The capacity of a battery has been defined as the amount of current it can deliver. The amount of current depends upon the following factors:
- No. of plates.
- Area of plates.
- Quantity of the electrolyte.
About 930 em of plate surface must be in contact with the electrolyte to produce 40-60 amperes of current. Six volts batteries have 15, 17,19 and 21 plates per cell; and twelve volts battery has 7, 9, 11 and13 plates per cell, depending upon the size of the battery. Always there is one more negative plate than the positive plate. Between two negative plates, there is one positive plate.
Since battery capacity and performance is reduced by low temperature, a minimum electrolyte temperature of 5°C is maintained as a general rule. However, in nuclear power plants, where the reduced capacity of battery could affect the safe shutdown of a reactor, a minimum temp of 15°C is maintained by thermostatically controlled heaters of the totally-enclosed tubular type batteries. Whilst batteries are capable of operating in an ambient temperature of 35°C, with occasional excursions to 40°C, these high temperatures are avoided, as they lead to greater evaporation of water from the electrolyte and a significant reduction in service life.
Battery Discharge Characteristics
Battery capacity is measured in milliamps x hours (mAH). For example, if a battery has 250 mAH capacity and provides 2 mA average current to a load, in theory, the battery Wl ill last 25 hours. However, the way the battery is discharged has an impact on battery life. By discharging the battery at the manufacturer- usually helps the battery deliver close to its nominal capacity. But the result cannot simply be extrapolated linearly to other discharge profiles. In many short-range wireless sensor networks, although the average current consumption of a device is low, the instantaneous current can be high. For example, in Figure 6.1, a transceiver with sleep current of 1 HA and peak active current of 20 mA wakes up every 2 seconds and stays active for 5 ms. The average current consumption is only 50 HA, but the peak current of 20 mA can have an adverse effect on the battery’s actual capacity, especially if there is not enough time between the time periods of high current discharge to let the battery at rest and recover.