Why did my battery fail?
Ultimately what has caused or will cause your battery to stop working will be one failed cell in 1 of the 28 modules in the pack. Here is how it works, one NiMH (nickel-metal hydride) cell has a voltage of 1.2 Volts. Figure 1 is an example of one module from a Prius battery. It is sealed unit that contains 6 (they are numbered) battery cells connected in series. When batteries are connected in series the capacity remains the same but their voltages add together. So when you add them up (1.2V x 6) you get a 7.2 Volt 6.5 Ah (Amp-Hour) module. Note: 6.5Ah is the amount of power that the module could hold when it was new.
Your battery is designed with 28 of these modules connected in series. Figure 2 shows a top view of a Prius battery with the safety cover removed. When you add all 28 modules together (7.2V x 28) you get a 201.6 Volt 6.5 Ah battery pack. (this is when it can be dangerous!)
As the hybrid system in your Prius charges and discharges the battery it (the battery) generates heat, a lot of heat! Toyota designed a very effective cooling system to remove this extra heat. A cooling fan draws in cabin air from an intake that is positioned on the passenger side of the car next to the back seat. If you have a Prius you should be very aware of it because it is important to keep that intake free from any blockages. That cabin air is fed thru some ducting and into the top of the battery casing. The air then flows thru the designed spaces between the modules and then out of the bottom of the battery case thru another piece of ducting that carries the heat out of the car. See Figure 3.
As hard as the engineers tried to design this system to provide even cooling to the entire battery, it just didn’t happen. Because of the way the modules are stacked together the sections on the end naturally stay cooler than those in the middle. All batteries depend on chemical reactions to work properly. Every battery has an optimal temperature for those reactions to take place. If you heat or cool a battery too far outside of its designed temperature band the chemical reactions slow down. The cells in center of the battery spent their life in harsher conditions than those on the outside so they are weaker and have lost more of their original capacity than the rest. Figure 4 shows a graphical representation of a batteries typical module capacity when we first test it.
This is the capacity imbalance that will result in the eventual failure of a hybrid battery pack. The computer system in a hybrid vehicle monitors the voltage in each group of modules as they charge and discharge. The inner modules will indicate full and empty very quickly because their capacity is so low, while the outer modules are actually doing very little. This heavy loading on the weakest modules is what eventually gets them to where they have almost no cycling range left in them. (This is where the rapid charging and discharging symptom comes from, it’s not really the whole battery that’s fully charging and discharging it’s just the weakest modules). Once that happens all it takes is parking the car over night with a low charge on the battery and the lowest cell in one of the weak modules will self-discharge enough in a short amount of time and cause what is called polarity reversal. It ruins that cell permanently and because the cell is in a sealed unit it renders the module bad. So if you remember a module is six 1.2V cells for a total of 7.2 Volts. It is now five 1.2V cells resulting in a 6 Volt module. Toyota designed the computer to prevent battery use if the voltage difference between the lowest and highest modules in the pack exceeded 1.2 Volts, in other words the car knows that a cell has failed and it will not work and has probably triggered a PA080 code and some low block voltage trouble code. The cooling fan may run on high to ensure that the battery gets maximum cooling while it is this malfunctioning state.