Differnces between 3rd gen Prius hybrid battery packs (2010t - 15) and 4th Gen (2016 to...) NimhThe biggest change is the Prius now comes with Lithium batteries that are approximately 35 pounds lighter. On the least expensive models the the Prius still has the Nimh batteries. All the rest of the comments pertain to the Nimh battery module.
Each battery module is 1.1 ounces lighter. That may not seem like much but times 28, the number of modules in the Prius and Prius V, equals 1.925 pounds. The modules are the same size and only the top is different. There is no differences in functionality. They are completely backwards compatible.
Thinner rods that hold the modules together. Since the older rods were hollow the saving is only 2.2 ounces per rod and a little more for the top rods. The new end connector on the computer side is much easier to work with. The older style rod had a flange which was held in place by two nipples. In taking apart a pack if you loosen the end bolt you could easily tear up a module with the flange spinning around. Not a nightmare for Nimh batteries but Lithium - a potential fire!
The case is also lighter. The most obvious change is the case does not extend out to hold the fan.
The safety plug is now much smaller and the fuse it contained is in a separate holder at the end of the pack. The safety plug no longer disconnects the pack near the middle but instead just disconnects the battery pack from the relay module. So now if you are stupid enough to put your hands at both ends of the pack you can get roughly a 215 volt shock even with the safety plug removed. Not sure why this was done except to save a few ounces of weight from slightly shorter cable.
Overall it looks like a lot of engineering to save weight and size. This enabled the battery to go under the rear seat and significantly increase the cargo capacity by 3 plus cubic feet and easily used capacity by even more. This also lowers the center of gravity for better handling.