2.1 Chemistry Comparison
Let's examine the advantages and limitations of today’s popular battery systems. Batteries are scrutinized not only in terms of energy density but service life, load characteristics, maintenance requirements, self-discharge and operational costs. Since NiCd remains a standard against which other batteries are compared, let’s 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 not environmentally friendly.
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 light weight is of prime importance. The Li-ion is more expensive than other systems and must follow strict guidelines to assure safety. Applications include notebook computers and cellular phones.
Lithium Ion Polymer (Li-ion polymer) — a potentially lower cost version of the Li-ion. This chemistry is similar to the Li-ion in terms of energy density. It enables very slim geometry and allows simplified packaging. Main applications are mobile phones.
Reusable Alkaline — replaces disposable household batteries; suitable for low-power applications. Its limited cycle life is compensated by low self-discharge, making this battery ideal for portable entertainment devices and flashlights.
Figure 2-1 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. Exotic batteries with above average ratings are not included.
NiCd NiMH Lead Acid Li-ion Li-ion polymer Reusable
Gravimetric Energy Density (Wh/kg) 45-80 60-120 30-50 110-160 100-130 80 (initial) Internal Resistance
(includes peripheral circuits) in mW
100 to 2001
200 to 3001
150 to 2501
200 to 3001
200 to 20001
Cycle Life (to 80% of initial capacity) 15002 300 to 5002,3 200 to
500 to 10003 300 to
Fast Charge Time 1h typical 2-4h 8-16h 2-4h 2-4h 2-3h Overcharge Tolerance moderate low high very low low moderate Self-discharge / Month (room temperature) 20%4 30%4 5% 10%5 ~10%5 0.3% Cell Voltage (nominal) 1.25V6 1.25V6 2V 3.6V 3.6V 1.5V Load Current
- best result
0.5C or lower
1C or lower
1C or lower
0.2C or lower
Operating Temperature (discharge only) -40 to
Maintenance Requirement 30 to 60 days 60 to 90 days 3 to 6 months9 not req. not req. not req. Typical Battery Cost
(US$, reference only)
Cost per Cycle (US$)11 $0.04 $0.12 $0.10 $0.14 $0.29 $0.10-0.50 Commercial use since 1950 1990 1970 1991 1999 1992
- 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 100mW.
- 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.