6.1 Overcharging protection
The overcharging of the battery can lead to permanent damage or life reduction of the battery. The overcharging may also liberate poisonous H2S gas. Figure 11 shows the different periods of the battery charging operation and overcharging protection.
Period o–a: This is the starting of the system. During this period, no power is generated by the solar panel. As the battery has some initial charge (assumed), it starts conducting, as seen in the initial positive current peaks. Generally it is neglected for the calculation (leakage current).
Period a–b: The system is set for the variation in the radiation from zero to its maximum so the ramp is seen in the power in Pin. As the power increases, the battery is charged and its power is shown as negative values in Fig. 11. The current also increases from zero to its maximum value (negative) depending upon the generated power from the solar panel.
Period b–c: Power generation by the solar panel is stable, so the charging current by the battery is also stable during this period. Hence the power taken by the battery in the Pbat curve is stable as can be seen.
Period c–d: Battery gets fully charged and its protection system for overcharging is activated to disconnect the battery from the supply. Thus the current is rapidly reduced to zero and so is the battery power Pbat. As there is only a very small load in the system, Pin reduces to very low value.
6.2 Deep discharging protection
Similar to the overcharging protection of the battery, deep discharging protection or over discharging protection is also very important. Up to the deep discharging point, e.g., after discharging of 95% of the total battery energy, further discharge of the battery to supply power to the load can lead to a sharp drop in its terminal voltage.
If a battery is discharged beyond a certain limit, it may not be charged again with normal charging. In addition, if deep discharging occurs frequently, the battery could lose its capacity and hence reduce its life time. To avoid this situation, deep discharging protection is provided.
If the load is removed from the battery before the point of deep discharging, the battery terminal voltage can be raised to the normal voltage. However, the voltage recovery of the battery will not be possible if the battery has high self-discharging or is connected to parasitic loads. Surrounding temperature also plays an important role in deep discharging and the cut-off voltage of the battery.
Figure 12 shows the different periods of the battery discharging operation and deep discharging protection.
Period o–a: The load is small and hence an only small current flow through the battery and the power (positive) taken from the battery is also small.
Period a–b: The load increases and hence the power taken from the battery also increases. The current is changed from low to a new high value.
Period b–c: At point “b”, the load again increases so the current and power also increase. At point “c”, deep discharging protection of the battery is activated, and the battery current and power reduce to zero.