I find the cause of my problem...finally!

Prius92 · Jul 14, 2025 at 4:08 PM

Hayslayer said: ↑

I've built probably 500 Gen 2 batteries and probably forgotten more about the Gen 2 battery then most people will ever know.
The temp sensor on top is the incoming air temp. The three thermistors on the bottom are there for a reason. Because air flows downward through the gaps between the modules and the bottom is going to be the warmest. The module you covered with expanding foam is one of the LEAST airflow modules of the pack. Think about how much air flows over the top of each module, through the gap and under the bottom..
Your photo shows some foam you jammed into the air chamber to act as a backer for the expanding foam, except that backer foam is also past the closest module, which exposed that module to being covered by your expanding foam, which expands really really well. It's likely the first foam insert was pushed one or two modules even further back, potentially allowing the expanding foam to cover the first 2 or 3 modules and blocking the cooling air passages that allow air to flow down between the modules. regardless, good luck. It will certainly provide a Wth moment for whoever opens the battery pack in the future. I've had plenty of those moments, so this DIY repair doesn't surprise me much at all..Pretty sure there's a thread somewhere on the forum about these type of repairs being discovered by rebuilders who frequent this forum...kind of like a wall of shame...maybe this will be posted there sometime in the future. Because of a simple little piece of plastic.........
Click to expand...

In all honesty the design of the battery in the 2nd gen is terrible. For starters the foam they used on air sealing areas is very low density and as such degrades in high heat environments and is damaged/torn easily.

The sheet metal they used is way too thin for the amount of weight involved and warps/bends easily.

You can't just buy the foam they used, either. The list price of the intake duct is $188, for literally no more than $10 worth of plastic and foam.

Personally, I think they should have used duct work that had flanges with rubber seals.

ChapmanF · Jul 14, 2025 at 5:12 PM

Prius92 said: ↑

You can't just buy the foam they used, either. The list price of the intake duct is $188, for literally no more than $10 worth of plastic and foam.
Click to expand...

That's the usual story for a lot of weird-shaped, largely-empty dealer auto parts:

$ 9.00 plastic
$ 1.00 foam
$ 30.00 shipping
$ 50.00 warehouse space
$ 98.00 cool logistics system that gets it to any dealer by 10AM tomorrow
–––––––
$188.00

Hayslayer · Jul 14, 2025 at 8:16 PM

Prius92 said: ↑

In all honesty the design of the battery in the 2nd gen is terrible. For starters the foam they used on air sealing areas is very low density and as such degrades in high heat environments and is damaged/torn easily.

The sheet metal they used is way too thin for the amount of weight involved and warps/bends easily.

You can't just buy the foam they used, either. The list price of the intake duct is $188, for literally no more than $10 worth of plastic and foam.

Personally, I think they should have used duct work that had flanges with rubber seals.
Click to expand...

It's all about mass production and using a system that is 'good enough'. I'm pretty sure Panasonic engineers and Toyota engineers know exactly what the end product needs to be and design the 'item' to meet those needs. I think a Gen 2 Prius HV battery typically lasting 13+ years and 200k+ miles is pretty reasonable. If they got fancy with everything, add 20k to the price of every car.
As for parts? That's why ebay, this enthusiast forum and many other enthusiast forums exist...............

I find the cause of my problem...finally!

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I find the cause of my problem...finally!

Prius92 Active Member

ChapmanF Senior Member

Hayslayer Member