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Toyota 2007 Hybrid battery packs dead vs. deminished

I have a 2007 Highlander Hybrid that I bought as a Certified Pre-owned vehicle.
One the whole, I have been happy with the vehicle, except that as it crossed the 50k mile marker, the fuel economy took a 20% nose dive from about 25mpg, to 20 mpg. I have taken it back to the dealer and they have not found anything wrong with it.

…none-the-less, I suspect my battery pack has begun to fade in its effectiveness.
The dealer has a black-box, red-light/green light test that they do which tells them the battery is green, but they seem to not know, or not be able to tell me what the threshold is that there battery diagnostics tests. This leads me to believe that the battery pack is beginning to fade and just like other batteries, it is holding its charge less and less but not yet fully failing.

Is there any way to actually test the battery pack and compare its current effectiveness to that of the original or a replacement pack?
Is there any way to actually get some information out of Toyota, beyond, well, you car passes our diagnostic test?

The battery is still under warranty, so in the perfect world, Toyota would replace the pack and my car would be back to great.
But, maybe that isn’t really the problem, so I want to avoid the worst possible case, where I pay $5k for new batteries and that isn’t the problem.

Help me Tappet Brothers… you’re my only hope…

I believe the battery packs have a 10 year warranty, so now to figure the drop in mileage, try another dealer and submit the drop in mileage as the problem and see if they come up with an answer. Assuming air, gas filter and all regular maintenance is up to date.

The chance of it being the battery is slim, they have been real world tested to 200+ K. It’s more likley your local gas station switched to winter blend gas plus it’s taking longer for your car warm up. Also note that the battery in your car never runs empty, and never gets fully charged that’s how it’s designed. It spends it’s life between 20 and 80% ish.

+1

From having experience driving my Mom’s hybrid (2012 Ford Fusion), let me tell you they get (comparitively speaking) lousy FE in the winter. A big part of their economy is shutting down the gas engine, and they won’t do that if they can’t maintain normal coolant temps.

First: is the temp gauge making it up to midpoint? Is the gas engine turning off at idle?

If that’s not it, try troubleshooting rather than getting “target fixated” on the batteries. If you have to “shotgun” parts at some point, start with the cheapest and easiest first. (I.e. not the battery pack!)

There are many many things that can cause a hybrid powertrain to lose some fuel economy, some of them mentioned here, some require significant time and detail to explain, some sound utterly ridiculous, and some may be unique to your particular circumstance and driving style. I’d closely monitor the fuel consumption using the odometer and gas pump method, and see if the drop in mileage continues, stabilizes, or improves.