Memory effect (accumulator)

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The memory effect is the capacity loss that occurs when a nickel-cadmium battery with sintered electrodes and other types of battery is frequently partially discharged . The battery seems to remember the energy requirement and over time, instead of the original, it only provides the amount of energy required for previous discharging processes. Electrically, the effect manifests itself in an early voltage drop . This means a reduction in the usable capacity of the accumulator, since consumers require a minimum voltage. If the cell voltage falls below this minimum requirement, the cell becomes unusable for use, although it can still deliver electrical energy.

Cause of the memory effect

The memory effect was first described by NASA in the 1960s . Sintered NiCd batteries in satellites were charged by solar cells at regular intervals, regardless of the degree of discharge . Over time, the batteries adapted to the charging rhythm; their capacity only lasted until the next charging cycle, although they were significantly larger in size.

The memory effect is very likely based on two processes.

  1. Crystal formation: When charging a NiCd battery, cadmium - microcrystals are formed . If the accumulator is only discharged to a certain "constant" degree, this favors the formation of larger crystals from microcrystals in non-discharged areas. Because the larger crystals have a smaller total surface area than smaller crystals with the same mass, they react worse when discharging, which is why the voltage collapses.
  2. Recrystallization: Older charging technologies ignore the battery level. They charge over a set period of time and overcharge a partially discharged battery. This leads to recrystallization on the Cd electrode. Due to the position of the cadmium within the electrochemical series , the recrystallization is associated with a lower output voltage and a consequent reduced capacity.

remedy

In the case of older NiCd batteries with memory effect, it was recommended to fully charge them in a cycle of up to five times in a row, to discharge them to a voltage of at least 0.85 volts per cell and then to charge them again. In this way, the chemical processes that had caused the effect could possibly be reversed. If this procedure didn't help either, the battery had to be rejected as defective.

Investigations on the memory effect in modern batteries

In 2001, the Center for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) examined commercially available NiCd , NiMH and lithium-ion batteries for their behavior in the event of multiple partial discharges. A decrease in the cell voltage after multiple partial discharges (ten partial discharge-charge cycles and more) was found, but this was always less than 0.05 volts. The previously published reduction in cell voltage by more than 0.1 volts could not be observed. The reduction of the cell voltage could be reversed by a single discharge to normal discharge voltage and recharging. Amazingly, NiCd and NiMH batteries behave very similarly, but with NiMH batteries one speaks of the battery inertia effect . Li + batteries only have this effect to a small extent. From this it can be concluded that none of the above-mentioned causes, in which cadmium is always involved, are possible for this effect.

The authors of the study come to the conclusion that the battery manufacturers have succeeded in largely eliminating the memory effect by choosing materials or changing the technology, and come to the following recommendations:

  • A complete discharge before each charge is not necessary.
  • Occasional discharging, for example after 50 partial discharge cycles, is recommended.

Rather, the battery life can be significantly increased by partial discharges - this applies to both NiMH and lithium batteries. With hybrid cars , more than ten times as many partial charging cycles are possible than if they were 100% discharged. The throughput of Pedelec batteries, for example, can be roughly tripled, which corresponds to more than twice the service life.

See also

Individual evidence

  1. ↑ Presentation: Der Akkumulator ( Memento from July 8, 2007 in the Internet Archive ) p. 13 (2004)
  2. ↑ The battery memory effect and battery inertia effect
  3. ↑ Thi Binh Phan, Andreas Jossen, Svoboda Vojtech, Harry Döring, Jürgen Garche: Deep discharge of batteries - causes, effects and avoidance (PDF; 441 kB)
  4. ↑ Caspar Grote, Renate Ester (Ed.) In: Batteries, charging concepts & power supply design . Design & Electronics, Munich 2001, pp. 31–44
  5. ↑ Service life in relation to the depth of discharge '
  6. ↑ Prediction of the aging of pedelec and e-bike batteries , report on a dissertation by Frieder Herb

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