Recently bought a 2008 RX 400H AWD with 185K miles on it. I have no idea whether the hybrid battery has ever been replaced. It's my understanding that the vehicle is sometimes supposed to drive powered by the hybrid battery.

When driving downwards on a slope for several minutes, the battery can go from just 2 bars, to around 75%-90%. That seems absurd to me, as if the battery's charge capacity has been entirely diminished so it only stores a fraction of what it originally could. The battery also gets depleted in a similar manner. This does not surprise me, but since I'm averaging 22 mpg, I'm not inclined to get a replacement battery.

Oddly enough, driving around until the battery is charged up and then parking it overnight results in diminished charged by morning. For example 80% down to 20%.

I am terrified about the hybrid battery possibly failing and shutting down the vehicle while I'm driving at speed on the highway. Are these just standard symptoms when the car is at this age or should I be looking into a replacement?

On that note, I'm curious whether the hybrid battery in the newer models work with my old model?

 

The hybrid batteries are pretty reliable. What you see as "empty to full" on the screen is really about 30-70% state of charge (SOG) for the batteries. I forget the exact number.

Your mileage is in the ball park. I will add with this car you really need to check it at the pump. My computer displayed MPG is always optimistic if I'm driving around town and pessimistic on the highway.

To drive on the battery only, you have to be very light on the throttle and below 35 mph. In my subdivision, I can get 3/4 of a mile doing that (speed limit is 20 mph). But that's it. With the way the car is designed, I cannot even pull into the garage from the driveway without the ICE starting.

I do not have your overnight battery depletion issue, so that might be an indicator of something. There is a way to calibrate the system manually, (holding the throttle to the floor with the brake on until the battery turns green). Search for that on here. Maybe that will help.

To my knowledge, there are no battery improvement or upgrades to be had. Years ago I remember a company making add-on batteries for the Prius. I think they listed a kit for the RX400h. Never heard of anyone trying it, but I don't pay that close attention.

 

I couldn't find a concrete thread on this after searching here and with Google. Could you please point me to it?

 

The ISC Learning TSB procedure is a bit controversial (or ISC reset). It was originally for the 2006-2007 cars, but also noted for the 2008. It can be done with TechStream or manually. The procedures for both are here on the first post of this thread (the three files are one pdf, so combine them):

https://www.clublexus.com/forums/hyb...all-years.html

The third page has the manual procedure if you don't have TechStream.

HTHs.