Duracell Ni-MH Pe rformance Characteristics, Capacity Effect of Discharge Rate and Temperature

Ni-MH Rechargeable Batteries

PerformanceCharacteristics(cont.)

Typically, when the current is higher and the temperature is lower, the operating voltage will be lower. This is due to the higher “IR” drop that occurs with increasing current and the cell’s increas- ing resistance at the lower temperatures. However, at moderate discharge rates (≈C/5), the effect of low temperature on the capacity of the nickel-metal hydride battery is minimal.

5.3Capacity: Effect of Discharge Rate and Temperature

The ampere-hour capacity of the battery is dependent on the discharge current and temperature, as can be observed in Figure 5.3.1. It should be noted that the delivered capacity is dependent on the cutoff or end voltage. The delivered capacity can be increased by continuing the discharge to lower end voltages. However, the battery should not be discharged to too low a cut-off voltage (less than 0.9 volts per cell) as the cells may be damaged (see Section 5.6). The recommended cutoff voltage for nickel-metal hydride batteries is 1.0 volt per cell.

Typically, optimum performance of the nickel- metal hydride battery is obtained between 0°C and 45°C (32°F and 113°F). The performance characteristics of the battery are affected moderately at higher tempera- tures. At lower discharge temperatures, performance decreases more significantly, caused primarily by the increase in internal resistance. Similarly, the effects of temperature on performance are more pronounced at higher discharge rates. The capacity of the battery decreases more noticeably as the current increases, particularly at lower temperatures.

FIGURE 5.3.1

	2.5	21°C (70°F)
	2.0	45°C (113°F)
Capacity (Ah)	1.5	0°C (32°F)
	1.0	-20°C (-4°F)
	0.5
	0

C/5 (0.48)	C (2.4)	2C (4.8)
	Discharge Rate (A)

Typical capacity of DURACELL DR30 batteries under constant current discharges at various temperatures.

[Conditions: Charge: 1C to -ΔV = 60mV @ 21°C (70°F); Discharge to 6.0V]

5.4 Energy Density

Energy density is the ratio of the energy available from a battery to its volume or weight. A comparison of the performance of various battery systems is normally made on practical, delivered energy density per-unit- weight or volume using production-based batteries and performance as opposed to theoretical energy density. Comparing energy densities, one must consider the influence of cell size, internal design, discharge rate and temperature conditions, as these parameters strongly impact performance characteristics.

7