Charging is the process of restoring a discharged battery to its original rated capacity. In order to be able to provide useful energy, the battery
CHARGING METHODS FOR NIMH BATTERIES 81
95 96 97 98 99 100 101
-10 0 10 20 30 40 50
Charge Temperature (°C) Capacity (% of Rated Capacity)
Capacity (%)
Figure 4–9 Variation of battery capacity during charging acceptance inefficiency.
pack must be charged via the proper charge method. Various methods of charging are used to charge traction batteries.
The sealing principle of the NiMH battery suggests that the capacity of the hydrogen storage negative electrode is greater than that of the positive electrode. Thus the negative electrode is not fully charged when the positive electrode is fully charged. Thus any oxygen generated at the positive electrode during the charge and overcharge is chemically con- sumed at the negative electrode. This phenomenon suppresses the gen- eration of battery internal pressure. However, during a rapid charge, internal pressure rises rapidly. The rise of the pressure actuates the safety vent and the electrolyte leaks. Loss of the electrolyte results in decrease in the electrolyte volume, lower discharge voltage, and lower cycle life.
The overvoltage (difference between the equilibrium voltage and the charge potential) tends to be greater when the charge current is higher.
For a lower charge rate of 300 mA, the potential of the negative elec- trode reaches the hydrogen generation potential at the SOC of 100%.
In contrast, as shown in Figure 4–10, at a higher charge rate of 1,000 mA, the potential of the negative electrode reaches the hydrogen gen- eration potential at 50% SOC.
The first method utilizes an overnight charge (typically a slow charge) using a maximum constant current at C/3 A. This charging is a two-step process:
82 ELECTRIC VEHICLE BATTERY CHARGING
0 0.5 1 1.5
100 350 550 1000
Charge Current (mA) Battery Internal Pressure (MPa)
Internal Pressure Vent Open Pressure Figure 4–10 Variation of battery pressure during charging.
1. The first step is at a higher current, I1(the end of this first step is determined by the criterion, c1)
2. The second step is at a lower current, I2(the end of this second step is determined by the criterion, c2)
In the overnight charge, the first step current (I1) and second step current (I2) ranges between I1L-I1Hand I2L-I2H, respectively. The allowed temperature rise is up to ranging between T1L, T2L, and T1H, T2Has the lower and upper temperature limits respectively. The first step charge is terminated once the temperature T1His reached, i.e., when the value of the slope dT/dt reaches the criterion c1(T) as shown in Figure 4–11 for a room temperature charge at the rate of 0.2 C.
The criterion c1(T) is a function of the actual battery temperature and is represented in Figure 4–12.
When the criterion C1 is reached, the first step of charge is stopped and the first step capacity charged is Ah1. With zero rest period between steps, the second constant current I2charge step is initiated. The end of charge criterion C2 is reached when the capacity charged Ah2during the second step may be represented as
Ah2 =mAh1+b where m and b are constant values.
CHARGING METHODS FOR NIMH BATTERIES 83
0 5 10 15 20 25 30 35 40 45 50
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 Time (hour)
Temperature (°C)
0 2 4 6 8 10 12
Slope dT/dt (C/hour)
Temp T1(°C) dT1/dt
Figure 4–11 Slow charge temperature profile for NiMH batteries.
Using this charge mode, the end of the first step of charge is detected close to the theoretical end of charge at room temperature, 22°C.
The second charge method utilizes a fast charge one step constant current rate allowing the cells to recharge to 40% of their capacity from 20 to 40% initial state of charge. The battery packs are typically charged for 10 to 20 hours at over 0.5C A but less than 1C A and temperature ranging from room temperature to 55°C. As charging the batteries at a current in excess of 1C A causes internal cell pressure to increase result- ing in the safety vent to be activated. This results in the electrolyte leakage. When the temperature of the batteries is under or over the com- mencement of the charge, rapid charge is terminated, and trickle charge is initiated.
Allowing high current to flow to excessively discharged or deep dis- charged batteries during the charge results in the formation of an elec- trode barrier that makes is difficult to restore the capacity of the traction batteries. It is important to first allow trickle current to flow, restore the battery voltage to its upper battery voltage limit control (1.8 V/cell approximately), and then proceed with the rapid charge current. This voltage is referred to as the rapid charge transition voltage restoration current and is normally 0.2 to 0.3C A.
84 ELECTRIC VEHICLE BATTERY CHARGING
0 2 4 6 8 10 12 14
-20 -10 0 15 20 40 45
Temperature (°C)
Criterion C1 (dT/dt)
C1 (C/hour)
Figure 4–12 The c1(T) criterion for NiMH batteries.
More precisely, when the battery voltage drops from its peak to 5 to 10 mV/cell during rapid charging, the charge is terminated and switched to trickle charge. In addition, the temperature of the traction batteries rises rapidly during the rapid charge. When the temperature rise of 2°C/min is detected, rapid charge is terminated and charge method is switched over to trickle charge as shown in Figure 4–13.
The overcharging of NiMH batteries, even with the trickle charge, causes a deterioration in the efficiency and cycle life. In order to prevent overcharging by trickle charging or any other charging method, the pro- vision of a timer to regulate the total charging time is recommended.