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Traction Battery Charging

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Hi All,

Great forum here with loads of helpful info.

Recently picked up a CPO 2008 RX-400h with 31,000 miles. I've noticed that while traveling in bumper to bumper traffic; once the hybrid battery drops to 2-purple bars, the ICE will turn on. This is presumably to charge the hybrid battery, however the energy monitor does not show this as happening.

It shows orange arrows from ICE to generator, orange arrows ICE to front wheels, and yellow arrows generator to wheels. Nothing goes to battery. It's strange that power would be "going to the wheels" when I'm standing on the brake. I've seen this happen multiple times. Once the vehicle begins to roll, then arrows direct toward the battery again in a stuttering back and forth dance. I've never been stationary long enough to see what would happen to the traction battery if I didn't move at all. Seems a bit weird.

Oddly, mimicking pieces of the ISC reset procedure, holding the brake and gas, WILL change the direction of arrows going into the battery while stopped and tick up a bar or two in about 15 seconds.

What seems odd is that during about a 15 minute traffic jam for a light, once the ICE came on to charge, it stayed on for about 10 minutes but didn't charge the hybrid battery at all. Average MPG dropped from 26-18 while I wasn't moving. Has anyone else experienced this have or any suggestions? Beyond this the car runs great and has no issues.

Thanks for any help!


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I've had my '08 400h for 3.5 years and with the type of driving I do, I've rarely seen the purple bar level for the battery. My recollection is that the monitor shows what you would expect when stopped with the purple bars showing - that the ICE kicks in charging the battery. Once the battery has received enough of a charge, the engine shuts off. We also have a TCH and it operates the same.

If no one else opines on this with a good explanation, I'd have the dealer check it out. It's CPO plus it still should be under original factory warranty.

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Turning off heat and AC/defrost will help keep the ICE from running when stopped. I noticed the ICE runs much more when the climate controls are set to AUTO with AC on, especially when using heat. Your avg MPG must have been recently reset if the avg dropped that much at one stop.

I wouldn't get too caught up with the arrows on the energy flow screen. They are only a basic approximation of what's going on. It doesn't always show when the ICE is turning. The hybrid system program is so complicated you will drive yourself crazy trying to understand what (and why) the energy flow screen is really doing.

Also do not shift to N or B mode during normal driving or sitting in traffic. Shifting to N will keep the ICE running unnecessarily. B mode will reduce mileage in most situations - it's only for steep hills to hold speed and reduce friciton braking.

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Aside from mimicking the ICE reset procedure, have you actually done the ISC reset? The way you're describing it doesn't sound right to me.

Thanks for the responses.

Lemon, I have actually done the full reset procedure as described elsewhere on this forum. The per tank difference I've found between properly set ISC values on identical driving is 24 vs 27 MPG. I found myself having to do it several times since I purchased. I got curious when I read information regarding the AUX battery dropping below 7 volts defaulting the ISC values.

I had the dealer check the battery and was told it tested good. Out of curiosity I bought a new 12-volt and installed it myself a few days ago. It's honestly like a different car, ICE comes on far less frequently. I'll take the original battery elsewhere and get it tested to be sure.

In response to Maseace, I had recently refueled and reset the ave MPG, which likely accounts for the dramatic drop.

I will keep an eye on the situation and post any further developments.

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