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Toyota will recall 16,680 RAV4 Primes due to faulty battery software

Too much time in EV mode in cold weather can drop the battery below its buffer.

Jonathan M. Gitlin – | 75
RAV4 Prime badge
Credit: Toyota
Credit: Toyota

Toyota's RAV4 Prime is one of the better plug-in hybrids we've tested. Its 18.1 kWh lithium-ion battery gives it a solid electric-only range of about 40 miles, and its highly efficient Atkinson cycle engine means more than 40 mpg even when the battery is tapped out. But owners of model-year 2021 RAV4 Primes have a trip to the dealership in their future.

Toyota is recalling 16,680 2021 RAV4 Primes to fix a software bug that could cause the lithium-ion traction battery to discharge too much, shutting down the hybrid system in the process.

To be more specific, the problem can occur in cold weather. If the RAV4 Prime has been driven continuously in "EV mode," (just using the plug-in hybrid battery, not the internal combustion engine) and the accelerator pedal is then press rapidly to accelerate the vehicle, it's possible for the battery to drop below a specified threshold. "If this occurs, the vehicle will display a warning message and the hybrid system will shut down, resulting in a loss of motive power," according to the recall report.

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Toyota's first inkling of a problem was a report from a European RAV4 Prime in March 2022. By April 2022, it had received two more reports of RAV4 Primes losing power, one in Japan and another in Europe. All three crossovers had their hybrid system electronic control unit (ECU) replaced, but the actual cause of the issue remained a mystery until Toyota noticed that all three vehicles were driven in cold weather in EV mode.

Testing later in 2022 revealed that rapidly pressing the accelerator when driving continuously in EV mode in cold temperatures replicated the issue:

Analysis of these results indicated that the HV battery voltage decreased more than expected and the demand output limit information was sent to the HEV ECU for battery protection. However, the calculation of the output limit value by the HEV ECU was not following the output limit demand, and the output limit value was above the output limit demand. In this condition, if the accelerator pedal is pressed rapidly, the actual output is above the output limit demand, causing the voltage of battery cell to rapidly drop below the threshold, resulting in the warning message and system ready-off.

The fix is an update for the hybrid system ECU, which will require a visit to a dealership. Owners should be notified by early April when this will happen.

Listing image: Toyota

Photo of Jonathan M. Gitlin
Jonathan M. Gitlin Automotive Editor
Jonathan is the Automotive Editor at Ars Technica. He has a BSc and PhD in Pharmacology. In 2014 he decided to indulge his lifelong passion for the car by leaving the National Human Genome Research Institute and launching Ars Technica's automotive coverage. He lives in Washington, DC.
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KaarlisK
With the Toyota RAV4/Prius and others that have a similar drivetrain, the hybrid system is the transmission. If the electric motors are off, the car cannot drive. The electric motors (which are both part of the transmission / "eCVT gearbox") are also the starter for the ICE and the "gearbox". Toyota uses a planetary gear + TWO electric motors to transfer power between the battery, the ICE and the roadwheels in any direction, while also providing an infinite number of gearbox ratios. But if the electric motors cannot run, then the ICE cannot run, because the electric motors not only start the ICE, they are also literally the physical connection between the ICE and the road wheels.
https://en.wikipedia.org/wiki/Hybrid_Synergy_DriveWhen Toyota says "Hybrid system", they mean the entire drivetrain, including the ICE.
View: https://www.youtube.com/watch?v=-dHeRJdrnI8
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