Hybrid vehicle technology may still be in its infancy, but it is steadily gaining in importance. All major car and truck manufacturers are currently developing hybrid models. Hybrid technology is a stepping stone to advanced electrically powered components that will be used in both hydrogen fuel cell vehicles and pure electric vehicles. Hybrid vehicles are already much more complex and technically sophisticated than ordinary cars or trucks.
This is most evident when comparing the circuits of hybrid and non-hybrid vehicles. While the electrical system of a conventional vehicle is operated almost exclusively at a voltage between 12 and 14 volts, in hybrid vehicles more than five times as high values can be achieved with a bandwidth between 12 and 650 volts. Direct current and alternating current are used, and variable voltages are quite common.
Hybrid vehicles are designed in such a way that measurements of high-voltage circuits are hardly necessary during operation. However, it can also be observed again and again that experienced hybrid vehicle technicians have to do just that. Even though articles like this are only intended as a supplement to the manufacturer's service information, it is worth mentioning general safety precautions that a professional technician should take into account when handling high-voltage circuits.
For the purposes of this article, a high voltage is understood to mean circuit voltages of over 50 volts. Although the high-voltage (HV) systems of most hybrid vehicles do not typically need to be disabled to perform routine maintenance, they must be shut down before disconnecting or removing high-voltage cables or components. For example, it can be helpful to disconnect HV components before diagnosing a grounding fault on the hybrid vehicle. For such work, a milliohmmeter or insulation measuring device is required, which will be discussed in more detail in another article.
Potentially dangerous electrical currents can come from the following four types of hybrid components: (1) the high-voltage battery, (2) the capacitors within the vehicle's rectifier assembly, (3) the electric motor or motors, also known as motor-generators, and (4) the high-voltage cables, usually orange, that connect these components.
One of the most common measurement data of a vehicle high-voltage system is the voltage value that is recorded after the system is shut down to check that it has been shut down properly. The following will discuss the steps that are generally required to ensure that a hybrid vehicle's HV system has been safely shut down.
Preparation
Direct measurement of circuits with potentially high voltage and current must be carried out in a planned and targeted manner. Even if it is to be expected that there will be no or almost no voltage during the following process (provided that the system has been properly switched off and is free of errors), you must always take into account the possibility that the circuit is active after all. Inform others if you are working on high-voltage circuits. Set aside all jewelry and other metallic objects that may fall out of a bag.
Turn the vehicle's ignition key to the OFF position and remove the key. If the vehicle has a keyless start function, disable it and make sure that the key fob is outside the vehicle area. Disconnect the vehicle's conventional 12V lead-acid battery. Before proceeding, adhere to the prescribed waiting time – this can be up to ten minutes, depending on OEM requirements – until the system's high-voltage capacitors are fully discharged.
To completely shut down or isolate the high-voltage circuits, you will need:
- an understanding of the manufacturer-approved shutdown procedures for the high-voltage system of the specific vehicle being serviced
- a pair of impeccable rubber or synthetic rubber insulating gloves approved for at least 1000 volts alternating current (class 0);
- a digital multimeter approved for at least 1000 volts, CAT III or CAT IV and
- Electrical cables approved for at least 1000 volts, CAT III or CAT IV, which are also equipped with at least one insulated alligator clip.
The 88 and 1587 automotive multimeters and Fluke cables are approved for 1000 volts, CAT III and CAT IV. Gloves, gauges, and wires must be approved for a voltage value that is higher than the highest voltage you want to measure. Rubber insulating gloves must be checked before each use in accordance with the manufacturer's instructions and tested at regular intervals. Most glove manufacturers also recommend or prescribe wearing additional leather protection over the insulating gloves for safety reasons.
Since all shutdown mechanisms in hybrid vehicles are directly connected to high-voltage cables, insulating gloves must be worn when removing HV service plugs (Ford, Toyota, Lexus) or switching off the HV safety switch (Honda).
After checking that the vehicle's ignition key has been removed, the keyless start function (if equipped) has been deactivated, and the 12V battery has been disconnected, you will usually put on a pair of rubber insulating gloves according to the manufacturer's specifications, and then disconnect the vehicle's HV system by either unplugging a service plug or turning a safety switch to the OFF position. By disconnecting the vehicle's 12V battery, you ensure that the HV battery pack is mechanically isolated by a properly functioning system until the 12V battery is reconnected.
If the vehicle has been properly shut down and there are no electrical faults:
- no voltage is generated by the motor-generators, as they do not rotate mechanically and do not have access to external power sources
- no current is available at the capacitors that has been discharged during the prescribed time interval
- there is no power on the vehicle's high-voltage cables, BUT:
- the vehicle's HV battery remains charged and therefore continues to be a potential source of power despite being disconnected from the rest of the system
Once all the necessary safety measures have been carried out, the manufacturer's service information will directly refer to a voltage value with a suitable measuring device to ensure that all high-voltage sources have been isolated or discharged.
Reading a measured value
If the vehicle's high voltage system has been properly shut down and you have put on the insulating gloves, the voltage value can be detected. Before doing so, turn on the meter, select "Volt DC," and check if the meter is working by measuring a known low-voltage power source, such as the vehicle's 12V battery. Faulty measuring devices can produce incorrect "zero voltage" values!
Since the high-voltage circuits are isolated from the chassis ground, the measured values are read directly at the HV connections. The service information from the manufacturer contains instructions on what to measure and where. The measured value is usually recorded at the "normally switched off" HV safety relays, which connect the HV battery pack to the rest of the system. However, if high-voltage cables need to be exposed or disconnected for maintenance, you must ensure that there is no voltage by reading a reading between the HV cable ends and between each HV cable end and the vehicle grounding.
Employees experienced in working under high voltage usually apply the "one-hand rule" when working on a high-voltage circuit. This means that only one hand can work on high-voltage circuits at a time. This is intended to reduce the risk of electric shock emanating from the current conducted by the employee, which can also lead to cardiac arrest. Such a measure provides additional protection to the rubber insulating gloves already in use.
In order to comply with the "one-hand rule" in a direct measurement of the high-voltage circuit, at least one electrical line must be equipped with an insulated alligator clip. Connect the wire to the alligator clip with one hand to one of the circuit connectors, and make contact between the wire and another connector to take the measurement. A cable or vehicle grounding may only be held or touched with one hand at a time.
Do not reapply the probe to high-voltage connectors as this will damage them. Do not use sprays (including cleaning sprays) in high-voltage circuits, as this can energize the air around the circuit and make it conductive. Keep in mind that the arc potential is significantly higher on a 600-volt circuit than on a 12-volt circuit. Immediately insulate disconnected exposed high-voltage terminals with electrical tape.
Put it all back together
High-voltage connections secured by threaded bolts or nuts shall be tightened to the torque specified by the manufacturer. The quality of the electrical connection and its corrosion resistance depends on the appropriate torque.
Check that you have not left any part or tool in the area where you are currently working. If a vehicle's high-voltage (HV) system has a removable service plug, make sure it is securely and correctly seated when you reattach it. Do not reconnect the 12V battery until the vehicle's HV service plug or switch is back in its original position and all removed access covers have been replaced.
Once the vehicle is reassembled and the 12V battery is reconnected, start it and look for codes and/or the READY status. Some hybrid systems always issue codes when the system has been disrupted or shut down. You may need to delete these codes and then check to see if they are still there. If necessary, start electrical systems such as power windows. Take the vehicle for a test drive to check if it is working properly. Finally, check your insulating gloves for damage before putting them away.
To recap
The safe measurement of high-voltage and high-current circuits requires discipline and awareness of the problem. Shortcuts are not permitted: the right procedures must be applied and the right equipment must be used. Remember that once the vehicle has been switched off, the first step of the high-voltage measurement procedure is to disconnect the 12 V battery and to reconnect it in the last step before the vehicle is restarted.
Take enough time to develop methods for safely measuring high voltages before venturing into hybrid technology, as this will lay the foundations for a competitive and profitable set of skills that will benefit you in the future.