Battery pack replacement is becoming more common as hybrid vehicles age. The second-generation Prius started with the 2004 model year and lasted until 2009. They have racked up many miles in the past decade or more, and the nickel-metal hydride battery is probably not providing the same range as before.
Inside a second-generation Prius battery pack are 168 1.2-volt cells that produce 201.6 volts when fully charged. Twenty-eight battery modules are wired in series and configured into 14 blocks. The battery pack has a heat management system to optimize the temperature under hot and cold conditions.
Any battery’s life comes down to the chemistry inside of it to store and discharge energy. Like all chemistry experiments, heat plays a part in the reaction inside the battery. Heat can degrade a battery and cause reduced capacity. Charge and discharge cycles can also take their toll on the battery.
The Battery ECU measures the state of charge, internal resistance and current flow to determine the battery’s condition.
Most codes that indicate the battery needs replacement will turn on the check engine light, and a message will appear in the information center.
These codes are what bring the customer to your shop.
Code P0A7F has the description of Hybrid Battery Pack Deterioration. The criteria for setting the code are based on profiles determined by Toyota engineers. The first is an increase in the internal resistance of the battery pack for one trip. The second criteria threshold is the state of charge of the battery. If the state of charge falls outside of the expected results for two trips, code P07AF is set. Also, the ECM will use the battery temperature sensor as part of the monitor’s enabling conditions. With this code, the only way to resolve it is to replace the battery pack.
The battery ECU can read the internal resistance and voltages of the battery blocks. All of the blocks should be within 0.3-volts of each other. If the values fall outside the parameters, a code will be set for a weak block. The monitor for these codes is continuous and sets immediately. Typically, the blocks in the center of the battery generate weak block codes.
The description for DTC P0A80 is Replace Hybrid Battery Pack. This code takes two trips to set and indicates the voltage difference in the blocks exceeds standards. The issue could be with the batteries or in the bus bar connections with the modules.
There are remanufactured battery packs on the market. The remanufacturing process involves removing the modules from the battery pack and analyzing them. Bad cells are discarded, and the good cells from several batteries with similar performance are reused, so the battery is balanced. They will also install new bus bars and check the overall battery performance.