to ~1/2 hr, even less with a fan, mist, and evaporation on the case surface.
For some use-cases the battery temp will need to be managed with a combination of moving air cooling and evaporative cooling. Alta can’t recommend spraying the battery with anything other than water.
The biggest factor in reducing the effects of thermal limiting are using map 1-3 instead of 4. The next largest factor is by utilizing a riding style with more momentum and less “point and shoot”. Any time dirt or sand is being pumped (roosted), heat is being generated at higher rates. Often this roosting is an indication of too much power being put to the ground relative to the conditions. In the case of the Redshift, riding style might need to be adjusted in order to get maximum range prior to Thermal Limiting.
Motor/Inverter Thermal Limiting
The Motor and Inverter are closely linked when referencing Thermal Limiting. The inverter is what is pulling the energy out of the battery and sending it to the motor when there is a request for torque from the rider. Both of these components are Liquid Cooled with the same cooling circuit on the Alta Redshift. Liquid Cooling doesn’t have as much as an effect with an Electric Motor as it does with an Internal Combustion Engine (ICE) due to the fact that the maximum temperatures aren’t nearly as high with the Electric Motor. The differential in temperature between ambient air and the thermal threshold isn’t great enough for an ambient air heat exchanger to be as effective as it is with an ICE.
The motor of the Redshift is most efficient at providing massive thrust at low RPM in short bursts under high loads. Due to the way the motor is constructed this efficiency is lessened when it is utilized for medium loads at high rpm. This means that after a few minutes at steady-state 70+mph speeds the motor will reach its Thermal Limit. When this happens, the Redshift will sense the Thermal Limit and reduce the output of the motor.
