Abstract: Low-temperature environments severely degrade the usable energy of an electric car. One important remedy is to recover waste heat from the electric drive system and use it to heat the traction battery directly. In this study I take a non-heat-pump electric car as the research object and quantify how the introduction of motor waste heat utilization affects the battery temperature, the discharge window, the recovered braking energy, and the winter driving range. My approach combines analytical heat balance models, three-dimensional CFD simulations, and full-vehicle experiments on a chassis dynamometer inside a climatic chamber. The simulation results indicate that, after two CLTC driving cycles, the battery average temperature reaches approximately 10°C when heated by the motor cooling water, while the maximum temperature difference remains below 5°C. These results satisfy the battery thermal management requirement. The experimental verification is performed with the same electric car by externally controlling the coupling valves and pumps. When motor waste heat is enabled, the battery temperature increases from 1°C to 13°C. The final state-of-charge at the end of the test is reduced from 7.0% to 5.75%, which corresponds to an additional usable energy of about 1 kWh. The average recovered braking energy intensity improves by 0.3 kWh/100 km. As a final result, the winter driving range of the electric car is extended by 5 km, from 281 km to 286 km, and the range decay rate is reduced from 48.9% to 48.0%. The findings demonstrate that waste heat utilization is a low-cost and technically effective solution for improving the winter performance of non-heat-pump electric cars.
1. Introduction
With the continuously increasing market share of new-energy cars, thermal management has become a pivotal supporting technology that affects almost every aspect of vehicle performance. For an electric car, thermal management directly influences energy consumption at high and low temperatures, power capability in hot and cold climates, cabin comfort, durability, and safety. Among these numerous performance dimensions, the impact on energy consumption at low temperatures is particularly important. Many electric-car owners complain that the driving range is dramatically shortened when the cabin heater is used in winter. This complaint has become one of the most painful issues in daily use of electric cars.
Many factors related to thermal management affect the low-temperature driving range of an electric car. The thermal management architecture, the air-conditioning control strategy, the battery thermal management system, and the heat source strategy are the main contributors. In recent years, many researchers inside and outside my organisation have investigated waste heat recovery from electric drive systems. Some studies have focused on thermodynamic analyses of waste heat utilization forms and quantified the influence of the ambient temperature and the waste heat quantity on the system coefficient of performance. Other studies have performed experimental research on heat-pump air-conditioning systems with waste heat recovery and showed that the system performance can be improved by using the heat rejected by the electric motor. However, those investigations mostly considered electric cars equipped with heat pumps.
At present, non-heat-pump electric cars are still the majority on the market. In the market segment of my research vehicle, the optional installation rate of a heat pump system is less than 10%. Therefore, studying the direct influence of motor waste heat on the battery loop and on the overall vehicle performance of a non-heat-pump electric car is practically more meaningful. The purpose of this work is to explore how a relatively simple coupling between the electric-drive cooling circuit and the battery cooling circuit can bring the waste heat of the motor into the battery pack, and to quantify the resulting benefits on battery temperature, usable energy, braking energy recovery, and final winter driving range.
The research described in this paper is based on an existing compact electric car that was already in production. The car has a lithium-ion high-voltage battery pack with a nominal capacity of about 74 kWh, an integrated permanent-magnet synchronous motor, a power-electronics inverter, and a DC-DC converter. The cabin heating system is a conventional positive-temperature-coefficient (PTC) air heater instead of a heat pump. In my work I have only added an intelligent control strategy that operates an already existing electromagnetic valve, thereby allowing the motor cooling water to flow through the battery cold plate. No additional hardware was installed in the vehicle. This aspect is particularly attractive from a cost perspective.
The structure of this article is as follows. First I describe the thermal management topology and the waste-heat-use control logic. Then I present a mathematical model that describes the heat generation of the electric drive system and the heat balance of the battery loop. After that, a three-dimensional CFD simulation is carried out to find the battery temperature field when motor waste heat is used. Next, I describe a full-vehicle experimental validation performed in a climatic chamber with a chassis dynamometer. Finally I discuss the results and draw conclusions.
2. Thermal Management Architecture and Control Strategy
2.1 System Architecture
The thermal management architecture of the electric car investigated in this work is a coupled liquid-cooling and liquid-heating system. The overall architecture contains two main water circuits: the electric-drive circuit and the battery circuit. They can be operated independently or connected by a single electromagnetic coupling valve. This valve is labelled Valve 1 in my descriptions. Each circuit is powered by its own electric water pump. The electric-drive circuit consists of the electric motor, the power electronics, the DC-DC converter, the main radiator, a thermostat, and the associated hoses. The battery circuit consists of the high-voltage battery pack with a cold plate, an interior heat-exchanger (chiller) for refrigerant-based cooling, an electric PTC heater for heating, the battery water pump, and the coupling valve. The two circuits share one expansion tank and the same type of coolant, which reduces component count and assembly complexity.
The main advantage of this topology is that when the electromagnetic coupling valve is open, the coolant heated by the motor and power electronics flows directly into the battery cold plate. No intermediate secondary heat exchanger is required, so the heat-transfer efficiency from the motor waste heat to the battery cells is high. Another advantage is that the battery circuit has a separate PTC heater. The PTC can be used to heat the battery even when the motor is cold, and it can also be combined with motor waste heat to accelerate the warm-up of the battery on extremely cold days. Finally, because the two circuits share one expansion tank, the system is simplified and maintains the same coolant concentration in both loops.
A summary of the main components and their functions in the research electric car is given in Table 1.
| Component | Loop | Function |
|---|---|---|
| Electric motor | Electric-drive circuit | Main propulsion and main waste heat source |
| Power electronics | Electric-drive circuit | Converts DC to AC; generates waste heat |
| DC-DC converter | Electric-drive circuit | Supplies low-voltage loads; generates waste heat |
| Main radiator | Electric-drive circuit | Rejects excess heat to ambient air |
| Thermostat | Electric-drive circuit | Controls bypass flow through radiator based on coolant temperature |
| Electromagnetic coupling valve | Both | Connects or disconnects the two water loops |
| Battery PTC heater | Battery circuit | Heats coolant before entering the battery cold plate |
| Chiller | Battery circuit | Cools battery coolant through the refrigerant loop |
| Battery pump | Battery circuit | Pumps coolant through the battery loop |
| Electric-drive pump | Electric-drive circuit | Pumps coolant through the electric-drive loop |
Table 1. Main components of the thermal management system in the investigated electric car.
2.2 Waste-Heat-Use Control Logic
In my research vehicle, the battery-pack heating function is classified into two categories: motor waste-heat heating and PTC heating. The motor waste-heat heating logic is implemented as follows. When the battery temperature is lower than 0°C, when the ambient temperature is lower than the battery temperature, and when the electric-drive loop coolant temperature is higher than the battery temperature by at least 8 K, then the battery circulation loop is coupled with the electric-drive circulation loop. In this situation, the motor coolant carries waste heat into the battery cold plate, thereby warming the battery cells. If the waste heat alone is insufficient to meet the battery heating demand, the battery loop is switched back to the small independent loop and the PTC heater directly heats the coolant inside the battery loop. The control logic can be summarized with the following condition:
\[
\text{Enable waste heat}\iff (T_{bat}<0^\circ C)\land(T_{amb}T_{bat}+8\,K).
\]
This logic can be embedded directly in the vehicle thermal management controller without any additional sensor. Since the motor-battery coupling valve and both pumps are already available in the vehicle architecture, the only necessary action is to modify the software calibration. This makes the implementation extremely cheap. Of course, the control logic must also consider the minimum temperature of the electric-drive loop, the maximum allowable cooling-water temperature of the battery, and the durability requirements of the electromagnetic valve. In my tests I intentionally simplified the logic to allow a clear quantification of the thermal effect.
2.3 Calculation of Waste Heat Power
The waste heat available in an electric car originates from the loss power of all heat-dissipating components in the electric-drive water loop. In this particular system, the electric motor, the power electronics and the DC-DC converter are the three principal heat sources. The motor heat loss is calculated from the motor efficiency map and the instantaneous operating point. The following formula gives the motor heat-generation power:
\[
P_{et}=P_m(1-\eta_m),
\]
where \(P_{et}\) is the motor heat generation, \(P_m\) is the motor input electric power, and \(\eta_m\) is the motor operating efficiency. The heat losses of the power electronics and the DC-DC converter are measured directly from the coolant-side temperature rise and flow rate during experiments. Table 2 lists the average heat-generation powers of these components under the CLTC driving cycle at -7°C ambient temperature.
| Component | Average heat generation power [W] |
|---|---|
| Electric motor | 1750 |
| Power electronics | 150 |
| DC-DC converter | 35 |
| Total | 1935 |
Table 2. Average heat generation power of the electric-drive components in the CLTC cycle at -7°C.
It is important to note that the total heat generation of 1935 W is the thermal energy available in the electric-drive circuit. Not all of this heat can be transferred to the battery. A portion of it is lost to the ambient air through hoses, the motor housing, the radiator during the periods when the thermostat is open, and the still cold pipe walls. Another portion increases the coolant temperature in the electric-drive loop. The actual heat that enters the battery cold plate depends on the coolant mass-flow rates of the two pumps, the size of the coupling orifice, the mixing temperature, and the heat capacity of the battery. In my analytical model I define the recovered waste heat as the heat transferred from the electric-drive coolant into the battery cold plate:
\[
\dot Q_{waste}=\dot{m}_{bat}\,c_p\,\bigl(T_{bat,in}-T_{bat,out}\bigr),
\]
where \(\dot{m}_{bat}\) is the mass-flow rate through the battery cold plate, \(c_p\) is the specific heat capacity of the coolant, \(T_{bat,in}\) is the coolant temperature at the battery inlet, and \(T_{bat,out}\) is the coolant temperature at the battery outlet. During the coupled mode, \(T_{bat,in}\) is elevated by the motor waste heat and therefore the battery receives a positive heat input.
3. Mathematical Model of the Battery Thermal Balance
To understand how the waste heat affects the battery and the vehicle energy consumption, I developed a zero-dimensional thermal balance model of the battery pack. The battery is considered as a lumped thermal mass with an average temperature \(T_b\). The energy balance equation can be written as
\[
m_b\,c_{p,b}\frac{dT_b}{dt}
= \dot Q_{waste}
+ \dot Q_{ptc}
+ \dot Q_{gen}
– \dot Q_{amb}
– \dot Q_{cool},
\]
where \(m_b\) is the battery mass, \(c_{p,b}\) is the specific heat capacity of the battery pack, \(\dot Q_{ptc}\) is the heat injected by the battery PTC heater, \(\dot Q_{gen}\) is the internal heat generated by the battery itself due to electrical losses, \(\dot Q_{amb}\) is the heat loss from the battery surface to the surrounding environment, and \(\dot Q_{cool}\) is the heat removed by the chiller or the radiator when active. In the waste-heat mode, \(\dot Q_{ptc}\) can be reduced or entirely avoided.
The internal heat generation of the battery is a function of the current \(I_b\) and the internal resistance \(R_b\):
\[
\dot Q_{gen}=I_b^2\,R_b(T_b,SOC).
\]
The internal resistance increases strongly at low temperatures. Therefore, if the battery temperature remains very low, not only does the battery produce less usable discharge power, but its internal losses also increase, reducing the energy efficiency of the entire electric car. When the battery is warmed by motor waste heat, \(R_b\) decreases, so the same discharge current produces less heat. This creates a positive feedback in terms of energy use: the warm battery becomes more efficient and thus retains more energy for driving.
The heat exchanged with the ambient can be approximated by
\[
\dot Q_{amb}=UA\,\bigl(T_b-T_{amb}\bigr),
\]
where \(UA\) is the overall heat-transfer conductance of the battery pack to the ambient. In winter, \(T_{amb}\) is much lower than the target battery temperature, so the ambient loss is a significant sink. The waste-heat power \(\dot Q_{waste}\) must first compensate for this loss before raising the battery temperature. In the measured ambient condition of -7°C, my calculations indicate that almost 400 W of the recovered heat was lost to the ambient and the rest contributed to the sensible temperature rise of the battery. This explains why the measured battery warm-up rate was moderate even though the waste-heat source had an average power of nearly 1 kW.
3.1 Improvement of Discharge Capability
One of the most important influences of battery temperature is on the permissible discharge and charge windows. Manufacturers normally restrict the battery operating voltage and current to protect the cells from lithium plating and excessive voltage drops at low temperatures. In my research electric car, the high-voltage battery has a certain discharge lower limit that is expressed as a state-of-charge limit. At low temperature, the discharge termination SOC must be kept higher to prevent the cell voltage from dropping below the cut-off voltage during high-power demands.
Figure 3 from the original vehicle calibration (not repeated here) shows the discharge power limit as a function of SOC and temperature for the battery cells used in the electric car. When the battery is cold, the discharge power limit is severely reduced at low SOC values. In contrast, when the battery is at about 13°C instead of 1°C, the cell resistance is lower and the discharge power limit is higher for the same SOC. As a result, the battery energy management unit can allow a deeper discharge before reaching the voltage limit.
I model the usable discharge energy as
\[
E_{usable}=\int_{SOC_{low}(T)}^{SOC_{high}} U_{oc}(SOC)\,Q_{bat}\,dSOC,
\]
where \(U_{oc}\) is the open-circuit voltage, \(Q_{bat}\) is the battery charge capacity, \(SOC_{low}\) is the lower SOC limit that depends on temperature, and \(SOC_{high}\) is the upper SOC limit. Since the waste heat increases \(T_b\), the lower SOC limit is reduced. In the experiment I measured that the lower SOC limit decreased from 7.0% to 5.75%. The corresponding additional energy is given by
\[
\Delta E_{usable}=\int_{0.0575}^{0.070} U_{oc}(SOC)Q_{bat}\,dSOC\approx 1.0\,kWh.
\]
This 1 kWh is a direct extension of the usable battery capacity in winter. It was not generated by the waste heat itself, but rather by the ability of the battery controller to safely exploit the stored energy that had previously been hidden behind a conservative SOC limit.
3.2 Improvement of Regenerative Braking Energy Recovery
Another important benefit of a warmer battery is improved regenerative braking. During regenerative braking, the electric motor operates as a generator and charges the battery. The maximum allowable charging current is limited by the battery charging power capability, which also depends on temperature. At low temperature, the charging power limit is drastically reduced, as indicated in the charging power map of the battery (again not reproduced here). Therefore, a cold electric car can collect only a small portion of the kinetic energy during braking; the rest is dissipated by the friction brakes.
When motor waste heat warms the battery, the maximum charge power at a given SOC increases. Consequently, the energy management system can set a higher negative torque limit during deceleration. In my measurements, the average regenerative-braking energy intensity increased by 0.3 kWh per 100 km. The total recovered energy during the entire test can be calculated by integrating the product of the braking power and the energy-recovery efficiency:
\[
E_{reg}=\int \eta_{reg}(T_b,SOC)\,P_{brake}\,dt,
\]
where \(P_{brake}\) is the deceleration power demand and \(\eta_{reg}\) is the regenerative efficiency, which is higher when the battery can accept a larger charge current. The 0.3 kWh/100 km improvement is modest but non-negligible over a full winter test cycle.
4. Simulation Study of Battery Temperature with Waste Heat Utilization
Before performing expensive vehicle tests, I carried out a three-dimensional CFD simulation to evaluate the spatial temperature distribution of the battery pack when the motor waste heat is introduced. The goal was not to model every cell locally, but to verify that the proposed coolant-side strategy would not create large in-plane temperature gradients that could degrade the battery consistency or accelerate local aging. In the simulation, I used the commercial CFD solver with a conjugate heat-transfer model. The battery pack was discretized with a structural mesh that resolved the cell layers, the cold plates, the aluminium heat-spreading fins, the thermal interface material, and the cooling channels. The coolant flow was assumed to be incompressible and turbulent. The transient solver was then coupled with a simplified vehicle model to produce the coolant inlet temperature boundary condition as a function of time.
4.1 Boundary Parameters
Table 3 reports the thermophysical properties of the materials used in the battery CFD model.
| Material | Density [kg/m3] | Specific heat [J/(kg K)] | Thermal conductivity [W/(m K)] |
|---|---|---|---|
| Aluminium | 2700 | 900 | 217.7 |
| Thermal interface material | 3000 | 1800 | 3 |
| Polypropylene (PP) | 910 | 1900 | 0.5 |
| Ceramic tile (JR) | 2400 | 1300 | 50 |
| Air | 1.16 | 1000 | 0.026 |
| Coolant | 1071 | 3300 | 0.38 |
Table 3. Material properties assigned to the battery CFD model.
The simulation boundary conditions were set as follows. The battery coolant inlet temperature was constant at 13°C, representing the steady-state mixed temperature of the electric-drive coolant during motor waste-heat operation. The coolant volume flow rate was set to 10 L/min. The ambient air around the battery pack was at -7°C and heat transfer by natural convection and radiation was allowed on the external surfaces. The battery initial temperature was set to 1°C to match the cold-soaking condition observed in the subsequent test. The load was represented by repeated CLTC driving cycles, with an electric current profile imposed on the cells.
4.2 Simulation Results
After two consecutive CLTC driving cycles, the simulated average battery temperature reaches approximately 10°C, starting from 1°C. This means that the waste heat flowing into the battery cold plate is sufficient to raise the average temperature by about 9 K. The maximum temperature difference within the entire battery pack is less than 5°C. Such a temperature gradient is acceptable for the cell chemistry used in this electric car. If the same heating task had to be performed solely by the PTC heater, the PTC would consume electrical energy that would otherwise be used for traction. The simulation therefore proves that the motor waste heat is able to provide the necessary heating capacity without requiring additional heating power from the battery.
I also extracted the internal temperature distribution of the module core and the cold plate surface. The steady-state pattern shows that the cells near the coolant inlet are slightly warmer than those near the outlet, but the difference is not excessively large. The coolant temperature rise across the battery pack is approximately 2–3 K, which is typical for the coolant-flow rate of 10 L/min. The results of the simulation were ultimately used as a basis for setting the test control strategy, because the simulation indicated that any intentional unevenness of the valve opening was not necessary.
One interesting observation from the CFD model is that the battery cold plate itself acts as a small secondary radiator when the loop is disconnected. If the coupling valve remains closed and the battery coolant is cold, the cold plate removes heat from the cells and transports it to the chiller if the refrigerant loop is active. In the waste-heat mode, the polarity of the heat transfer is reversed: the cold plate now injects heat into the cells. This is exactly the desired functionality for winter warm-up. The simulation helped me understand that the time constant of the battery thermal mass is on the order of 30 minutes, so the effect of the motor waste heat is clearly seen only after at least one CLTC cycle.
5. Experimental Validation
5.1 Experimental Implementation
To demonstrate the feasibility of the waste-heat-use concept without changing the original vehicle software, I performed a full-vehicle test using an external control arrangement. The electric car was placed in a climatic wind tunnel with a chassis dynamometer. The ambient temperature was held constant at -7°C. The vehicle was soaked for at least 12 hours before the test. During the test, I directly controlled the electromagnetic coupling valve and both pumps from outside the vehicle. The valve was operated by an external voltage switch. Two PWM signal generators were used to control the electric-drive water pump and the battery water pump. This arrangement allowed me to manually close the coupling loop and to establish a well-defined mass-flow ratio between the two circuits.
The test procedure followed a standard electric-car energy-consumption and range test that uses repeated CLTC cycles. The test was identical, except that I was able to switch the waste-heat utilisation on at a predetermined moment. In one vehicle state the coupling valve was kept closed for the entire test; this was the baseline. In the second vehicle state the coupling valve was opened after the motor-loop water temperature reached a certain threshold; this was the waste-heat-enabled case. All other boundary conditions, including tyre pressure, wind speed, loading and battery initial SOC, were kept equal.
5.2 Test Procedure
The test began with a fully charged battery. The electric car was discharged by operating the driver model on the chassis dynamometer over repeated CLTC cycles. The battery coolant temperature, motor inlet/outlet temperatures, and SOC were recorded at a frequency of 10 Hz. After about 65 minutes of driving, the motor coolant temperature reached 17°C. At this point, I manually turned on the electromagnetic coupling valve and the second water pump, thus directing the motor waste heat into the battery cold plate. The coolant flow through the battery was kept at approximately 10 L/min. From that moment on, the battery temperature increased steadily. The temperature traces of the battery with and without waste heat utilization are shown in the discussion below.
5.3 Temperature Results
Figure 6 in the original data set shows the comparison of battery temperature curves; in words, the battery temperature without waste heat utilization remained almost frozen at approximately 1°C for the entire test, whereas with the waste-heat-enabled case it gradually rose from 1°C to 13°C when the test was terminated. The final battery temperature was therefore 12 K higher than the baseline. This increase was achieved solely by the motor waste heat.
During the test, I measured the actual heat flow into the battery using the coolant flow rate and the temperature difference between the battery inlet and outlet. The resulting recovered waste-heat power was calculated as
\[
\dot Q_{waste,actual}
= \frac{1}{\Delta t}\int_{0}^{\Delta t}\dot{m}\,c_p\,\bigl[T_{bat,in}(t)-T_{bat,out}(t)\bigr]dt.
\]
Over the period from the activation of the valve until the end of the test, the average recovered power was between 0.8 kW and 1.0 kW. This is consistent with the available heat generation of the motor and power electronics. The theoretical maximum would be close to the total heat generation of 1935 W, but the actual value is lower because some heat is stored in the thermal mass of the electric-drive circuit and some is rejected to the environment through the radiator and the hoses. Still, the recovered power is sufficient to eliminate or greatly reduce the need for the battery PTC heater.
6. Overall Effects on the Battery and the Vehicle
6.1 SOC Window and Usable Battery Energy
The most direct effect of the increased battery temperature is the relaxation of the discharge termination criterion. In the baseline test without waste heat, the test terminated when the battery reached the minimum allowed SOC of 7.0%. The vehicle controller then limited the discharge power to protect the battery. In the waste-heat-enabled test, the warmer battery allowed the test to continue until the SOC reached 5.75%. This reduction of the lower SOC limit by 1.25 percentage points corresponds to an additional usable energy of about 1 kWh. The battery management system calibrations are intentionally conservative at low temperatures because a cold cell has a higher internal resistance and a larger voltage drop under load. When the waste heat raises the mean cell temperature, the voltage drop becomes smaller and the same minimum voltage is reached later in the discharge process.
I summarize the results in Table 4.
| Parameter | Without waste heat utilization | With waste heat utilization | Difference |
|---|---|---|---|
| Battery temperature at test end [°C] | 1.0 | 13.0 | +12.0 |
| Termination SOC [%] | 7.0 | 5.75 | -1.25 |
| Battery usable energy [kWh] | 72.2 | 73.2 | +1.0 |
| Vehicle driving range [km] | 281 | 286 | +5 |
| Range decay rate [%] | 48.9 | 48.0 | -0.9 |
| Regenerative braking energy intensity increase [kWh/100 km] | — | — | +0.3 |
Table 4. Measured influence of motor waste heat utilization on battery and vehicle performance.
6.2 Effect on Energy Recovery
The warmer battery also increases the available charging power during deceleration. At low temperatures, the maximum charging current is limited to a low value by the battery management system, which means that the electric motor cannot provide the desired braking torque. The extra braking force must then come from the hydraulic brakes, converting kinetic energy to heat instead of storing it in the battery. After the battery warm-up achieved by waste heat, the regenerative braking window expands. My energy analysis shows that the recovered electrical energy per distance travelled increases by 0.3 kWh/100 km. Over a total test of roughly 286 km, this contributes approximately 0.86 kWh of additional electrical energy returning to the battery. Although this contribution is smaller than the 1 kWh benefit from the SOC window relaxation, it is still valuable in winter range extension.
Mathematically, the total electrical energy consumed from the battery over the range test is
\[
E_{total}=E_{used}-E_{reg}+E_{aux},
\]
where \(E_{used}\) is the energy absorbed by the powertrain at the battery terminals, \(E_{reg}\) is the regenerated energy, and \(E_{aux}\) is the energy used by auxiliary devices, including the cabin heater, PTC battery heater if active, pumps, and controllers. The motor waste heat reduces \(E_{aux}\) by turning off the battery PTC heater and it also increases \(E_{reg}\). The combination of these two mechanisms reduces \(E_{total}\) for a fixed vehicle-speed profile, thereby increasing the driving range.
6.3 Energy Efficiency Impact
To discuss the vehicle-level energy consumption, I define the average consumption from the grid as
\[
C_{avg}=\frac{E_{battery,initial}-E_{battery,remaining}}{\int v\,dt} \times 100,
\]
where \(v\) is the vehicle speed. The measured range decay rate is defined as the ratio of the range loss relative to the nominal range under a warm ambient condition:
\[
R_{decay}=\frac{R_{nominal}-R_{winter}}{R_{nominal}}\times 100\%.
\]
With the nominal range of the electric car assumed to be 550 km under a mild CLTC condition, a winter baseline range of 281 km gives \(R_{decay}= (550-281)/550=48.9\%\). With waste heat utilization, the winter range is 286 km, giving \(R_{decay}= (550-286)/550=48.0\%\). The improvement of 0.9 percentage points might seem small at first glance, but it corresponds to 5 km of additional real-world driving, which is meaningful for users with a short commute or occasional access to charging stations.
6.4 How the Range Extension Is Achieved
The 5 km range extension can be decomposed into three parts. The additional 1 kWh of usable energy from the deeper discharge window contributes roughly 3 km if the specific energy consumption is around 0.3 kWh/km. The improved regenerative braking energy contributes about 0.9 kWh of additional returned energy, which would be equivalent to approximately 2.7 km if the total efficiency were unchanged. However, the actual total improvement is only 5 km because some of the additional recovered energy is used to supply the still-running cabin heater and other auxiliaries. There is also a small effect of the reduced internal resistance of the battery: a warm battery loses less energy to heat for the same current, which improves the round-trip efficiency of the traction system. The decomposition is given in Table 5.
| Contribution mechanism | Approx. effect [km] |
|---|---|
| Deeper discharge window (+1 kWh usable capacity) | +3.0 |
| Improved regenerative braking (+0.86 kWh over test) | +2.7 |
| Reduced internal losses and auxiliary heating | +0.5 |
| Additional cabin-heater energy consumption over longer time | -1.2 |
| Net range extension | +5.0 |
Table 5. Approximate decomposition of the range improvement due to waste heat utilization.
This decomposition is only approximate because the actual interactions are nonlinear. For instance, the cabin heater consumes energy in proportion to the total test time, so if the car travels farther, the heater consumes more energy in total. Nevertheless, the measured net range extension remains positive and demonstrates the effectiveness of the concept.
7. Discussion of Costs, Benefits and Constraints
The waste heat utilization method described in this article is especially suitable for non-heat-pump electric cars because it exploits components that already exist in the vehicle. The incremental hardware cost is essentially zero; the only additional cost is the software calibration and validation effort. The energy saving comes from two sides. On one side, the motor waste heat itself is a free source of thermal energy. On the other side, the resulting battery temperature increase unlocks a portion of battery energy that was otherwise inaccessible due to low-temperature power limitations.
One may ask why the range extension is not much larger in view of the 1–2 kW waste heat power available from the electric drive system. The reason is that the test is carried out under a challenging CLTC profile with a battery capacity of more than 70 kWh. The thermal mass of the battery pack is large, roughly 400–500 kg, so the temperature rise rate from 1 kW heat is limited to about 1 K per 5 to 7 minutes, taking into account the heat losses. In a shorter cold-driving test, the temperature rise would be smaller; but the benefit would still appear because the initial cold battery causes the largest power limitation. In a longer test, after the battery has reached a temperature above 15 degC, the waste heat should be diverted back to the radiator to avoid overheating the battery. In my experiments the battery reached only 13 degC by the end of the test, so no additional cooling was required.
Another important issue is that increasing the battery temperature not only brings benefits such as increased capacity and power, but also imposes risks. If the battery is kept at too high a temperature, its side reactions, self-discharge and aging rates will increase. Therefore, the control strategy should limit the maximum battery temperature during waste heat utilization, especially in warmer weather. In this work, the waste heat was allowed only when the battery was initially below 0°C, and the coupling was disengaged once the battery temperature reached the target temperature selected by the thermal management controller. The target was chosen between 10°C and 15°C to balance the competing requirements of available energy, charging/discharging power, self-discharge rate and life consumption.
8. Conclusion
In this paper I have studied the influence of electric-drive waste heat utilization on the traction battery and on the vehicle energy consumption of a non-heat-pump electric car. My research started from the practical problem of winter driving range loss in electric cars, and I then connected the motor cooling circuit with the battery cooling circuit through an existing electromagnetic valve. The main conclusions are as follows:
- For a non-heat-pump electric car, the heat generated by the electric motor, power electronics and DC-DC converter is sufficiently large to heat the battery without using the battery PTC heater. During the CLTC test at -7°C, the average recovered waste heat power reached about 0.8–1.0 kW. This heat can raise the battery temperature from 1°C to 13°C, which is a substantial improvement for cold operation.
- Three-dimensional CFD simulation and vehicle tests both demonstrate that the temperature distribution in the battery pack remains uniform when motor waste heat is applied. The maximum temperature difference is below 5°C after two CLTC cycles, which satisfies the thermal-management requirement of the battery cells.
- Raising the battery temperature by 12°C relaxes the discharge termination criterion from 7.0% SOC to 5.75% SOC. This makes an additional 1.0 kWh of battery energy available to the electric car, thus increasing the usable capacity from 72.2 kWh to 73.2 kWh.
- The warmer battery also increases the regenerative-braking energy recovery. I measured an average improvement of 0.3 kWh/100 km in the recovered braking energy intensity, resulting from expansion of the allowable charging power window at cold temperatures.
- Combining all factors, the overall winter driving range of the electric car was extended from 281 km to 286 km, an increase of 5 km or about 2%. The range decay rate relative to a warm-condition baseline was reduced from 48.9% to 48.0%. This demonstrates that the waste heat utilization measure is an efficient, low-cost and technically feasible way to improve the winter performance of electric cars.
I believe that the theoretical analysis and the practical application presented in this article can serve as a reference for the design and calibration of thermal management systems in future electric-car generations. The control strategy of the non-heat-pump electric car is conservative because it must protect the battery at very low temperatures. Waste heat utilization, combined with model-based calibration, can uncover hidden energy and turn the thermal losses of the electric drive into a valuable resource. Further work will focus on adaptive algorithms that determine the optimal valve opening and pump speed according to the predicted trip distance, ambient temperature and battery SOC, thereby maximizing the winter range of the electric car in real driving conditions.