A common problem we often inquire about with hybrid vehicles is a fault code in the Battery Management System (BMS) stating: the precharging time of the high voltage on-board electrical system is too long. Fault codes such as 0C7800 or P0C78 may be current in BMS ECU, but the exact fault code varies between manufacturers. Some of the most common faults related to precharge issues are listed below.

Precharge fault codes can be very tricky to diagnose and troubleshoot. A fault in any of the hybrid components on the hybrid system may cause a long pre-charge time in the BMS (Battery Management System). The most common problem that causes a pre-charge time too long problem is a faulty hybrid battery, with the second most likely cause being a problem with the DC/DC converter.

  • P0C78: Hybrid Battery System Precharge Time Too Long

While hybrid battery and DC/DC converter are the most likely causes that trigger the hybrid battery system pre-charge time too long fault code, it is possible that the issue can be any of the electronic equipment of the hybrid system.

What does precharge too long mean in hybrid and electric vehicles?


This circuitry limits the inrush current to a controlled amount to charge the downstream circuit. It is an essential function for the proper functioning and maintenance of components in high-voltage applications. Pre-charging extends the lifespan of electric parts and improves the reliability and safety of the whole system.

A pre-charge circuit allows the voltage to reach a certain level orderly before the main contactors are closed. Contactors are the main switch that turns on the hybrid system. In hybrid or electric vehicles, the pre-charge circuit is used every time the car is turned on.

Common Causes of Precharge Problems


When the pre-charge circuit fails to close properly (the resistor is open, or the K1 relay is not closing), the greater inrush current will occur at the end of the pre-charge cycle, damaging the contactors, the capacitor, and the main fuse. Therefore, the reliable operation of the pre-charge circuit is paramount, or its operation should be verified before the contactors are tested.

  • Faulty Hybrid Battery – A common problem is the hybrid battery itself. The BMS and contactors are often integrated into the hybrid battery and can fail. Faulty BMS, HV electronics control unit.
  • Defective DC to DC Converter – Second most common problem that can cause precharge issues on hybrid vehicles. Malfunction of the onboard charger/DC-DC converter.
  • Damaged resistor – To ensure that the power surge does not cause problems with the pre-charge, the pre-charge resistor must withstand high currents and be rated for high power. Since the pre-charge duration is short, a high-current resistor is not required—a malfunctioning pre-charge resistor. Pre-charge resistor. SMRB, SMRP, SMRG, and the pre-charge resistor are located in the hybrid battery junction block assembly in the HV battery pack.
  • HV Precharge relay – Built into the Power Relay Assembly (PRA), the Pre-Charge Relay starts to charge the capacitor of the inverter soon after ignition “ON”, then it goes out when the capacitor has been fully charged. SMRs (System Main Relays) are the relays that connect or disconnect the high-voltage power system following commands from the power management control ECU.
  • Extreme temperatures – Extremely hot or cold temperatures can affect the operation of the hybrid system and cause the precharge to take too long.
  • Stuck pre-charge contactor – The prospect of welding a pre-charge contactor decreases since it closes into a radio-controlled circuit with a known low-inrush current.
  • Stuck closed malfunction of a pre-charge relay stuck malfunction on the HV battery negative side.
  • Damaged harness or connector.
  • Loose wire(s), connector (s), or a fault in the high-voltage wiring harness.
  • Blown main high-voltage fuse.
  • The energization circuit system/high-voltage circuit of the main contactor (P), (N) or the charging contactor fails (loose high-voltage wiring harness wire or bus bar), or the main contactor (P), (N) or the charging contactor itself fails.
  • Malfunction of the electric heater <Vehicles with electric heater>
  • Malfunction of the AC inverter <Vehicles with AC power supply>
  • Malfunction of the A/C control unit (A/C-ECU). or malfunction of the A/C compressor.
  • Malfunction of the power electronic module and drive unit.
  • Malfunction of the REMCU or the PHEV-ECU.
  • The System Main Relay harness is open or shorted.
  • System Main Relay circuit poor electrical connection

How does hybrid precharge work?

When you turn on the ignition, the hybrid battery connects to the rest of the hybrid circuit via the contactor. A second before this connection is made, the voltage of the various hybrid components is brought up to match that of the high-voltage battery.

Without powering up the rest of the hybrid system, there will be sparks at the main contactor in the hybrid battery. Hybrid batteries carry high loads, so the inrush current can quickly peak at 100A, leading to problems with the hybrid system.

A pre-charge circuit limits that inrush current. Precharge circuits are essential for hybrid systems to avoid high inrush currents during power-ups. Current spikes of hundreds of amps can easily damage system components and, as mentioned earlier, cause contactors to weld closed, which is not desirable. To prevent this problem, the rest of the hybrid system is pre-charged to get the voltage to match the high-voltage battery before closing the main contactors.

When you first turn on the ignition during the pre-charge cycle, the voltage should reach at least 95% of the hybrid battery voltage in less than one second.

Components of Precharge Circuit

The pre-charge circuit consists of the minimum of:

  • A pre-charge resistor to limit the inrush current
  • A contactor (high power relay) across the pre-charge resistor to bypass the resistor during normal operation

Additionally, the pre-charge circuit may have:

  • A pre-charge relay (K1) keeps the load from being powered through the pre-charge resistor when the system is off.
  • A contactor is in line with the other end of the battery to isolate the load when the system is off.

The pre-charge circuit is usually composed of a smaller contactor in series with a resistor and is connected in parallel with the main current-contactor. The pre-charge circuit is usually found on the positive leg but could also be on the negative side.

Hybrid System Pre-charge Cycle

The sequence of events to pre-charging a system is typically comprised of the following steps:

  1. Close the main negative contactor.
  2. Close the precharge contactor.
  3. Monitor the voltage to ensure it is rising as expected. Voltage for hybrid components must match the hybrid battery voltage with +/- 5% variance.
  4. When the voltage has equalized, close the Main Positive contactor.
  5. Open the Precharge contactor.
  6. Start the hybrid system.

Smart pre-charge circuits have a timer that will stop the pre-charge if the voltage does not reach a certain voltage in under one second. Some also have a feature that will prevent precharges when certain occurrences occur during a certain amount of time to avoid overheating resistors.

If the voltage rising curve is understood, the voltage over time can be measured during precycle to ensure that it is within an acceptable and expected range.

If you detect that a pre-charged contactor is not closing, it may be easy to diagnose as the voltage will not begin rising. Likely, the voltage rises too slowly because of a dead short or short circuit caused by leaving the downstream loads connected. A short or dead short circuit will cause the voltage to rise too quickly.


s400 hybrid precharge high voltage battery

When the car is turned on, a high-voltage system with downstream components can be exposed to a significant incoming current. If this current is not limited and controlled, it can cause damage to the system or other components. If the hybrid system can not be powered up, a precharge fault code may be triggered and stored in the BMS (Battery Management System).

An advantage of pre-charging is that it is useful in diagnosing faulty components, system issues, or electrical defects. For example, attempting to pre-charge a short will fail as the system knows that the downstream voltage is not increasing and automatically initiates the discharge.

Common Precharge Fault Codes

Here are some of the most common fault codes that come up when there is a problem with the hybrid vehicle pre-charge circuit taking too long. These codes are from various vehicle manufacturers.

  • P0AE7-224 Hybrid Battery Precharge Contactor Control Circuit High
  • P0AE2-773 Hybrid Battery Precharge Contactor Circuit Stuck Closed
  • P0C78 if the condenser is pre-charged improperly when the drive battery system starts. Hybrid Battery System Precharge Time Too Long
  • P0AE6-225 Hybrid Battery Precharge Contactor Control Circuit LowP0C78 | Toyota
  • P0AE411 – Hybrid/EV Battery Precharge Contactor Circuit Short to Ground SMRs (System Main Relays) are the relays that connect or disconnect the high-voltage system by the hybrid vehicle control commands ECU. Short to ground in the SMRP circuit or Primary circuit of SMRP is malfunctioning. Check – 1. Wire harness or connector, 2. HV battery junction block assembly 3. The hybrid vehicle control ECU.
  • P0C78 – Hybrid Battery System Precharge Time Too Long
  • P0AE3 – Hybrid Battery Precharge Contactor Circuit Stuck OpenP0AE1: Hybrid Battery Precharge Contactor Circuit
  • P1B77: High Voltage Precharging Fault
  • P0AE2 code. What means Hybrid Battery Precharge Contactor Circuit Stuck Closed in auto. Engine flaw Hybrid Battery Precharge
  • P0D26 Battery Charger System Precharge Time Too Long
  • P0C78 Hybrid/EV Battery System Precharge Time Too Long
  • P1B77: High Voltage Precharging Fault
  • P0AE6: Hybrid Battery Precharge Contactor Control Circuit Low
  • P0AE3: Hybrid battery pre-charge contactor – circuit stuck open
  • P0C77: Hybrid/EV Battery System Precharge Time Too Short
  • P0C78: Hybrid/EV Battery System Precharge Time Too Long
  • C1705 EPS ECU Precharge Circuit Fault – Hyundai or Kia EPS CMs may turn on a warning lamp if the voltage is too low due to a malfunction in the charging circuit.- Common causes include: Poor connection in the power circuit, Poor connection in the ground circuit, Overcharging
  • P0AE2-773 Hybrid Battery Precharge Contactor Circuit Stuck Closed, Hybrid Battery Precharge Contactor Circuit Stuck Closed SMRP (pre-charge relay) is stuck closed due to a mechanical malfunction or other malfunction.
  • P0AE1 Hybrid Battery Precharge Contactor Circuit
  • P0AE411 Hybrid/EV Battery Precharge Contactor Circuit Short to Ground Short to ground in the SMRP circuit: Primary circuit of SMRP is malfunctioning.Wire harness or connector. HV battery junction block assembly. Hybrid vehicle control ECU

Frequently Asked Questions

How long does pre-charge take?

The continuous rated current of the pre-charge contactor is not as important as the length of time it takes to reach it, typically a matter of seconds, if not longer. The pre-charge circuit, contactor, and resistor must be rated for the full battery voltage. The pre-charge contactor and resistor must also handle the pre-charge current and power dissipation.

Where is the pre-charge located on a hybrid vehicle?

In most cases, the series resistor is placed after the contactor to make it easier to limit the number of connections that can be continually energized by the battery pack when the system is turned off. Placing the pre-charge and main contactors near the battery pack reduces the risk of exposure to a high voltage system.

Why is a hybrid system pre-charge necessary?

The welded contacts of contactors are one of the most common failure modes of contactors and, therefore, of the hybrid battery. Hundreds of amps can pass through the closed contactor if the circuit has a short circuit. This is a rare failure and is usually very easy to diagnose.
For example, if an electrical short circuit interrupts the connection, possibly an imperceptible spanner that had slipped over the terminals will just have occurred because of a sudden shock. An extreme impact can cause the contact points to open up, followed by a successful reunification within the next moment.
Over time, the arc that forms during the brief opening of the contactor generates a series of tiny hot spots which melt. When the contactor opens and the hot spots cool, the contactor is then welded. Most often, the contactor welding that occurs is due to inrush currents.
In some instances, the cause of welded contacts is overlooked and not easily recognized or understood. Often, the contactor’s trouble is that contacts are welded, but it is much more likely that the contacts are welded when the contactor shuts.
A pre-charge resistor reduces the power supply, which will cause system damage while the fuse clears the fault. A fault indicator or alarm code can be used to inform the operator or technician.

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