10. Getting the Most from your Batteries
A common difficulty with portable equipment is the gradual decline in battery performance after the first year of service. Although fully charged, the battery eventually regresses to a point where the available energy is less than half of its original capacity, resulting in unexpected downtime.
Downtime almost always occurs at critical moments. This is especially true in the public safety sector where portable equipment runs as part of a fleet operation and the battery is charged in a pool setting, often with minimal care and attention. Under normal conditions, the battery will hold enough power to last the day. During heavy activities and longer than expected duties, a marginal battery cannot provide the extra power needed and the equipment fails.
Rechargeable batteries are known to cause more concern, grief and frustration than any other part of a portable device. Given its relatively short life span, the battery is the most expensive and least reliable component of a portable device.
In many ways, a rechargeable battery exhibits human-like characteristics: it needs good nutrition, it prefers moderate room temperature and, in the case of the nickel-based system, requires regular exercise to prevent the phenomenon called ‘memory’. Each battery seems to develop a unique personality of its own.
10.1 Memory: myth or fact?
The word ‘memory’ was originally derived from ‘cyclic memory’, meaning that a NiCd battery can remember how much discharge was required on previous discharges. Improvements in battery technology have virtually eliminated this phenomenon. Tests performed at a Black & Decker lab, for example, showed that the effects of cyclic memory on the modern NiCd were so small that they could only be detected with sensitive instruments. After the same battery was discharged for different lengths of time, the cyclic memory phenomenon could no longer be noticed.
The problem with the nickel-based battery is not the cyclic memory but the effects of crystalline formation. There are other factors involved that cause degeneration of a battery. For clarity and simplicity, we use the word ‘memory’ to address capacity loss on nickel-based batteries that are reversible.
The active cadmium material of a NiCd battery is present in finely divided crystals. In a good cell, these crystals remain small, obtaining maximum surface area. When the memory phenomenon occurs, the crystals grow and drastically reduce the surface area. The result is a voltage depression, which leads to a loss of capacity. In advanced stages, the sharp edges of the crystals may grow through the separator, causing high self-discharge or an electrical short.
Another form of memory that occurs on some NiCd cells is the formation of an inter-metallic compound of nickel and cadmium, which ties up some of the needed cadmium and creates extra resistance in the cell. Reconditioning by deep discharge helps to break up this compound and reverses the capacity loss.
The memory phenomenon can be explained in layman’s terms as expressed by Duracell: “The voltage drop occurs because only a portion of the active materials in the cells is discharged and recharged during shallow or partial discharging. The active materials that have not been cycled change in physical characteristics and increase in resistance. Subsequent full discharge/charge cycling will restore the active materials to their original state.”
When NiMH was first introduced there was much publicity about its memory-free status. Today, it is known that this chemistry also suffers from memory but to a lesser extent than the NiCd. The positive nickel plate, a metal that is shared by both chemistries, is responsible for the crystalline formation.
In addition to the crystal-forming activity on the positive plate, the NiCd also develops crystals on the negative cadmium plate. Because both plates are affected by crystalline formation, the NiCd requires more frequent discharge cycles than the NiMH. This is a non-scientific explanation of why the NiCd is more prone to memory than the NiMH.
The stages of crystalline formation of a NiCd battery are illustrated in Figure 10-1. The enlargements show the negative cadmium plate in normal crystal structure of a new cell, crystalline formation after use (or abuse) and restoration.
Lithium and lead-based batteries are not affected by memory, but these chemistries have their own peculiarities. Current inhibiting pacifier layers affect both batteries — plate oxidation on the lithium and sulfation and corrosion on the lead acid systems. These degenerative effects are non-correctible on the lithium-based system and only partially reversible on the lead acid.