Lead-acid batteries suffer from deep discharge for several reasons. An increasing depth of discharge results in a decreasing acid concentration and due to the increased sulphate solubility in accelerated sulphation (Section 188.8.131.52.2), corrosion (Section 184.108.40.206.3) and higher sensitivity to freezing (Section 220.127.116.11.7). Further, the mechanical stress is increased because of the changes in the specific volume of the active materials and in long battery strings, the risk of reverse charging of single cells (Section 18.104.22.168.6) increases. Therefore, the maximum depth of discharge should be limited during normal operation.
While choosing the appropriate DOD for the operation strategy, the data-sheet information given by the manufacturers should be analysed. They often give the number of cycles during the lifetime of a battery as a function of the depth of discharge. However, for the system design the number of cycles is not the most important parameter. The level of capacity throughput that can be realised during the battery lifetime is of more relevance. A cycle with 50% DOD means that only 50% of the capacity is used and therefore the overall capacity throughput for, for example, 200 cycles with 50% DOD is equivalent to 100 cycles with 100% DOD. However, from the point of view of the system design a battery which is limited to 50% DOD during normal operation must have double the size with respect to a battery with 100% DOD during normal operation. This is worthy of mention because batteries are always limited by the capacity throughput on one hand and by operation life on the other hand. Therefore, it makes no sense to operate a battery which is, for example, rated for 10 000 cycles at 20% DOD in autonomous power supply systems even though this might promise the highest capacity throughput. Assuming that on a daily basis 10 000 cycles take place, it would take more than 25 years to achieve this. However, the battery lifetime would not last that long due to other ageing processes.
Figure 20.24 shows for two different batteries the cycle life as a function of the DOD (data taken from data sheets) and the resulting capacity throughput. It is obvious that for the battery Type 2, the capacity throughput is almost independent of the DOD but for battery Type 1 there is a strong dependency leading to higher throughputs at lower DODs.
Figure 20.24 Number of cycles and overall charge transfer (capacity throughput) in units of the rated capacity during the battery lifetime as a function of the depth of discharge during cycling. Data from data sheets from battery manufacturers
For practical purposes, the following ‘rules’ can be used, which have proved their suitability in the field. In Classes 1 and 2 (Section 20.3.2), the maximum DOD should be 60-70% and in Classes 3 and 4, 80-90%. The lower values are for flooded batteries and the higher values are for VRLA batteries. Low-cost ‘solar batteries’ should be operated to a maximum of 50% DOD. It is very important to take into account that the mentioned values for the DOD are given on the basis of the C10 capacity. Using, for example, 80% of the C100 capacity means using more than 100% of the C10 capacity and this is hazardous.
Control of the maximum DOD can be realised either by deep-discharge disconnecting voltage or on the basis of the state of charge. Most commercial charge controllers control the maximum DOD by the voltage. The drawback of this method is that the discharged capacity up to a certain voltage limit depends very much on the discharge current. Table 20.5 shows typical end – of-discharge voltages up to which 100% of the C10 capacity has been discharged from the battery. This shows the problem of an efficient DOD control. If the maximum DOD is assured by a high
Table 20.5 Typical end-of-discharge voltages up to which 100% of the C10 capacity has been discharged from the battery at different discharge currents at room temperature
State of charge normalised to measured C20-capacity
voltage limit even at small currents (e. g. 1.95 V/cell), the available capacity at higher currents is very limited . Figure 20.25 shows a more pronounced version of the problem. A given voltage limit might be appropriate for one battery type, but for another with the same technology (flat plates, lead-acid and flooded electrolyte) the discharge curve looks quite different and the same voltage limit results in significant differences in the maximum DOD with respect to the minimum SOC defined to protect the battery as shown in the figure. The difference in SOC for the same voltage limit could be as high as 25%. Taking into account different battery technologies, the differences in SOC are even higher. In autonomous power supply systems, low and high currents occur (Figure 20.5, ).
Two solutions for the problem are available. One is a current-compensated end-of-discharge voltage threshold and the other is the use of state-of-charge determination. The latter solution is the most appropriate for autonomous power supply systems. There are various methods for state-of-charge determination in lead-acid batteries, which are appropriate for autonomous power supply systems .