Self-discharge
Self-discharge referred to by self-running processes that lead to the fact that batteries and accumulators discharged more or less quickly, even if there is no electrical load is connected. The rate of self-discharge rate determines how much of the originally stored charge amount (capacity) after storage is still available. The self-discharge is one of the most important characteristics of batteries or accumulators. Knowledge of the self-discharge is important to select appropriate battery systems for certain applications.
General
Basically self-discharge occurs on all batteries and accumulators. Causes side reactions or internal short-circuiting. Side reactions in the electrodes cause the electrochemically active material is consumed and then no longer available for the discharge reaction are available. Side reactions are often caused by impurities or accelerated. Internal short circuits occur when the electrochemically active materials fall into the anode and cathode in electrical contact with and can react with each other directly. This is the case with defective or a defective separator between the anode and cathode.
The difference resulting from the annual self-discharge capacity loss can be from less than 1% and 100% of original capacity. The amount of self-discharge may depend on many factors. The electrochemical system is essential for the self-discharge. In some systems with low self-discharge during storage form protective layers ( passive films) on the electrode surfaces that slow the further self-discharge. The rate of self-discharge is a function of the storage time. Usually they will asymptotically with increasing storage time. The self-discharge is strongly temperature dependent, their speed increases with higher temperature. As a rule of thumb that an increase in temperature by 10 ° C doubles the rate of self-discharge. Reduced self-discharge at a lower temperature is the reason why the storage is recommended at low temperatures for some battery systems. The amount of self-discharge often depends on the degree of discharge, which is the ratio of the removed even at a discharge capacity to the initial capacity decreases.
Primary batteries
The self-discharge determines the service life or shelf life at primary batteries. The higher the self-discharge rate, the greater is the loss of the useful capacity in the period between manufacture and use. In case of very long duration discharges ( with small loads ), the usable capacity may be limited by self-discharge. Therefore, may have ( for example, in heart pacemakers ) only a very small self-discharge rate of primary batteries for such applications.
Secondary batteries (accumulators)
The rate of self-discharge is typically in the fully charged condition and decreases with increasing time. The self-discharge of batteries is significantly larger than of primary batteries (see Tables; details here per month, not per year). The rate of self-discharge depends on both the electrochemical system as well as the design or performance of the battery.
Sanyo has launched in August 2006, a modified called Eneloop NiMH battery on the market, which has a self-discharge rate of only 15% per year. The batteries also have a different discharge curve than conventional batteries, it is more similar to that of primary batteries. This means that digital cameras can be operated with a charge much longer, sometimes up to three times longer than conventional batteries of the same capacity. The new NiMH batteries are also sold by other manufacturers under their own brand names (see list). Due to the low self-discharge rate of these cells, for example, a digital camera continue to operate even after several months of non-use simple without need to recharge the batteries.
To compensate for the self discharge and to keep the batteries always ready for use, they must be recharged periodically or continuously re- loaded after loading with the trickle charge. The latter is, for example, in the case of uninterruptible power supplies that are constantly connected to the application to the electricity grid.