Self-discharge
Self-discharge refers to self-running processes that lead to batteries and accumulators discharging more or less quickly, even if no electrical consumer is connected. The speed of self-discharge determines what proportion of the originally stored amount of charge (capacity) can still be used after storage. Self-discharge is one of the most important characteristics of batteries and accumulators. Knowledge of self-discharge is important in order to select suitable battery systems for certain applications.
General
Basically, self-discharge occurs in all batteries and accumulators. The causes are side reactions or internal short circuits . Side reactions in the electrodes mean that the electrochemically active material is consumed and is then no longer available for the discharge reaction. Side reactions are often caused or accelerated by impurities. Internal short circuits occur when the electrochemically active materials in the anode and cathode come into electrical contact and can thus react directly with one another. This is the case with a defective or defective separator between the anode and cathode.
The annual capacity loss resulting from self-discharge can be between less than 1% and 100% of the original capacity. The extent of self-discharge can depend on many factors. The electrochemical system is essential for self-discharge. In some systems with low self-discharge, protective layers ( passive films ) form on the electrode surfaces during storage , which slow down further self-discharge. The speed of self-discharge depends on the storage time. As a rule, it decreases asymptotically with increasing storage time . Self-discharge is highly temperature-dependent; its speed increases with higher temperature . As a rule of thumb , a temperature increase of 10 ° C doubles the rate of self-discharge. The reduced self-discharge at lower temperatures is the reason why storage at lower temperatures is recommended for some battery systems. The extent of self-discharge often depends on the degree of discharge, which is the ratio of the capacity already taken during a discharge to the original capacity.
The self-discharge can be expressed in the equivalent circuit as a parallel connection of a resistor to the galvanic cell.
Batteries
The self-discharge of batteries , also known as primary cells, determines the shelf life and is usually related to one year for batteries. The self-discharge of primary cells is significantly smaller than that of rechargeable batteries (secondary cells). In the case of very long discharges with small loads, the usable capacity can be limited by self-discharge. For this reason, primary batteries for applications such as cardiac pacemakers should have a very low self-discharge rate.
| Battery system | Self-discharge per year at 20 ° C |
|---|---|
| Lithium iodine battery | by 0.06% |
| Lithium iron sulfide battery | 1-2% |
| Lithium manganese dioxide battery | 1-2% |
| Lithium thionyl chloride battery | 1-2% |
| Zinc-air battery | 3% (if sealed) |
| Alkaline manganese battery | about 6 % |
| Mercury Oxide-Zinc Battery | 4% |
| Silver oxide zinc battery | 6% |
| Zinc-manganese dioxide cell | 7-10% |
Accumulators
The self-discharge of accumulators is usually significantly greater than that of primary batteries and is usually related to a period of one month. The self-discharge rate depends both on the electrochemical system and on the design or design of the accumulator.
| Accumulator system | Self-discharge per month at 20 ° C |
|---|---|
| Lithium-ion battery | approx. 4% |
| Lithium polymer battery | 4-5% |
| NiMH accumulator with low self-discharge (LSD-NiMH) | 1-3% |
| Lead accumulator | 4-6% |
| Nickel-cadmium battery | approx. 20% |
| Nickel-metal hydride battery | approx. 25% |
In contrast to normal NiMH batteries, NiMH accumulators with low self-discharge have a similar self-discharge behavior to primary batteries. With newer generations of this battery the self-discharge is only 10% to 15% per year. Therefore, when using these batteries z. B. a digital camera can simply be operated even after several months of non-use without having to recharge it first. In contrast to conventional batteries, such batteries are supplied already charged and are immediately ready for use (“Ready to use”). This would not be possible with batteries without this special technology.
literature
- David Linden, Thomas B. Reddy (Eds.): Handbook of Batteries . 3. Edition. McGraw-Hill, New York 2002, ISBN 0-07-135978-8 .
Individual evidence
- ^ Edward J. Prosen, Jennifer C. Colbert: A Microcalorimeter for Measuring Self-Discharge of Pacemakers and Pacemaker Power Cells . In: Journal of Research of the National Bureau of Standards . tape 85 , no. 3 , 1980, doi : 10.6028 / jres.085.010 .
- ↑ a b c d Umweltbundesamt (Ed.): Batteries and accumulators . October 2012, p. 20 ( Umweltbundesamt.de [PDF; 3.8 MB ]).
- ^ Li-Polymer Battery. (No longer available online.) Archived from the original on April 19, 2014 ; accessed on October 12, 2014 .
- ↑ Jürgen Rink: Endurance runner: Long-term test of NiMH batteries with reduced self-discharge. (No longer available online.) In: heise mobil. August 14, 2009, archived from the original on January 16, 2014 ; accessed on January 16, 2014 .
- ↑ SANYO eneloop product introduction. (PDF) (No longer available online.) Sanyo , archived from the original on June 17, 2009 ; accessed on February 15, 2018 .
- ^ Battery Performance Characteristics. In: mpoweruk.com. Retrieved February 15, 2018 .