Tool Q&A: Want to do hybrid repairs?

Tools, equipment and advice from getting started with hybrid repairs to rebuilding HV batteries.


Always be sure when diagnosing these vehicles that the HV computer has the latest software. For example, Honda Civic hybrids with a good HV battery state of charge and an out of date calibration ID (fancy terminology for the version of software on the vehicle) need a software update as per a TSB. If you cannot read the PID, you cannot diagnose the IMA (Integrate Motor Assist) light problem on that vehicle and might be replacing an expensive battery instead of updating software.

Q: How can you rebuild a Toyota Prius' hybrid battery?

A: You can rebuild a hybrid battery by diagnosing which cells are bad. Then you can replace a bad cell with a known good cell. With the Prius, simply replace a Gen 1 cell with another Gen 1 cell, or a Gen 2 with another Gen 2 cell. Gen 1 cells have a lot of problems, so it is possible to change over the whole battery back of Gen 1 cells to Gen 2, but they cannot be mixed and matched.

These batteries can also be taken apart and load tested if necessary. Usually, not all the batteries go bad at once, just a few do, so with a load tester and a multimeter, you can compare the batteries to one another.

Here's how you test HV battery cells step-by-step:

  1. Check the voltage on each cell and make a notation on each cell that is below 6.9V to 7V. (Figure 1)
  2. Put a normal battery charger on any of the low voltage cells at the lowest setting possible for about 20 seconds. The moment that HV battery cell feels a little warm (not hot, warmer than room temperature), stop charging. Note: The battery cells' ground and power sides will flip flop, so be sure to connect your charger properly.
  3. Load test each HV battery cell with a simple load tester. They should hold for about five seconds before the voltage dips down. Simply compare the cells to one another. The bad ones really stick out. (Figure 2)
  4. Replace the bad battery cells with known good cells (from a junkyard HV battery, for example).

Gen 2 battery cells can be used in place of Gen 1 cells on a Gen 1 Toyota Prius. If you don't want a comeback on a Gen 1 battery, it is best to change over every single battery cell with a Gen 2 battery cell. Believe it or not, this works like a charm; it's the HV battery rebuilder's secret.

Some experts question the validity of rebuilding HV battery packs at all and have brought up the following concerns:

  • There is no perfect method to test individual battery cells. Some experts also claim you cannot use terminal voltage to determine the state of health or the performance of a nickel battery.
  • Profession HV battery rebuilding companies test cells using a power test and an energy test. A power test will load the batteries to determine battery internal resistance while the energy test will load the batteries to determine battery capacity.

It is recommended only to attempt this procedure after proper hands-on training.

Q: Is it worth tooling up for hybrid service?

A: This is a tough one. A hybrid customer is not exactly a big-spending customer. After all, he spent money on a car purely so he can save money on gas, so who says he wants to spend a lot on repairs? Furthermore, hybrids do not need frequent brake or transmission service either. They are not a huge moneymaker unless you can corner the market on HV battery and transmission rebuilding. However, it is always good business not to send work back to the dealer. Also, more importantly, most tools you use for hybrid service cross over to other vehicles.

Q: What common tests can I do with my meter?

A: Hybrids require the same sort of voltage drop testing as a regular car. For example, a Toyota Prius HV battery often just has an issue with the copper bus bars corroding, causing internal voltage drops in the HV battery. (Figure 3) As you can see in the picture, ultrasonically cleaning this in the Launch X-SonicClean did the trick. For demonstrative purposes, we cleaned half the corrosion off the copper bus bar so you can see before and after.

The one special test you will need to know is an "insulation test," which requires a MegOhm meter capable of sending a high voltage pulse. Be aware of the capabilities of your insulation tester. Some experts say using 1,000V on a system that should be tested at 500 or 250 volts can damage the electronics in the system being tested. Make sure to check your tool's settings before doing any tests.

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