Spontaneous discharge (Glossary)

The so-called spontaneous or self discharge reflects the extent to which a battery discharges itself over time. It is one of the most important characteristics of a battery and is decisive for the choice of an optimum battery system for a certain application.

Batterieentladung / Discharging of a Battery

In primary batteries the spontaneous discharge affects the general service life and above all the energy loss in the period between manufacture and use. It is thus useful and important not only for the user but also the logistics of trade and storage.

Primary batteries for very long-lived applications (e.g. pacemaker) or long storage times between manufacture and use should have only a very low spontaneous discharge rate. The silver oxide-zinc batteries that have been in use since the beginning of the 70ies in pocket calculators, hearing aids and quartz clocks have a service life of 3 - 4 years. Thanks to an extremely low spontaneous discharge rate of approx. 1% per year, the service life of lithium primary batteries could be increased to 6 to 8 years, and that of lithium-iodine batteries in pacemakers up to 15 years.

In secondary batteries the spontaneous discharge determines the intended purpose of the accumulator, in other words how long it can keep its charge after being disconnected from the charger or whether it is at all suitable for separate operation.

The spontaneous discharge of accumulators for uninterruptible power supplies (UPS), for example, is irrelevant since these energy stores remain connected to a power supply with a float charge until they are used. Other accumulators, for example in mobile phones, should have the highest standby time possible.

The spontaneous discharge essentially depends on the battery, its age and design, though also on the operating temperature, whereby an increase in temperature of 10 degrees roughly doubles the spontaneous discharge. It is strongest with a full charge and decreases during discharge.

To compensate spontaneous discharge and permanently keep accumulators ready for use at a maximum charge level they are supplied with the so-called float charge, which is normally 0.1 V above the no-load voltage, after charging.

There are special technical solution for extreme requirements on a low spontaneous discharge of batteries or accumulators, such as thermoelectric batteries (battery not activated until directly before use) or atomic batteries.

© Marc Stenzel

	See also:
		Glossary overview Memory effect (Glossary)

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