Battery technology

9.5.1 Primary batteries

So-called primary batteries, where chemical energy is irreversibly converted into electrical power, typically have much higher energy densities than rechargeable or secondary batteries. For low-energy missions, such as planetary atmosphere probes, primary batteries are often the system of choice. They are convenient in that they impose few ancillary requirements such as attitude, and are robust.

The highest energy density cells commonly used at present are lithium thionyl chloride (Li SOCl2) or lithium sulphur dioxide (Li SO2). Lithium thionyl chloride cells were used on the DS-2 Mars Microprobes, and were qualified for 20 000 g impact; LiSO2 cells were used on the Galileo and Huygens probes.

Performance of the cells depends on several factors. The most obvious is temperature, both in use (typically for only a few hours), and during the storage period before the mission itself (typically several years). Ideally the cells are stored at low temperatures, to minimize the self-discharge which can increase by as much as 20% for a 5 K increase in temperature, while their capacity in operation is improved by moderate temperatures, perhaps by 10% for a 10 K increase in temperature. Usually the only way to estimate the in-flight performance confidently is to store cells from the same batch as the flight cells at cruise temperatures and measure the capacity.

Primary cells often build up an oxide or similar coating on their electrodes. This coating usefully protects the electrodes during a long storage or cruise period, but retards ion migration and thus limits the in-use battery performance. It is usual, therefore, to burn off this coating, or 'depassivate' the batteries, by applying a brief short circuit to the battery, drawing a large current shortly before the prime mission itself.

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