The Toyota Prius contains a 1.310 kWh battery, composed of 28 modules. Each battery module is made of six individual 1.2 V 6.5 Ah NiMH (Nickel Metal Hydride) cells in series forming a 7.2 V 6.5 Ah module. Each module contains an integrated charge controller and relay. These modules are connected 28 in series to form a 201.6 V 6.5 Ah battery, also known as the Energy Storage System (ESS). The computer controlled charge controller and battery management computer systems keep this battery between 38% and 82% state of charge, with a tendency to keep the average state of charge around 60%. By shallow cycling the battery, only a small portion of its net available energy storage capacity is available for use by the hybrid drive system. However the shallow computer controlled cycling dramatically improves the cycle life, thermal management control and net long term calendar life of the battery. Independent tests show no significant loss of battery performance over 10 years and 200,000 km of use.

(a) What voltage will the ESS supply when discharging at maximum charge (82%)? What about when discharging at minimum charge (38%)?

(b) How much total energy is available from the ESS when the computer system has the battery at maximum charge given the charging limits of 38% and 82%? Assume that the discharge curve is linear between 38% and 82%.

(c) The Prius can drive for 20 minutes as an all electric vehicle using only the energy available in the ESS between charging limits. What is the average current used when driving? The average power?

(d) Find the energy required to charge the ESS from 38% minimum charge to 82% maximum charge, again assuming that the charge curve is linear over the charging region. Explain the difference to the energy available from the discharge.

(e) What average power is required to charge the battery from minimum charge to maximum charge in three hours?

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B01 1.7 1.5 1.3 1.1 0.9 0.7 0.5 0 10 Charge 20 NiMH Charge Discharge Characteristic Discharge 30 40 50 State of Charge 60 70 80 90 100 Figure 1. NiMH Charge Discharge Characteristic Figure 1 above shows the charge / discharge curve for an individual cell in the ESS. Note that the cells have a different voltage across them when they are charging (absorbing energy) to when they are discharging (providing energy). B01 1.7 1.5 1.3 1.1 0.9 0.7 0.5 0 10 Charge 20 NiMH Charge Discharge Characteristic Discharge 30 40 50 State of Charge 60 70 80 90 100 Figure 1. NiMH Charge Discharge Characteristic Figure 1 above shows the charge / discharge curve for an individual cell in the ESS. Note that the cells have a different voltage across them when they are charging (absorbing energy) to when they are discharging (providing energy).