Hybrid Vehicle Service Saftey

During the initial introduction of Hybrid vehicles, there were many rumors about the perceived danger of servicing them. While the batteries contain harmful or even lethal energy, proper service practices will render these vehicles safe to service. These vehicles should not be serviced in a careless manner and HV gloves should be worn when called for by the manufacturer service information.

Test your Hybrid safety knowledge:

Hybrid vehicle high voltage relays are:

  1. Normally open
  2. Normally closed

The relays are normally open and very sophisticated software ensures that if one sticks ON, the vehicle will not power up. These are checked on every vehicle start up for proper operation. Disabling the HV plug is a back-up safety measure, which few people realize. Unless the relays have stuck, there is no voltage coming out of the battery pack when the system is shut down.

Hybrid vehicle high voltage relays are on:

  1. the positive cable
  2. the negative cable
  3. both cables

Any of these answers are correct; Honda has the relay only on the positive cable, possibly because it contains all of its DC voltage behind the rear seat. Toyota has relays on both of its cables but high voltage DC is applied from the battery to the engine compartment. The Ford Escape likewise applies high DC voltage to the engine compartment to be inverted on top of the transaxle.

what about the capacitors that store voltage even after shutdown?

Hybrid Capacitors

Honda Hybrid capacitors are located behind the rear seat under a metal shield that cannot be removed without disabling the high voltage. Toyota states it takes a minimum of five minutes before the 273.6 volts of electricity is sufficiently discharged. The Pico capture below was obtained after turning the key OFF on a Gen 1 Prius. Discharge time was slightly more than 500 ms. But, in the event of a failure in the DC to DC converter which is generally the route for high voltage capacitors to dissipate voltage it is possible the time would be delayed. The safest approach is to use the voltmeter to confirm the voltage is dissipated before touching anything without High Voltage Safety Gloves.

Hybrid Vehicle Safety; Working With the Batteries

  • The Gen 1 Prius Battery Recall

    The Prius battery recall was enacted to prevent potential failures. It involved the possibility for a DTC P3009 (high voltage leak) due to small amounts of NiMh electrolyte leaking around the battery studs that connected to the bus bars. The solution was to clean the contamination, install resin insulators at both ends of the battery pack, replace the bus bars and add a sealing resin to each of the bus bar to battery module nuts.

  • Toyota Gen 2 Battery Pack Bus Bar

    The small wires with connectors provide the voltage readings for each pair of prismatic battery packs. Nominal voltage of each pair is 14.4 volts. The HV Battery ECU uses this to help calculate battery state of charge, a vital parameter used by several ECU’s including PCM, ABS, and HV Control.

High Voltage Lineman’s Gloves

The lineman’s gloves are big and clumsy but it makes sense to wear them when dealing with lethal voltage. The biggest problem with vehicle safety is that like anything, experience can cause carelessness. At a dealer franchise we visited, the technician we interviewed could not find a single pair of HV gloves. The technician stated that HV gloves were not used which subjects the technician to extreme danger of electrical shock. Note that the battery recall document is 6 pages in length and contains numerous safety issues. Toyota recommends wrapping the sockets used to loosen the bus bar nuts with vinyl electrical tape. This is a place where the HV gloves should be worn because the electrolyte leaks actually can carry the high voltage into the case in a significant amount to give the technician a shock. The danger is small, but it is real.

Fluke 1507 Insulation Test Meter

A good quality meter will usually meet the minimum requirement of Cat III. A recommendation is made for obtaining CAT III leads in many Hybrid classes. Although the voltage can be as high as 330 volts with a terrific potential for amps, it should be remembered that it is not like ignition voltage. It is not going to arc like a spark plug. CAT Ill leads differ in construction so that less metal is exposed, thus there is less chance of shorting. The insulation test meter is used to test for potential voltage leakage. Like a high powered ohmmeter, it is used only on discharged circuits, not ones with voltage in them. Toyota publishes specifications in their service information as to the setting of the tool (50 volts through 1000 volts) and the specific resistance allowable. The most common reason for checking for voltage leaks is the DTC P3009. Note from the chart that DTC P3009 is not considered by Toyota to be a serious enough condition to affect the operation of the vehicle in any way.

This post was written by: Martin Hand


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About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.


  • Nannette roach says:

    How do I discharge a capacitor on a honda

    • Martin Hand says:

      This can be very dangerous, I wouldn’t attempt to discharge it unless you know what your doing. It’s as simple as applying a power source or resistors to drain the capacity. It will drain on its own over time. You can check its charge with a voltmeter.

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