The customer brought in a 2004 Toyota Prius with 103k miles, complaining of unresponsiveness, sluggishness on acceleration, and decreased fuel economy (from 43mpg to 26mpg) for about the last week and a half. On a few occasions the battery cooling fan has turned on high, causing noise that the customer had never previously noticed; nothing was blocking the battery fan's intake. There are no warning indicators on the dash. The multifunction display usually indicates the battery SOC to be very low on start-up. The Toyota dealer had not been able to determine a cause of the symptoms, but did find a rodent nest in the cabin air filter. We don't have complete information as to what the dealer did to diagnose the problem.

On a test drive the car did seem somewhat sluggish. More noticeable was the frequency at which the engine cycled on and off while parked with little electrical load (heat and A/C were off). The engine would cycle on for about 8 seconds a little more frequently than once per minute. This cycle SOC between about 39% and 52%.

No DTCs were stored for any module.

The most striking piece of live data is the Delta SOC of 82.5%. This is outside of Toyota's given reference range of 0% to 60%. I would expect this value to be close to 0% in a healthy battery, and I find it hard to believe that individual battery modules that differ in SOC by 80% would not cause a DTC to set. As I watched the data, Delta SOC did occasionally update, but was generally near 80%.

Another piece of data that stood out, but I'm less clear in understanding, is Win and Wout. I normally expect these both to be around +/-20kw. I'm guessing that Win maximum power (current x voltage) the HV ECU is allowed to flow into the battery. And Wout is the maximum amount of power that it's allowed to draw from the battery. Over the course of watching data over several test drives I saw Wout remain fairly constant around 20kW, but Win varied from -25kW to -7.5kW. I imagine that Win and Wout are adjusted based on battery voltages and temperatures; in this case maybe quickly rising temperatures or voltages caused Win to restrict current flow. It seems odd though, again, that this sort of wide varying number wouldn't cause a code to set.

More data that seems unusual: On a test drive, SOC plateaued at 99.9% for almost a minute. I was under the impression that it shouldn't really go much above 80%.

Battery module voltages were typically within a range of 0.13V of each other when in park, ready on (about 16.6V to 16.8V). When cranking the engine the difference between min and max modules was about .5V.

With all of these HV behaviors seeming a little strange to me, I didn't really think much about engine operation until I was told to do so by Identifix. The car is due for a tune-up soon, but nothing jumped out at me as being particularly bad. Fuel trims averaged close to zero with STFT peaking to -20% at brief WOTs. LTFTs were from 0 to 4%. I hadn't thought specifically to test MAF, but looking back CLVs did go up to 80%.

We disassembled to the point that we could visually inspect the battery and ventilation fan and ducts. We found small amounts of electrolyte leakage at several of the (-) bus bar terminals (photos available), but none in the rubber vent hoses on top of the modules. There was no discoloration or evidence of swelling or heat damage. The fan and cooling ducts were all clear and operational-- rodents had not gotten into this system. We later demonstrated the battery cooling fan noise for the customer to verify that its proper operation was consistent with the noise that was heard before (even if there might have been an abnormal high temperature condition that caused it to turn on high).

We advised the customer that their HV battery was likely to cause a DTC to set in the near future and that it would require replacement. The decreased capacity of the battery, or at least some of its modules, to store energy would explain the symptoms of sluggishness and decreased fuel economy.

We would appreciate help and advice regarding how we might further test or evaluate the car or the battery specifically.

Thank you.

John Mayer
Technician
Hawthorne Auto Clinic
Portland, Oregon, USA