NMH Batteries used by Toyota Hybrids are very rebuildable and Infinitive supplies a rebuild/exchange service.
Here is a video on testing the 28 cells and the rebuild: (NB: Only 2 cells are defective)
Keep in mind that the biggest risk to a NMH Traction Battery is heat. Modern ones have cooling fans with intake vents usually locates close to the back seat so its important to keep the inside of the car free from dust (including animal hair) and to not cover the fan vents with a doggy blanket etc.
The bigger Toyota NMH Batteries have 28 cells while the smaller ones such as the Yaris(Vitz) have 20.
