I found the missing 4" (vertical) in the trunk. It is below the battery.

then they should have made the smaller flat, and the bigger could stick up 3", in stead of down 3".

 

And if they started from the PHEV, the rear floor could have also been different for a bigger battery. Though I admit that I don't know how the suspension and E-four fit underneath.

The concept of TNGA, a universal platform that can be used in multiple models, isn't new.
Chrysler's K-Car: The First Modular Platform? - Autotrader
VW's new modular platform could spark a revolution | Autoweek
Volvo Details New SPA Modular Platform Architecture – News – Car and Driver | Car and Driver Blog

Looking at the Prius and Prime, it just appears that Toyota is limiting the amount of modular in TNGA to save more cost.

Are you saying Toyota left this empty space to allow expansion of the battery pack? If they could make the pack fit down into that space, why not do so from the beginning? The Volt and Energis have been out for years. Sedan hybrids with the battery in the trunk have been out for even longer. Loss of cargo and passenger space being seen as a negative was a known factor. Toyota should have at least heard how the PiP not losing cargo space over the Prius was considered a plus of the model.

Mid-cycle refreshes tend to be cosmetic tweaks and shifting of features. If there is something major, like AWD or another engine choice, the people would expect the cost to reflect it.

I remember a Toyota critter saying they could move the charger once the seating criticism started, and North America is yet to get the PV option. But that is an option. I would have thought giving priority to placement of standard equipment in the design. If an option, like a subwoofer, needs more space, why punish your customers that don't want it by taking away space from their car for the option. That assumes the Prime development underwent to usual order, though.

 

Note that those vents actually appear to have blowers (centrifugal fans) in them.

 

Nope. The NiMh battery is located between the front seats under the armrest.

 

the pip cooling fans are integrated into the battery case, iirc.

 

Battery charger.

 

I agree with your assessment, with one additional thought

Why contemplate altering the bottom pan, when you have a battery pack which could be altered in shape?

 

Note that more complex battery pack shapes can increase cost of the pack assembly significantly. That's part of why I'm personally for the VW/Hyundai layout of an under-seat battery and cargo area fuel tank.

 

Altering the battery pack shape is harder than it sounds. It's probably got twice the volume or more of the spare tire depression, it uses large rectangular battery cells, it has to be manufacturable, and it has to be able to be monitored and temperature controlled.

The right way to do this is to put it entirely under the rear seat, extending rearward to fit its size. But the gas tank is in the way.

 

If #3 is the NiMH battery, what is #4?

 

#4 is the solar battery ECU.

 

Why not hinge the battery pak, throw in some air shocks, tilt the battery and throw in a spare.

 

It's kinda heavy. The little NiMH pack in the hybrid is about 100 pounds.

I also think the compact spare is wider than the indent for it is deep.

 

That's a redesign and requalification. They were shoe horning a big battery into a car not designed for it so they just threw it in the trunk.

As stated above what they should have done is design G4 for plugin from day one and adapt that for hybrid.

 

Priorities.

You can't do it all in a single upgrade and we know there are more to come. Toyota chose to deliver a robust battery for an affordable cost, at the tradeoff of it not fitting as well. The result was an impressively efficient vehicle so competitive, it makes you wonder how likely a next-gen model without a plug will be.

Successful business requires a balance of priorities. As much as the engineers here would like more, it simply doesn't make sense after you come to realize the actual customers are dealers.

Think about it. What incentive is there for a dealer to purchase and stock Prime on their lot? It won't return much of a profit and their staff must do more to prep & sell the vehicle. Battery design doesn't even come into play. Dealers are much better off simply selling guzzlers. Why spend extra resources knowing you'll get less in return?

People here need to step back and look at the bigger picture.