Posted to Technical Discussion Forum on 10/10/2012
Gen II Prius, Why no MIL with Delta SOC = 82% !?
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
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
We would appreciate help and advice regarding how we might
further test or evaluate the car or the battery
Hawthorne Auto Clinic
Portland, Oregon, USA
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