Secondary Battery Technologies Offer Tradeoffs for Energy Storage

Battery-powered systems are now widespread thanks to developments of new battery chemistries that make it possible to provide energy to an increasingly wide range of systems. Secondary batteries have become important sources of energy, providing an often greener solution for electronics systems than primaries as they can be recharged instead of demanding new batteries, and the old ones discarded when they run down.

If not used as the main power source, secondary batteries can perform a vital role in many consumer and light-industrial applications, providing essential backup power in the event that the main power source fails. They may act as the energy source to ride-out power cuts or to allow primary batteries to be swapped out without losing precious data stored in volatile memory. The use of rechargeable technologies allows them to be kept fully charged and ready to support the system, using energy from the main source. This ensures that uptime does not degrade over time as it would with the use of primary, non-rechargeable batteries.

There are four main types of rechargeable batteries now in common use for powering electronic and electrical equipment: the sealed lead-acid battery; the nickel-cadmium (NiCd) battery; the nickel-metal hydride (NiMH) battery; and the lithium-ion battery. Each of them has their own individual characteristics, but they also have some common properties.

Most battery chemistries are non-linear in the way they supply power. The capacity of a battery is related to the current it provides; typically, the more current that is drawn, the lower the effective capacity of the battery. For example, a battery might have a capacity of 1000 mAh for a current draw of 5 mA. However, the same battery might have a capacity of 500 mAh for a current draw of 200 mA.

Further, the chemistry and construction of a secondary cell have a strong bearing on the way in which it can be recharged and discharged and its ability to sustain a high enough voltage to power the circuit. Some chemistries can provide high discharge or charge rates, but their output voltages can drop quickly at some point, possibly beyond the ability of the load circuit to continue performing. As a result, the load is unable to use all the energy available. A further consideration is how quickly the energy can be restored to the battery once the main power supply has been reactivated, and whether the cell can be charged regularly or it is best to perform a deep discharge followed by a recharge.

The memory effect in batteries is particularly noticeable for NiCd cells, which suffer reductions in their useable capacity if they experience repeated shallow cycling. For example, if the interruptions to power are short lived and frequent and the batteries are recharged each time, the memory effect will lead to a fall in their effective maximum capacity.

Figure 1: The typical discharge curve of a secondary battery.

The NiCd is one of the oldest known rechargeable battery chemistries, discovered at the end of the 19th century. At the time, it was the only realistic competitor to the lead-acid battery. Over time, energy density improved and surpassed that of the lead-acid chemistry. Cells comprise a nickel oxide-hydroxide positive electrode plate and cadmium negative plate kept apart by a thin separator. They contain an alkaline electrolyte, typically potassium hydroxide.

The cause of the memory effect is due to a change in the arrangement of active materials on the electrodes. Ideally, these are arranged as a very large number of tiny crystals across the surface of the electrode, which leads to a high mass-to-surface area ratio, improving contact with the electrolyte. Over time, and encouraged by short charging cycles, these crystals can coalesce into larger and larger crystals, reducing the total surface area.

A possible side effect of the discharge cycle is the generation of unwanted gaseous byproducts. Products such as Sanyo Energy’s Cadnica range of NiCd batteries are designed to minimize gas production, but the individual cells contain self-resealing vents to prevent the gas from building up inside the unit. The Cadnica batteries are designed to withstand overcharging and overdischarging. Sanyo's electrode plate manufacturing process and current collectors minimize internal resistance, letting the batteries take advantage of a key characteristic of the NiCd chemistry, which is a high discharge rate when high currents are needed.

Figure 2: Construction of a Sanyo Energy Cadnica NiCd battery.

Compared to NiCd, the newer NiMH battery chemistry exhibits a higher self-discharge of around 50 percent, but offers an overall higher energy density. A further characteristic of NiMH batteries is that they produce heat during charging, limiting the rate at which they can be charged safely. Charging is therefore more complicated, and will generally need to include a temperature-sensing circuit. A useful characteristic of the NiMH is that it does not suffer as much from the memory effect as the NiCd chemistry.

Although the oldest form of battery technology in widespread use in electronic systems, the lead-acid battery still has a number of advantages, such as a very flat discharge curve. The lead-acid battery may be most familiar as a liquid-filled tank that needs to be handled with care. However, sealed units that replace the liquid with a gel-type electrolyte are now commonplace, particularly for smaller loads. The electrodes used a lead alloy composition that is designed to prevent gas forming during charging, a side effect encountered in liquid lead-acid batteries. Like some NiCd batteries, there is usually a safety valve to prevent pressure building up inside if the charging process does create some gas.

The gel-cell battery generally has a lower capacity per kilogram than other battery chemistries, but because of its flat discharge curve, providing a usable voltage output over much of its range, this battery type has a very low self-discharge rate of 5 percent or so per month, although this tends to be non-linear. However, there is a voltage beyond which the battery should not be pushed before the characteristics change enough to prevent it being charged to full capacity.

In contrast to NiCd batteries, which do not suit frequent light charging, sealed lead-acid batteries such as the Cyclon® series from Enersys, will demonstrate longer life if they are not deep discharged frequently but instead are used lightly and then topped up when the main power source is restored. Lithium rechargeables, such as the lithium-ion chemistry, also suffer when discharged too far but benefit from a relatively-flat discharge curve up to the point where their output voltage drops quickly.

Figure 3: The relationship between discharge depth and lifetime in an Enersys Cyclon sealed lead-acid battery.

As the lightest metal available with a high electrochemical potential, lithium can, in principle, achieve high energy densities. However, the reactivity of the metal poses the risk of fire and explosion. So the element is used in its oxidized form to reduce the danger. As a result, Lithium-ion chemistries use materials such as lithium-cobalt dioxide instead of the metal itself.

Typically, lithium-ion batteries have a negative electrode of aluminum, coated with a lithium compound such as lithium-cobalt dioxide, lithium-nickel dioxide, or lithium-manganese dioxide. The positive electrode is generally copper coated with carbon. The electrolyte is usually a lithium salt, such as lithium-phosphorus hexafluoride, dissolved in an organic solvent.

A lithium-ion battery can achieve an energy density twice that of a NiCd and does not suffer from the memory effect. A further benefit is a low self-discharge rate similar to that of sealed lead-acid batteries. However, there needs to be a balance between relatively high cost and a more complex charging regime, the batteries cannot be trickled or float-charged and need to be protected from overcharging and excessive discharging. Pushing the batteries past their limits poses a safety risk. Although, where space is a premium, the lithium-ion battery has clear advantages.

The high energy density has seen the production of coin-cell secondary battery forms from suppliers such as FDK, Panasonic, and Seiko Instruments. However, that is not the only form. The Enerchip™ from Cymbet is a cell that can be handled like any other surface-mount IC, enabling it to be assembled and soldered automatically on the circuit board.

The capacity of an Enerchip is less than 1 mAh, but it suits battery backup applications, for example, sustaining the system in sleep while the main battery power source is swapped, in a wide variety of applications where size and low-cost production are important. The main competition for the Enerchip is not so much other batteries, but the supercapacitor. However, the lithium-based electrochemical construction of the chip provides a much lower self-discharge rate, resulting in less need to consume energy simply to keep sufficient charge on the backup.

Innovations in battery chemistry and design in recent years are likely to see further developments in secondary battery technology, extending their range into fields currently dominated by other power sources, including larger supercapacitors and primary batteries. It is important to understand how each chemistry will work with a given system architecture as this article has shown.