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In HEVs (Hybrid Electric Vehicle), NiMH batteries are used. The energy and power are available from the battery. It is mainly measured by the SoC (State of Charging) and the temperature of the battery.

The HV (High Voltage) battery conditions such as temperature, voltage, and amperage and the SoC calculations are monitored constantly by the battery ECU.

The function of the HV battery is the repetition of the charging and discharging cycles while the vehicle is in motion. The discharging cycle is during acceleration and charging during regenerative braking.

Mostly the battery is charged by the motor/generator when the engine power is necessary. The battery ECU (Electronic control unit) calculated SoC (State of Charging) and a signal is sent on charging/discharging requests to the vehicle ECU (Electric Control Unit) so that the SoC can be maintained at a median level.

In the case of the Toyota Prius, The SoC target is 60% with a band of ±20%. During the vehicle on move, the vehicle ECU received the request from the battery ECU when the SoC drops below 40% (a controlled region).  After that, a request is sent to the engine ECU so that engine power can be increased to charge the battery.

However, in the case of SoC to be above 80% (controlled region), the batter ECU will send a request to the vehicle ECU so that the motor receives a request so that the battery power can be discharged by using the motor to drive the vehicle.

The available power from the battery is related closely to the battery SoC. When the SoC is in a range of 55-80%, the vehicle is allowed with high power from the battery. The approach is mainly designed for battery life.

So, in the case of a Hybrid electric vehicle, if a new or even a large battery back is added to the replace or add an extra charge to the available original battery pack. The main goal is the use of battery energy as much as possible.

The typical range for the SoC is to drop from 100% to 30% for the Li-ion battery pack. If the battery is operated below 30% that there will be a negative impact on the battery health or the battery life.

Source:- Hybrid Electric Vehicles by Chris Mi