The Temperature Dependence of Battery Capacity

Table 3–1 Current and future VRLA battery specifications.

Midterm Current Pb-acid Specifications Specific Energy 45Wh/kg 80Wh/kg Energy Density 95Wh/L 135Wh/L Specific Power 245W/kg 150–200Wh/L Recharge Time <10hrs. <6hrs. Cycle Life (C/3) 600 600

Figure 3–1 The estimated Peukert plot at 80°F for an 80Ahr battery.

86

Time (min)

84

82

80

78

76

74

70 80 90 100 Current (A)

Estimated Peukert at 80 °F

For example, an 80Ahr VRLA battery, Peukert constants n may vary between 1.123 to 1.33 and K may vary between 138 to 300 respectively. The graph in Figure 3–1 is a Peukert plot at room temperature, 80°F.

THE TEMPERATURE DEPENDENCE OF BATTERY CAPACITY

The useful Ahr capacity available from the VRLA is dependent on battery temperature and may be represented by the following equation

Ct = C77 ¥ (1 - 0.065(77 - t))

where t is the temperature in °F, Ct is the battery capacity at t °F and C77 is the capacity of the battery at 77°F (room temperature). For example, C3 capacity at 32°F, for a 80Ahr VRLA battery is expressed as C3 (32°F).

C3(32°F) = 80 ¥ (1 - 0.0065(77 - 32)) = 56.6Ah

Similarly C0.1 at 80°F for a 80Ahr VRLA battery is 36.3Ahr. Thus C0.1 at 120°F is 45.74Ahr.

Thus under a constant current discharge and variation of temperature the battery pack capacity changes the performance of the electric vehicle (EV). This is observed as a variation of the driving distance before an EV recharge.

As illustrated in Figure 3–2, the graph is the estimated VRLA battery capacity with respect to the battery pack temperature. A 80Ahr VRLA battery above room temperature, 77°F, exhibits a larger than rated battery capacity. (See Figure 3–3.) This increase is larger at higher temperatures. A fully charged battery pack when discharged at 100°F can deliver approximately two times the rated battery pack current than a battery pack at room temperature under similar discharge conditions.

Figure 3–2 Variation of estimated VRLA battery capacity with temperature.

Battery Capacity (Ah)

160 140 120 100 80 60 40 20 0 400 350 300 250 200 150 100 50 40 30 20 10

Discharge Current (A)

T = 100 T = 90 T = 80 T = 70 T = 60 T = 50 T = 40 T = 30 T = 20 T = 10 T = 0 T = -10 T = -20