Modern electric cars are no longer limited to low-voltage accessories. The traction system, air-conditioning compressor, DC/DC converter, and cabin heater all draw power from a large battery pack whose normal operating voltage can be far above the conventional 12V level. This high-voltage network is needed to reduce cable mass and improve system efficiency, but it also introduces a serious challenge when the system is first connected to the battery. If a controller is connected directly to the battery through a contactor, the large capacitive storage inside the controller can behave almost like a short circuit during the first few milliseconds. The resulting current spike may damage fuses, weld relay contacts, and reduce the lifetime of power electronic components. Therefore, every reliable electric car needs a carefully designed high-voltage precharge circuit. In this paper, I explain the principle, calculation method, component selection, and simulation verification of a precharge circuit using a concrete electric car project as an example.
Keywords: electric car; high-voltage precharge circuit; precharge resistor; precharge relay; circuit simulation; capacitor inrush current; thermal verification.
1. Why an Electric Car Needs a Precharge Circuit
The main inverter of an electric car contains a DC-link capacitor. Its function is to stabilize the bus voltage and handle the high-frequency current ripple produced by the motor drive. The capacitance value can be very large. In many modern electric cars, the total DC-bus capacitance is higher than 1000 microfarads. The total resistance of a direct high-voltage connection path is often only the battery internal resistance plus contact resistance. In the example discussed in this article, the battery internal resistance is approximately 60 milliohms, and the contact resistance is approximately 30 milliohms. The total path resistance is therefore about 90 milliohms.
$$R_{\text{parasitic}} = R_{\text{battery}} + R_{\text{contact}} \approx 60\,\mathrm{m\Omega} + 30\,\mathrm{m\Omega} = 90\,\mathrm{m\Omega}$$
If the battery voltage is 403 V, then the theoretical initial current caused by directly closing the main contactor is:
$$I_{\text{inrush}} = \frac{V_{\text{bat}}}{R_{\text{parasitic}}} = \frac{403}{0.09} \approx 4478\,\mathrm{A}$$
Such a current will not persist for a long time because the capacitor charges quickly, but even a short current pulse of several kiloamperes is enough to stress every component in the circuit. I have seen simulation results showing a direct-connect transient of roughly 4000 A in an electric car high-voltage loop. The consequences include contact welding, cracked capacitors, damaged fuses, and premature failure of power modules. A precharge circuit solves this problem by inserting a controlled resistance into the charging path before the main positive contactor is closed.
| Item | Value | Effect on electric car components |
|---|---|---|
| Battery voltage | 403 V | Nominal high-voltage bus value |
| Parasitic series resistance | 90 mΩ | Forms the only limiting resistance in a direct closure |
| DC-bus capacitance | 1100 μF | Large capacitor stores energy and tends to absorb a large current step |
| Predicted direct inrush current | About 4000–4500 A | Unacceptable for relays, fuses, and capacitors |
2. High-Voltage Precharge Architecture in an Electric Car
The high-voltage circuit of an electric car normally contains a battery pack, a main positive relay, a main negative relay, a fuse, a precharge relay, a precharge resistor, and the capacitive loads inside the high-voltage controllers. The precharge branch is usually placed in parallel with the main positive relay. This branch consists of two primary components: the precharge relay and the precharge resistor. The resistor is placed in series with the precharge relay to limit the current while the capacitor is being charged.
The power-on sequence for an electric car can be described in the following steps.
First, the main negative relay is closed. At this moment, the high-voltage positive side is still open, so no charging current flows. Second, the precharge relay is closed. Now the battery voltage is applied to the DC bus through the precharge resistor. Because the resistor is present, the initial current is limited to a moderate value instead of several kiloamperes. Third, the capacitor voltage rises progressively according to an RC charging characteristic. The precharge relay remains closed until the capacitor voltage has reached a voltage close to the battery voltage. Fourth, when the voltage difference between the battery and the capacitor is smaller than a defined threshold, the main positive relay is closed. Since the capacitor is already precharged, only a small equalizing current flows. Finally, the precharge relay is opened and the high-voltage system enters normal operation.
$$I_{\text{precharge}} = \frac{V_{\text{bat}}}{R_{\text{precharge}}}$$
The purpose of the precharge circuit is not to charge the capacitor to an exactly equal voltage. It is sufficient to bring the capacitor voltage close enough to the battery voltage so that the current produced when the main positive relay closes is within the safe operating range of the relay, fuse, and controllers. In this electric car design, the target was to finish precharging when the capacitor voltage was not more than 10 V below the battery voltage.
3. Basic Equations for the Precharge Circuit
During precharge, the precharge resistor, the parasitic resistance, and the load capacitor form a first-order RC circuit. The differential equation that describes the voltage across the capacitor is:
$$V_{\text{bat}} = R i(t) + v_{C}(t)$$
where \(i(t)\) is the charging current and \(v_C(t)\) is the instantaneous capacitor voltage. The relationship between current and capacitor voltage is:
$$i(t) = C \frac{dv_C(t)}{dt}$$
Combining these equations gives:
$$RC \frac{dv_C(t)}{dt} + v_C(t) = V_{\text{bat}}$$
If the capacitor is initially discharged to \(V_0\), the general solution for the capacitor voltage is:
$$v_C(t) = V_{\text{bat}} – \left(V_{\text{bat}} – V_0\right)e^{-\frac{t}{RC}}$$
At the beginning of a normal electric car start, the high-voltage capacitor is usually discharged and \(V_0 = 0\). The voltage waveform then simplifies to:
$$v_C(t) = V_{\text{bat}}\left(1 – e^{-\frac{t}{RC}}\right)$$
The charging current in the precharge loop is:
$$i(t) = \frac{V_{\text{bat}} – v_C(t)}{R} = \frac{V_{\text{bat}}}{R} e^{-\frac{t}{RC}}$$
The maximum current appears at the instant the precharge relay is closed:
$$I_{\max} = \frac{V_{\text{bat}}}{R}$$
The instantaneous power dissipated by the precharge resistor is important for component selection. Since the current is falling exponentially, the power waveform is also exponential:
$$P_R(t) = i^2(t) R = \frac{V_{\text{bat}}^2}{R} e^{-\frac{2t}{RC}}$$
The total energy converted into heat in the resistor during a precharge event can be found by integrating the instantaneous power:
$$E_R = \int_{0}^{t_c} P_R(t)\,dt = \frac{1}{2} C V_{\text{bat}}^2 \left(1 – e^{-\frac{2t_c}{RC}}\right)$$
If the precharge interval is long compared with the time constant, the resistor energy approaches the energy stored in the capacitor:
$$E_R \approx \frac{1}{2} C V_{\text{bat}}^2$$
The required precharge time to reach a final capacitor voltage \(V_{C,final}\), starting from an initial voltage \(V_0\), is obtained by solving the voltage equation:
$$t_c = RC \ln\left(\frac{V_{\text{bat}} – V_0}{V_{\text{bat}} – V_{C,\text{final}}}\right)$$
For the common case \(V_0 = 0\), the denominator is simply the allowable voltage difference at the end of precharge:
$$t_c = RC \ln\left(\frac{V_{\text{bat}}}{\Delta V}\right)$$
where \(\Delta V = V_{\text{bat}} – V_{C,\text{final}}\). This is the equation I used to calculate the allowable range of the precharge resistor in the electric car high-voltage system.
4. Design Inputs and Precharge Resistor Range
The design inputs of the example electric car are summarized in the following table. The capacitance in this project was taken as 1100 microfarads because the total bus capacitor includes the motor controller capacitor and the capacitors of other high-voltage loads. The battery voltage used for worst-case calculation was the maximum operational system voltage. The allowable precharge time was defined between 0.1 second and 0.5 second. The precharge ending criterion was defined as a maximum remaining voltage difference of 10 V between the battery and the DC bus.
| Parameter | Symbol | Value |
|---|---|---|
| Maximum battery system voltage | \(V_{\text{bat}}\) | 403 V |
| Total equivalent DC-bus capacitance | \(C\) | 1100 μF |
| Battery internal resistance | \(R_{\text{battery}}\) | 60 mΩ |
| Total contact resistance | \(R_{\text{contact}}\) | 30 mΩ |
| Required precharge time window | \(t_c\) | 0.1 s to 0.5 s |
| Allowed residual voltage difference | \(\Delta V\) | 10 V |
| Required capacitor voltage at end | \(V_{C,\text{final}}\) | ≥ 393 V |
The first step is to find the minimum allowable precharge resistance. A smaller resistance causes a faster charge, but also produces a larger current and a larger instantaneous power pulse. If the precharge time must not be shorter than 0.1 second, then the resistance must not be too small. From the precharge-time equation, the minimum resistance is:
$$R_{\min} = \frac{t_{\min}}{C \ln\left(\dfrac{V_{\text{bat}}}{\Delta V}\right)} = \frac{0.1}{1.1\times10^{-3} \ln\left(\dfrac{403}{10}\right)} \approx 25\,\Omega$$
Similarly, if the precharge time must not be longer than 0.5 second, the resistance must not be too large:
$$R_{\max} = \frac{t_{\max}}{C \ln\left(\dfrac{V_{\text{bat}}}{\Delta V}\right)} = \frac{0.5}{1.1\times10^{-3} \ln\left(\dfrac{403}{10}\right)} \approx 123\,\Omega$$
Therefore, any precharge resistance value from roughly 25 ohms to 123 ohms satisfies the voltage and timing requirements of this electric car. From the product catalogues available for high-voltage precharge resistors, I selected a resistor with a nominal resistance of 47 ohms and a rated continuous power of 40 W. This value lies comfortably within the calculated range. The resulting RC time constant is:
$$\tau = R_{\text{precharge}} C = 47 \times 1.1\times10^{-3} = 0.0517\,\mathrm{s}$$
The actual precharge time required to reach the target voltage difference is:
$$t_c = R_{\text{precharge}} C \ln\left(\frac{V_{\text{bat}}}{\Delta V}\right) = 0.0517 \times \ln\left(\frac{403}{10}\right) \approx 0.191\,\mathrm{s}$$
This time is well within the required window of 0.1 second to 0.5 second. The initial current produced when the precharge relay is closed is:
$$I_{\max} = \frac{V_{\text{bat}}}{R_{\text{precharge}} + R_{\text{parasitic}}} = \frac{403}{47 + 0.09} \approx 8.57\,\mathrm{A}$$
This current is easily handled by a small high-voltage relay.
| Design Requirement | Calculated Value | Selected Value |
|---|---|---|
| Precharge resistance lower limit | ≈ 25 Ω | — |
| Precharge resistance upper limit | ≈ 123 Ω | — |
| Selected precharge resistance | — | 47 Ω |
| Selected continuous power rating | — | 40 W |
| Calculated precharge time | 0.191 s | 0.19 s |
| Maximum precharge current | 8.57 A | Relay selected above this value |
5. Thermal Verification of the Precharge Resistor
One of the most common mistakes in electric car precharge design is to consider only the continuous power rating of the resistor. During a single precharge event, the average power in the resistor is far above the rated continuous power because the event lasts only a fraction of a second. In this design example, the resistor energy dissipated in a single precharge event can be estimated from the capacitor energy relationship. The energy stored in the capacitor is:
$$E_{C} = \frac{1}{2} C V_{\text{bat}}^2 = 0.5 \times 1.1\times10^{-3} \times 403^2 \approx 89.3\,\mathrm{J}$$
Most of this energy is absorbed by the precharge resistor during the charging process. The actual energy dissipated by the selected 47-ohm resistor until the voltage difference falls to 10 V is obtained from the exact resistor energy equation:
$$E_R = \frac{1}{2} C V_{\text{bat}}^2 \left(1 – e^{-\frac{2t_c}{RC}}\right)$$
$$E_R = 0.5 \times 1.1\times10^{-3} \times 403^2 \times \left(1 – e^{-2 \times 3.696}\right) \approx 89.2\,\mathrm{J}$$
The average resistor power during the 0.191-second precharge interval is therefore:
$$P_{\text{avg,one cycle}} = \frac{E_R}{t_c} = \frac{89.2}{0.191} \approx 467\,\mathrm{W}$$
This value is much higher than the 40 W continuous rating. However, a pulse-rated precharge resistor can withstand a high overload for a short time because the thermal capacitance of the resistor body absorbs the energy and the temperature rise remains acceptable. The manufacturer’s pulse withstand curve showed that the selected resistor can withstand approximately 1600 W for 0.2 second. The actual average power of 467 W over the same time interval is well below that limit. Thus, the resistor can safely handle a single precharge event.
In a real electric car, the user may attempt to power off and power on several times in a short period. I therefore checked the continuous precharge strategy used in the vehicle control software. The strategy allowed five precharge attempts in a row, with a 3-second pause between consecutive attempts. After the fifth attempt, the system must wait at least 10 seconds before another precharge is allowed. During the short pause, the high-voltage capacitor is discharged through the system discharge resistor. The residual voltage at the beginning of the next precharge is normally small enough to be treated as zero.
In the five-cycle group, the total precharge energy is:
$$E_{\text{total}} = 5 \times 89.2 = 446\,\mathrm{J}$$
The energy is released in five precharge intervals plus four 3-second cooling intervals. The total time of this group of attempts is:
$$T_{\text{group}} = 5 \times 0.191 + 4 \times 3 = 12.955\,\mathrm{s}$$
The average power over that group is:
$$P_{\text{avg, group}} = \frac{446}{12.955} \approx 34.4\,\mathrm{W}$$
This value is below the 40 W continuous rating. If the 10-second rest after the fifth precharge is included, the average power would be even lower. Therefore, the selected 47-ohm 40-watt precharge resistor is adequate for both single precharge and repeated precharge operation in this electric car.
| Thermal Check Item | Calculated or Simulated Value | Acceptance Criterion | Result |
|---|---|---|---|
| Resistor energy in one precharge | ≈ 89 J | Resistor pulse curve | Pass |
| Average power during one precharge | ≈ 467 W | 1600 W for 0.2 s | Pass |
| Peak resistor power at first instant | ≈ 3.45 kW | Manufacturer allows 3 kW for 0.05 s | Pass |
| Average power during five consecutive precharges | ≈ 34.4 W | 40 W continuous | Pass |
6. Selection of the Precharge Relay
The precharge relay must be matched to the selected precharge resistor. In this electric car project, the maximum current through the precharge relay is limited by the resistance of the precharge loop:
$$I_{\text{precharge, max}} = \frac{V_{\text{bat}}}{R_{\text{precharge}} + R_{\text{parasitic}}} \approx \frac{403}{47.09} = 8.57\,\mathrm{A}$$
Selecting a relay with a current rating of 10 A provides a reasonable margin above the calculated peak current. The relay also has to withstand the nominal battery voltage on its contacts and must have sufficient clearance for high-voltage automotive applications.
The precharge relay is not asked to interrupt an inductive load or a full load current. After the precharge operation is complete, the main positive relay is closed first. Then the precharge relay is opened. At that moment, the voltage difference across the precharge relay contacts is small, so the opening current is low. The main challenge for the precharge relay is therefore the contact life during closing and the ability to carry the repeated pulse current without overheating.
A 10 A high-voltage relay is therefore suitable for this precharge circuit. The precharge relay does not need to interrupt the 100 A or higher current handled by the main positive relay. Its main task is to safely make the circuit and to carry the limited precharge current until the capacitor is charged.
| Selection Parameter | Value | Design Margin |
|---|---|---|
| Maximum calculated precharge current | 8.57 A | — |
| Selected precharge relay current rating | 10 A | Approximately 17% margin |
| Battery system voltage | 403 V | Relay rated for automotive high voltage |
| Current path at the moment the relay opens | Near zero | Low contact erosion |
7. Simulation of the Electric Car Precharge Circuit
After the analytical design was completed, I built a transient simulation model of the electric car high-voltage circuit. The model contained a battery voltage source, battery internal resistance, main positive relay, main negative relay, precharge relay, precharge resistor, load capacitance, voltage measurement blocks, and current measurement blocks. The simulation used the same parameters as the analytical calculation: a 403 V battery, a 47-ohm precharge resistor, and a bus capacitance of 1100 microfarads.
The simulation results showed that the capacitor voltage reaches approximately 393 V in about 0.2 second. At that moment, the voltage difference between the battery and the capacitor has dropped to about 10 V. The main positive relay is then closed and the precharge relay is opened. The voltage waveform has a small transient at the switching instant because the main positive relay closes into a very small remaining voltage difference. The resulting current spike is approximately 100 A. That current is within the safe contact current range for the main positive and main negative relays. It is also within the normal rated current range of the high-voltage controllers.
I also simulated the condition where the precharge branch is removed and the main positive relay is closed directly. The result shows an instantaneous transient current of approximately 4000 A. This huge current damages the fuse, the relay contacts, and the power electronic components in a real electric car. It clearly demonstrates why the precharge circuit is necessary.
The simulation results also provide useful information about the thermal stress on the precharge resistor. The simulated resistor energy during the precharge process was about 105 J, slightly higher than the approximate analytical value of 90 J. The small difference appears because the simulation includes parasitic resistance, a more detailed voltage transition, and the exact switching timing. The selected resistor can still withstand this energy. The simulated peak resistor power was in the region of 3 kW during the first few milliseconds. The resistor manufacturer’s pulse characteristic showed that the resistor can withstand this power level for roughly 0.05 second, while the effective duration of the high-power portion is much shorter than 0.05 second.
| Comparison Item | Analytical Design | Simulation Result |
|---|---|---|
| Precharge time | 0.191 s | ≈ 0.2 s |
| Capacitor voltage at end of precharge | 393 V | ≈ 393 V |
| Residual voltage difference | 10 V | ≈ 10 V |
| Peak current when main positive relay closes | ≈ 111 A by simple resistance calculation | ≈ 100 A |
| Peak current without precharge circuit | ≈ 4500 A by simple resistance calculation | ≈ 4000 A |
| Resistor energy in one precharge | ≈ 90 J | ≈ 105 J |
| Resistor peak power | ≈ 3.45 kW | ≈ 3 kW |
One important observation from the simulation is that the current spike after the main positive relay closes is not exactly equal to the simple quotient of the voltage difference divided by parasitic resistance. This is because the parasitic inductance of the cables and the internal busbar delays the rise of the current and changes the dynamic response. Still, the simulation confirms that the peak current is in the order of 100 A rather than several kiloamperes. This value is acceptable for the electric car’s main relay and control components.
8. Relay Stress and Expected Service Life
One of the main reasons for using a precharge circuit in an electric car is to protect the main relays. A relay is most stressed when it makes or breaks a high current. If the main positive relay is closed while the capacitor is completely discharged, the initial current is enormous. The relay contacts may weld together because the micro-arc that occurs during contact bounce is sustained by a very high fault-like current.
After precharge, the current at the moment of main positive relay closure is controlled to a much lower level. In this design, the final voltage difference of about 10 V, combined with the parasitic resistance of about 90 milliohms, gives a simple calculated closure current of about 111 A. The relay supplier’s life data for this class of relay shows that the main positive and main negative relays can tolerate thousands of operations at this current level. In practice, a current level of about 100 A to 150 A is within the service-life range of many automotive high-voltage DC relays. This is why the end-of-precharge voltage criterion is so important. It is not necessary to equalize the capacitor voltage perfectly. It is only necessary to reduce the closure current to a level that the relay can safely tolerate many times during the lifetime of the electric car.
The precharge relay is stressed differently. Every time the precharge relay closes, it makes the precharge circuit and starts the charging current. The initial current is only about 8.6 A because the 47-ohm resistor limits the current. The precharge relay opens after the main positive relay is closed, so it opens at near-zero current. Therefore, the precharge relay selection is governed by its continuous current carrying capability and its make operation, not by its ability to interrupt high load current.
| Component | Main Stress | Typical Controlled Value | Selection Logic |
|---|---|---|---|
| Main positive relay | Closing current after precharge | ≈ 100 A | Keep below weld current and within contact life |
| Main negative relay | Closing current during precharge completion | ≈ 100 A | Must support the bus current and repeated closures |
| Precharge relay | Making current through resistor | ≤ 8.57 A | 10 A rated relay gives margin |
| Precharge resistor | Pulse power and thermal accumulation | Single pulse energy 89 J | Pulse curve and average power verification |
9. Additional Design Considerations
There are several other practical points that should be considered in the design of a high-voltage precharge circuit for an electric car.
The first is the position of the precharge branch. The precharge resistor and relay are usually connected in parallel with the main positive relay. This is the most common arrangement because it allows the negative side to be closed first and keeps the positive side protected until the capacitor is charged. However, some designs place the precharge branch across the main negative relay. Both topologies are possible, but the design equations remain the same because the battery, resistor, and capacitor form the same RC network.
The second point is the discharge behavior between precharge attempts. After the main relays are opened, the DC-bus capacitor does not instantly drop to zero. There is normally a discharge resistor inside the controllers, but the discharge time can be several seconds. If the driver attempts to restart the electric car while the capacitor still holds a high voltage, the initial voltage \(V_0\) is not zero. The precharge time will be shorter because the capacitor is already partially charged. The formula including \(V_0\) should be used for such a case. However, in the continuous-restart strategy considered in this design, the pause time between attempts was sufficient for the capacitor to discharge to a low voltage.
$$t_c = RC \ln\left(\frac{V_{\text{bat}} – V_0}{V_{\text{bat}} – V_{C,\text{final}}}\right)$$
The third point is the initial voltage detection fault. A robust electric car control system should check whether the DC-bus capacitor voltage is too high before starting the precharge. If the precharge resistor is selected for a fully discharged capacitor and the bus is already at a high voltage, the precharge current will be smaller than normal. This condition is not dangerous for the resistor, but it may indicate a discharge fault or an unexpected bus voltage source. The control system should therefore monitor the precharge current and capacitor voltage to detect abnormal states.
The fourth point is the fuse coordination. The fuse in the high-voltage circuit must allow the precharge current to flow without melting. During precharge, the current is moderate, but the duration is long enough to heat the fuse element. The fuse must also withstand the repeated precharge current pulses without degradation. The selected fuse rating should be larger than the maximum steady-state load current of the electric car, but the precharge current pulse should be checked against the fuse’s time-current characteristic.
The fifth point is the ambient temperature. The allowable pulse power of the precharge resistor is usually given at a reference temperature. In an electric car, the resistor may be installed close to the battery pack or power electronics, where the ambient temperature can be high. If the ambient temperature is high, the thermal margin of the resistor is reduced. I recommend using a resistor with a pulse rating that is sufficiently above the worst-case requirement to account for elevated temperatures.
10. Simulation Model Structure
The transient simulation model used for this precharge circuit was kept simple but representative. It included the following elements:
| Model Element | Symbol | Value or Representation in Model |
|---|---|---|
| Battery pack | \(V_{\text{bat}}\) | 403 V DC source |
| Battery internal resistance | \(R_{\text{battery}}\) | 60 mΩ |
| System contact resistance | \(R_{\text{contact}}\) | 30 mΩ |
| Precharge resistance | \(R_{\text{pre}}\) | 47 Ω |
| Load capacitance | \(C\) | 1100 μF |
| High-voltage cables | \(L_{\text{cable}}\) | Small parasitic inductance included in simulation |
| Main positive relay | — | Closed after precharge target is reached |
| Main negative relay | — | Closed at the beginning of the power-on sequence |
| Precharge relay | — | Opened after main positive relay closes |
The model was used to compare three important cases. The first case is the normal precharge operation. The second case is a fault condition in which the precharge relay remains closed while the main positive relay is also closed. This case normally causes no significant stress because the voltage difference is already small, but it is still useful to confirm that the precharge resistor is not continuously carrying load current after the main contactor is closed. The third case is the worst-case production fault of closing the main positive relay without precharge. This final case is not an operating state; it is a diagnostic simulation used to show the value of the precharge loop and to set requirements for relay interlocking logic.
11. Simulation Results and Verification
The simulation results confirmed that the selected electric car precharge circuit meets all design targets. During normal precharge, the bus voltage increases smoothly. The voltage approaches 393 V after about 0.2 second. The remaining voltage difference is around 10 V, which is exactly the control target. The current through the precharge branch falls from its initial value of 8.6 A to a small value at the end of the precharge event. This behaviour is very close to the calculated RC charging curve.
When the main positive relay closes after precharge, the simulation shows a transient current spike of about 100 A. The spike is influenced by the parasitic inductance in the high-voltage circuit and by the switching instant, but it is far lower than the direct-connect transient current. The main positive relay and main negative relay can withstand this current level for the number of operations expected during the life of the electric car.
In contrast, the simulation without the precharge circuit shows a peak current of approximately 4000 A. The capacitor begins to charge almost immediately, but the initial current is constrained only by the battery internal resistance, the wiring resistance, the relay contact resistance, and the very small parasitic inductance. Such a large current will create substantial electromagnetic interference, possible contact welding, fuse element damage, capacitor heating, and semiconductor stress. This comparison is very useful for communicating the importance of the precharge design to other engineering teams.
| Operating Condition | Peak Bus Current | Capacitor Voltage after 0.2 s | Evaluation |
|---|---|---|---|
| Normal precharge followed by main positive relay closure | ≈ 100 A | ≈ 393 V | Safe and acceptable |
| Direct main positive relay closure without precharge | ≈ 4000 A | Charging extremely fast | Unacceptable for relay and components |
| Repeated precharge attempts with 3 s pause | Resistor average power ≈ 34 W | Capacitor discharged between attempts | Within 40 W resistor rating |
12. Final Remarks on the Design Method
I have found that the design of a precharge circuit for an electric car can be organized into a very clear method. First, the system voltage range and the total bus capacitance must be defined. Second, the allowable precharge time and final voltage difference must be set by the vehicle power-on strategy. Third, the precharge resistor range can be calculated from the RC charging equation. Fourth, a catalogue resistor should be selected inside that range with sufficient pulse energy capability. Fifth, the thermal stress must be checked for both one precharge event and a sequence of repeated precharge attempts. Sixth, the precharge relay should be selected based on the maximum current through the precharge resistor. Seventh, the complete circuit should be verified with a transient simulation. Finally, the design should be confirmed by measuring real voltage and current waveforms on a prototype electric car.
In this project, I selected a 47-ohm resistor because it lies between the calculated lower limit of about 25 ohms and the upper limit of about 123 ohms. The resistor has a continuous power rating of 40 W. A single precharge event deposits about 90 J of energy in the resistor with an average power of about 467 W during the event. The resistor can survive this because the pulse duration is only about 0.2 second and the manufacturer’s pulse curve allows a much higher power for that duration. During a five-cycle precharge sequence with 3-second pauses, the average power is about 34 W, which is below the 40 W rating. The maximum precharge current is only 8.6 A, so a 10 A precharge relay is sufficient.
The simulation verified that the capacitor reaches approximately 393 V in about 0.2 second and that the current spike after the main positive relay closes is about 100 A. The simulation also confirmed that omission of the precharge circuit would produce an inrush current of about 4000 A, which would be destructive to many components. These results prove that the precharge loop is not only a desirable feature but also a mandatory part of a safe electric car high-voltage system.
13. Conclusion
A high-voltage precharge circuit is essential for protecting capacitors, relays, fuses, and power electronics in an electric car. During the power-on process, a precharge resistor limits the initial current and allows the DC-bus capacitor to charge gradually. Once the capacitor voltage is close to the battery voltage, the main positive relay can be closed without dangerous inrush current.
I have presented the complete design procedure for the precharge circuit of a typical electric car. The method includes theoretical derivation, resistance-range calculation, resistor selection, thermal verification, relay selection, and transient simulation. The analytical results and simulation results agree well. The chosen 47-ohm precharge resistor and 10 A precharge relay meet all the requirements for normal operation, repeated restart, and component lifetime.
The design method described in this article can be reused for other electric car platforms with different voltages, capacitances, and precharge time requirements. The equations are general. By adjusting the battery voltage, bus capacitance, target voltage difference, and allowable time, a designer can quickly obtain a safe precharge resistance value. The same simulation approach can then be used to verify the final component selection and to demonstrate that the electric car high-voltage system is robust under both normal and abnormal power-on conditions.