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Caution When Doing Isc Reset

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Hey guys,

I recently had to disconnect the battery in my RX400h after removing the steering wheel for some polishing, so i decided to perform the ISC reset (even though i personally don't think it does anything, at least for the '08 model). But nevertheless i did it this evening.

I had just driven a good 20 miles on the highway so the engine temp was nice and warm (per the instructions of the ISC reset). I did the reset in my garage- held the car in D with my foot on both the brake and the gas until the battery (per the info screen) was completely charged with the 8 green bars. I then turned the car off, and turned it back on and pressed the gas to start the engine, and waited a couple of minutes for the engine to turn off, then turned the ignition off and went inside my house.

About 5 hours later, i had to make a short trip. Right after i started it up, i instantly heard this "running" sound, like a fan or small electric motor, coming from the back seat area. I turned the AC off, but the sound was still there. I got out and realized it was coming from below the rear seats, behind the small grates near the floor. Apparently the fans that circulate the air around the battery (the hybrid battery is under the rear seats) were running at FULL capacity as i could literally feel the air rushing past those grates. After feeling around i also noticed that the floor below the rear seats was HOT, just about as hot as if the car were sitting in the sun on an 80 degree day (even though this was nighttime with an air temp of about 60 degrees).

So i let the car sit running in the driveway for about 5 minutes but the fans didn't slow down at all (it sounded like a jet engine was running down there). I checked the energy monitor and the battery was still fully charged, so i drove the car around for about a mile and the battery level QUICKLY dropped to only 2 bars, and the fan noise mysteriously terminated. Drove the car around for another good half hour, and everything seems to be back to normal, the battery level back up to the 5 or 6 blue bar level.

My guess is that leaving the hybrid battery fully charged creates too much heat in that compartment. I've NEVER heard those fans running so quickly, i had no idea what was going on! But i guess the Rx400h did a good job cooling it down once the car was started. My only concern is why they couldn't be cooled while the car was off. What if i hadn't turned the car on tonight? Could that heat have built up and caused damage to either the battery or the car, or both?

So moral of the story? Don't do the ISC reset and leave the hybrid battery fully charged! :censored:

What are your thoughts about this?

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Interesting. On my Prius, when the batteries became overcharged when driving downhill, the ICE would go into an automatic start/stop cycle to bleed excess charge from the hybrid batteries. But the battery cooling blower never came on. Was the engine running when you let it sit with the battery cooling blowers on full blast?

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If you read the instructions/TSB, you're only supposed to run it for 30 seconds, and NO MORE THAN 40 SECONDS. It doesn't matter if the battery is fully charged (full bars), as long as you go the 30 to 40 seconds.

Where on Earth did you ever get the idea that you're supposed to run it until the charge indicator is full?

My guess is that parking with a full charged battery is not a problem, but you may have OVERCHARGED yours, hence the overheating.

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It charged pretty quickly, definitely didn't take 40 seconds. I've done the reset before but never noticed the fans coming on afterwards.

The engine was running when i had it sitting in the driveway afterwards, everything seemed normal except for the fan speed.

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  • 3 weeks later...
  • 2 weeks later...

OK, when charging a battery, part of the logic process that goes into determining if a battery is fully charged is a combo of measuring resistance and heat (among other factors, but the important criteria remains temp and resistance). So, whenever charging a battery, especially quickly or with a lot of current, the by product of doing so is heat. So, is it normal to quickly charge a battery and have the battery give off heat? Your proof is in your original post.

As for the auxiliary fans running, I must say that I have never heard any fan for that matter turn on or blow out from where the hybrid batteries are located. You might want to investigate the matter further, because the Lexus system is designed to never fully charge the batteries for obvious life cycle reasons and to prolong there durability.

When the system senses the batteries approaching 80% or so, the system will prevent further energy to be stored by the hybrid batteries. So, if for some reason you were capable in your attempt to reset the ISC, and caused the hybrid batteries to overheat in some shape, way, or form. I would book an appointment and have your system inspected and checked out.

Or, if you have an OBD II scanner, see if you can see any kind of fault code and if so keep that knowledge with you when you have your vehicle serviced/inspected. To me, it doesn't sound normal.



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