Secondary batteries

Among secondary batteries, nickel-cadmium have been the most common for Earth satellites; more recently nickel-hydrogen cells have become popular owing to their rather higher specific energy. Silver-zinc batteries offer rather higher specific power than the other types, at the expense of a lower cycle life. Silver-zinc batteries were therefore used on Mars Pathfinder, since its technology demonstration objectives were expected to be fulfilled within a few weeks (as, indeed they were, although contact became unreliable after the battery failed).

Careful control of the charge state of batteries is crucial to prolonging their life, particularly in the case of NiCd cells. In general, life is maximized by shallow charge cycles (i.e. a low depth-of-discharge (DoD) - not using all the capacity of the cells) although this has the effect of slowly degrading the cell capacity (the 'memory effect'). Occasional battery reconditioning - a very deep discharge, which carries some risk of cell damage - can recover most of this lost capacity. Battery specific energy can vary by a factor of 2, so the values in Table 9.1 should be considered representative.

These figures refer essentially to the cells themselves. Batteries will have casings (usually rather substantial ones, since they are massive components which therefore need secure attachment), cabling, diodes, etc. As an example, the LiSO2 batteries used on Huygens have a rated capacity of 2400 Wh, and a mass of 24kg - a specific energy half that quoted above; similarly the Sojourner battery had a capacity of 150 Wh and a mass of 1.24kg.

Table 9.1. Battery parameters

Battery type

Chemical formula

Specific energy Whrkg"1

Cycle life 75%-25% DoD

Silver-zinc

AgZn

100

75-2000

Nickel-cadmium

NiCd

30

800-30000

Nickel-hydrogen

NiH

60

4000-30000

Lithium-sulphur dioxide

LiSO2

200

Lithium-thionyl chloride

LiSOCl2

200

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