What's the Best Battery?

We often get puzzled by announcements of new batteries that are said to offer very high energy densities, deliver 1000 charge/discharge cycles, 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 wear out prematurely. Another battery may be built for long life, but it is big and bulky. A third battery may provide all the desirable attributes, but the price would be too high for commercial use.

If you want to learn more, please visit our website lithium ion battery replacement for nicd.

Battery manufacturers understand customer needs and offer packs that best suit specific applications. The mobile phone industry, for example, emphasizes small size, high energy density, and low price, with longevity coming second.

The inscription of NiMH on a battery pack does not automatically guarantee high energy density. For instance, a prismatic Nickel-Metal Hydride battery for a mobile phone is made for slim geometry, providing energy densities of about 60Wh/kg and around 300 cycles. In comparison, a cylindrical NiMH offers energy densities of 80Wh/kg and higher but has moderate to low cycle counts. High durability NiMH batteries, which endure 1000 discharges, are usually packaged in bulky cylindrical cells with a modest 70Wh/kg.

Compromises also exist with lithium-based batteries. Li-ion packs for defense applications far exceed the energy density of commercial equivalents. Unfortunately, these super-high capacity Li-ion batteries are deemed unsafe for public use, and their high price makes them inaccessible to the commercial market.

In this article, we look at the advantages and limitations of commercial batteries. We exclude the so-called miracle batteries that exist only in controlled environments. We scrutinize batteries in terms of energy density, 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 traditional battery type.

Nickel Cadmium

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 include two-way radios, biomedical equipment, professional video cameras, and power tools. The NiCd contains toxic metals and is environmentally unfriendly.

Nickel-Metal Hydride

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

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

Lithium Ion (Li-ion) — fastest-growing battery system. Li-ion is used where high energy density and lightweight are of prime importance. The technology is fragile, and a protection circuit is required to assure safety. Applications include notebook computers and cellular phones.

Lithium Ion Polymer

Lithium Ion Polymer (Li-ion polymer) — offers the attributes of the Li-ion in ultra-slim geometry and simplified packaging. Main applications are mobile 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.

NiCd

NiMH Lead Acid Li-ion Li-ion polymer Reusable
Alkaline

Gravimetric Energy Density

(Wh/kg) 45-80 60-120 30-50 110-160 100-130 80 (initial) Internal Resistance
(includes peripheral circuits) in mΩ 100 to 200 1
6V pack 200 to 300 1
6V pack <100 1
12V pack 150 to 250 1
7.2V pack 200 to 300 1
7.2V pack 200 to 2000 1
6V pack Cycle Life (to 80% of initial capacity) 1500 2300 to 500 2, 3 200 to
300 2500 to 1000 3 300 to
500 50 3
(to 50%) 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) 1.25V 6 1.25V 6 2V 3.6V 3.6V 1.5V

Load Current

- peak
- best result 20C
1C 5C
0.5C or lower 5C 7
0.2C >2C
1C or lower >2C
1C or lower 0.5C
0.2C or lower Operating Temperature (discharge only) -40 to
60°C -20 to
60°C -20 to
60°C -20 to
60°C 0 to
60°C 0 to
65°C Maintenance Requirement 30 to 60 days 60 to 90 days 3 to 6 months 9 not req. not req. not req. Typical Battery Cost
(US$, reference only) $50
(7.2V) $60
(7.2V) $25
(6V) $100
(7.2V) $100
(7.2V) $5
(9V) Cost per Cycle (US$) 11 $0.04 $0.12 $0.10 $0.14 $0.29 $0.10-0.50 Commercial use since 1950 1990 1970 (sealed lead acid) 1991 1999 1992

Figure 1: Characteristics of commonly used rechargeable batteries

1. Internal resistance of a battery pack varies with cell rating, type of protection circuit, and number of cells. The protection circuit of Li-ion and Li-polymer adds about 100mΩ.
2. Cycle life is based on a battery receiving regular maintenance. Failing to apply periodic full discharge cycles may reduce the cycle life by a factor of three.
3. Cycle life is based on the depth of discharge. Shallow discharges provide more cycles than deep discharges.
4. The discharge is highest immediately after charge and then tapers off. The NiCd capacity decreases by 10% in the first 24h, then declines to about 10% every 30 days thereafter. Self-discharge increases with higher temperatures.
5. Internal protection circuits typically consume 3% of the stored energy per month.
6. 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.
7. Capable of high current pulses.
8. Applies to discharge only; the charge temperature range is more confined.
9. Maintenance may be in the form of 'equalizing' or 'topping' charge.
10. Cost of battery for commercially available portable devices.
11. 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.

The Nickel Cadmium (NiCd) battery

The NiCd prefers fast charges to slow charges and pulse charge to DC charge. All other chemistries prefer a shallow discharge and moderate load currents. The NiCd is a strong and silent worker; hard labor poses no problem. In fact, the NiCd is the only battery type that performs well under rigorous working conditions. It does not like to be pampered by sitting in chargers for days and being used only occasionally for brief periods. A periodic full discharge is important because, if omitted, large crystals will form on the cell plates (also referred to as memory) and the NiCd will gradually lose its performance.

Among rechargeable batteries, NiCd remains a popular choice for applications such as two-way radios, emergency medical equipment, and power tools. Batteries with higher energy densities and less toxic metals are causing a diversion from NiCd to newer technologies.

Advantages and Limitations of NiCd Batteries

Advantages

Fast and simple charge — even after prolonged storage.

High number of charge/discharge cycles — if properly maintained, the NiCd provides over 1000 charge/discharge cycles.

Good load performance — the NiCd allows recharging at low temperatures.

Long shelf life – in any state-of-charge.

Simple storage and transportation — most airfreight companies accept the NiCd without special conditions.

Good low-temperature performance.

Forgiving if abused — the NiCd is one of the most rugged rechargeable batteries.

Economically priced — the NiCd is the lowest cost battery in terms of cost per cycle.

Available in a wide range of sizes and performance options — most NiCd cells are cylindrical.

Limitations

Relatively low energy density — compared with newer systems.

Memory effect — the NiCd must periodically be exercised to prevent memory.

Environmentally unfriendly — the NiCd contains toxic metals. Some countries are limiting the use of the NiCd battery.

Has a relatively high self-discharge — needs recharging after storage.

Figure 2: Advantages and limitations of NiCd batteries.

The Nickel-Metal Hydride (NiMH) battery

Research on the NiMH system started in the 1970s as a means of discovering how to store hydrogen for the nickel hydrogen battery. Today, nickel hydrogen batteries are mainly used for satellite applications. They are bulky, contain high-pressure steel canisters, and cost thousands of dollars per cell.

In the early experimental days of the NiMH battery, the metal hydride alloys were unstable in the cell environment, and the desired performance characteristics could not be achieved. As a result, the development of NiMH slowed down. New hydride alloys were developed in the 1980s that were stable enough for use in a cell. Since the late 1980s, NiMH has steadily improved.

The success of NiMH has been driven by its high energy density and the use of environmentally friendly metals. The modern NiMH offers up to 40 percent higher energy density compared to NiCd. There is potential for even higher capacities, but not without some negative side effects.

NiMH is less durable than NiCd. Cycling under heavy load and storage at high temperatures reduces its service life. NiMH suffers from high self-discharge, which is considerably greater than that of NiCd.

NiMH has been replacing NiCd in markets such as wireless communications and mobile computing. In many parts of the world, buyers are encouraged to use NiMH rather than NiCd batteries. This is due to environmental concerns about the careless disposal of spent batteries.

Experts agree that NiMH has greatly improved over the years, but limitations remain. Most of the shortcomings are native to the nickel-based technology and are shared with NiCd batteries. It is widely accepted that NiMH is an interim step to lithium battery technology.

Advantages and Limitations of NiMH Batteries

Advantages

30–40 percent higher capacity over a standard NiCd. The NiMH has the potential for even higher energy densities.

Less prone to memory than NiCd. Periodic exercise cycles are required less often.

Simple storage and transportation — transportation conditions are not subject to regulatory control.

Environmentally friendly — contains only mild toxins; profitable for recycling.

Limitations

Limited service life — if repeatedly deep cycled, especially at high load currents, the performance starts to deteriorate after 200 to 300 cycles. Shallow rather than deep discharge cycles are preferred.

Limited discharge current — although a NiMH battery is capable of delivering high discharge currents, repeated discharges with high load currents reduce its cycle life. Best results are achieved with load currents of 0.2C to 0.5C (one-fifth to one-half of the rated capacity).

More complex charge algorithm needed — the NiMH generates more heat during charge and requires a longer charge time than NiCd. The trickle charge is critical and must be controlled carefully.

High self-discharge — the NiMH has about 50 percent higher self-discharge compared to NiCd. New chemical additives improve the self-discharge but at the expense of lower energy density.

Performance degrades if stored at elevated temperatures — the NiMH should be stored in a cool place and at a state-of-charge of about 40 percent.

High maintenance — battery requires regular full discharge to prevent crystalline formation.

About 20 percent more expensive than NiCd — NiMH batteries designed for high current draw are more expensive than the regular version.

The Lead Acid Battery

Invented by the French physician Gaston Planté in 1859, lead acid was the first rechargeable battery for commercial use. Today, the flooded lead acid battery is used in automobiles, forklifts, and large uninterruptible power supply (UPS) systems.

During the mid-1970s, researchers developed a maintenance-free lead acid battery that could operate in any position. The liquid electrolyte was transformed into moistened separators, and the enclosure was sealed. Safety valves were added to allow venting of gas during charge and discharge.

Driven by different applications, two battery designations emerged. They are the small sealed lead acid (SLA), also known under the brand name of Gelcell, and the large valve-regulated lead acid (VRLA). Technically, both batteries are the same. (Engineers may argue that the word ‘sealed lead acid’ is a misnomer because no lead acid battery can be totally sealed.) Because of our emphasis on portable batteries, we focus on the SLA.

Unlike the flooded lead acid battery, both the SLA and VRLA are designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging would cause gassing and water depletion. Consequently, these batteries can never be charged to their full potential.

The lead acid is not subject to memory. Leaving the battery on float charge for a prolonged time does not cause damage. The battery’s charge retention is best among rechargeable batteries. Whereas the NiCd self-discharges approximately 40 percent of its stored energy in three months, the SLA self-discharges the same amount in one year. The SLA is relatively inexpensive to purchase, but operational costs can be more expensive than NiCd if full cycles are required on a repetitive basis.

The SLA does not lend itself to fast charging — typical charge times are 8 to 16 hours. The SLA must always be stored in a charged state. Leaving the battery in a discharged condition causes sulfation, a condition that makes the battery difficult, if not impossible, to recharge.

Unlike the NiCd, the SLA does not like deep cycling. A full discharge causes extra strain, and each cycle robs the battery of a small amount of capacity. This wear-down characteristic also applies to other battery chemistries in varying degrees. To prevent the battery from being stressed through repetitive deep discharge, a larger SLA battery is recommended.

Depending on the depth of discharge and operating temperature, the SLA provides 200