When the charging is in procedure, it is very important to decide when to terminate the charging. This is because of two main reasons. One is to avoid undercharge, i. e., making sure the battery is fully charged, not partially, in order to use the full capacity of the batteries. The other one is to avoid overcharging which is very dangerous, especially in the case of high energy density Lithium-based EV/PHEV battery packs. If not terminated on time, the overcharging of batteries can lead to over gassing of the cells, especially in liquid electrolyte cells which results in increase in the volume of individual cells, a situation that cannot be tolerated in a rigidly packed battery pack. Another issue is overheating of the cells, especially in Lithium-based batteries which can easily lead to the explosion and firing of the whole pack, since; Lithium is a very active material and easily combines with oxygen in the air. The only thing needed to begin the combination is enough heat.
Choosing different termination criteria leads to different termination methods. Selecting the type of termination of charging process depends on different factors such as the application and the environment where the battery is used. The conventional termination methods that can be used are mentioned here:
(1) Time: Using time is one of the simplest methods which is mainly used as a backup for fast charging or normally used for regular charging for specific types of batteries. This method is very simple and inexpensive, but because of diminishing battery capacity over time due to aging, the time should be reset for a reduced capacity aged battery to avoid overcharging of old batteries. Therefore, the charger will not work well for new batteries and will lead to life time reduction.
(2) Voltage: As mentioned before, voltage can be used as a termination factor, i. e., terminating the charging process when the battery voltage reaches a specific value. This method has some inaccuracies, because real open circuit voltage is obtained when the battery is left disconnected for some time after the charging. This is because chemical actions happening inside the battery need some time to stabilize. Nevertheless, this method is widely used. In addition, this technique is usually used with constant current technique to avoid overheating damage to the battery.
(3) Voltage Drop (dV/dT): In some chemistries like Ni-Cd when charged using constant current method, the voltage increases up to the fully charged state point and then the voltage begins to decrease. This is due to oxygen build-up inside the battery. This decrease is significant, so the negative derivative of the voltage versus time can be measured to indicate overcharge. When this parameter becomes negative, it shows that we have passed the fully charged state and the temperature begins to rise. After this point the charging method can be switched to trickle, or float charge, or terminated completely.
(4) Current: In the last stages of charging, if constant voltage method is used, the current begins to decrease as the battery reaches fully charge state. A preset current value such as C/10 rate can be defined and when the current goes below this value the charging would be terminated.
(5) Temperature: In general, increase in temperature is a sign of overcharge. However, using temperature sensors highly adds to the cost of system. Nevertheless, for some chemistries such as Ni-MH, methods such as voltage drop is not recommended, because the voltage drop after full charge state is not significant to be relied on. In this case, temperature increase is a good indication of overcharge and can be used.