We often get puzzled by announcements of new batteries that are said to offer very high energy densities,deliver 1000 charge/discharge cycle and are paper-thin.Are they real? Perhaps -but not in one and the same battery.While one battery type may be designed for small size and long runtime,this pack will not last and wear out prematurely.Another battery may be built for long life,but the size is big and bulky.A third battery may provide all the desirable attributes,but the price would be too high for commercial use.
Battery manufacturers are well aware of customer needs and have responded by offering packs that best suit the specific applications.The mobile phone industry is an example of clever adaptation.Emphasis is placed on small size,high energy density and low price.Longevity comes in second.The inscription of NiMH on battery packs do not automatically guarantee high energy density.
Compromises exist on lithium-based batteries.Li‑ion packs are being produced for defense applications that far exceed the energy density of the commercial equivalent.Unfortunately,these super-high capacity Li‑ion batteries are deemed unsafe in the hands of the public and the high price puts them out of reach of the commercial market.
A prismatic Nickel-Metal Hydride battery for a mobile phone,for example,is made for slim geometry.Such a pack provides an energy density of about 60Wh/kg and the cycle count is around 300.In comparison,a cylindrical NiMH offers energy densities of 80Wh/kg and higher.Still,the cycle count of this battery is moderate to low.High durability NiMH batteries,which endure 1000 discharges,are commonly packaged in bulky cylindrical cells.The energy density of these cells is a modest 70Wh/kg.
In this article we look at the advantages and limitations of the commercial battery.The so-called miracle battery that merely live in controlled environments is excluded.We scrutinize the batteries not only in terms of energy density but also longevity,load characteristics,maintenance requirements,self-discharge and operational costs.Since NiCd remains a standard against which other batteries are compared,we evaluate alternative chemistries against this classic battery type.
Nickel Cadmium (NiCd) — mature and well understood but relatively low in energy density.The NiCd is used where long life,high discharge rate and economical price are important.Main applications are two-way radios,biomedical equipment,professional video cameras and power tools. The NiCd contains toxic metals and is environmentally unfriendly.
Nickel-Metal Hydride (NiMH) — has a higher energy density compared to the NiCd at the expense of reduced cycle life.NiMH contains no toxic metals. Applications include mobile phones and laptop computers.
Lead Acid — most economical for larger power applications where weight is of little concern.The lead acid battery is the preferred choice for hospital equipment,wheelchairs,emergency lighting and UPS systems.
Lithium Ion (Li‑ion) — fastest growing battery system.Li‑ion is used where high-energy density and lightweight is of prime importance.The technology is fragile and a protection circuit is required to assure safety.Applications include notebook computers and cellular phones.
Figure 1 compares the characteristics of the six most commonly used rechargeable battery systems in terms of energy density,cycle life,exercise requirements and cost.The figures are based on average ratings of commercially available batteries at the time of publication.
Figure 1: Characteristics of commonly used rechargeable batteries
- Internal resistance of a battery pack varies with cell rating, type of protection circuit and number of cells. Protection circuit of Li‑ion and Li-polymer adds about 100mΩ.
- Cycle life is based on battery receiving regular maintenance. Failing to apply periodic full discharge cycles may reduce the cycle life by a factor of three.
- Cycle life is based on the depth of discharge.Shallow discharges provide more cycles than deep discharges.
- The discharge is highest immediately after charge,then tapers off.The NiCd capacity decreases 10% in the first 24h,then declines to about 10% every 30 days thereafter.Self-discharge increases with higher temperature.
- Internal protection circuits typically consume 3% of the stored energy per month.
- 1.25V is the open cell voltage. 1.2V is the commonly used value.There is no difference between the cells; it is simply a method of rating.
- Capable of high current pulses.
- Applies to discharge only; charge temperature range is more confined.
- Maintenance may be in the form of ‘equalizing’ or ‘topping’ charge.
- Cost of battery for commercially available portable devices.
- Derived from the battery price divided by cycle life.Does not include the cost of electricity and chargers.
Observation: It is interesting to note that NiCd has the shortest charge time,delivers the highest load current and offers the lowest overall cost-per-cycle,but has the most demanding maintenance requirements.