Battery Basics

[Steve in PR]

Nickel-cadmium – Ni-Cad

Nickel-cadmium is used where long life, high discharge rate and extended temperature range is important. Nickel-cadmium contains toxic metals.

It is a strong and silent worker; hard labor poses little problem. In fact, nickel-cadmium is the only battery type that performs well under rigorous working conditions. All other chemistries prefer a shallow discharge and moderate load currents.

Nickel-cadmium 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 so important that, if omitted, large crystals will form on the cell plates (also referred to as memory) and the nickel-cadmium will gradually lose its performance.

There is shift towards batteries with higher energy densities and less toxic metals but alternative chemistries cannot always match the superior durability and low cost of nickel-cadmium.

Here is a summary of the advantages and limitations of nickel-cadmium batteries.


Advantages

• Fast and simple charge, even after prolonged storage.
• High number of charge/discharge cycles - if properly maintained, nickel-cadmium provides over 1000 charge/discharge cycles.
• Good load performance - nickel-cadmium allows recharging at low temperatures.
• Long shelf life - five-year storage is possible. Some priming prior to use will be required.
• Simple storage and transportation - most airfreight companies accept nickel-cadmium without special conditions.
• Good low-temperature performance.
• Forgiving if abused - nickel-cadmium is one of the most rugged rechargeable batteries.
• Economically priced - nickel-cadmium is lowest in terms of cost per cycle.
• Available in a wide range of sizes and performance options - most nickel-cadmium cells are cylindrical.

Limitations

• Relatively low energy density.
• Memory effect - nickel-cadmium must periodically be exercised (discharge/charge) to prevent memory.
• Environmentally unfriendly - nickel-cadmium contains toxic metals. Some countries restrict its use.
• Relatively high self-discharge - needs recharging after storage

Nickel-metal-hydride – NiMH

Nickel-metal hydride is less durable than nickel-cadmium. Cycling under heavy load and storage at high temperature reduces the service life. Nickel-metal hydride suffers from high self-discharge, which is higher than that of nickel-cadmium.

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

Here is a summary of the advantages and limitations of nickel-metal hydride batteries.

Advantages

• 30-40% higher capacity than standard nickel-cadmium. Nickel-metal-hydride has potential for yet higher energy densities.
• Less prone to memory than nickel-cadmium - fewer exercise cycles are required.
• Simple storage and transportation - transport is not subject to regulatory control.
• Environmentally friendly - contains only mild toxins; profitable for recycling.

Limitations

• Limited service life - the performance starts to deteriorate after 200-300 cycles if repeatedly deeply cycled.
• Relatively short storage of three years. Cool temperature and a partial charge slows aging.
• Limited discharge current - although nickel-metal-hydride is capable of delivering high discharge currents, heavy load reduces the battery's cycle life.
• More complex charge algorithm needed - nickel-metal-hydride generates more heat during charge and requires slightly longer charge times than nickel-cadmium. Trickle charge settings are critical because the battery cannot absorb overcharge.
• High self-discharge - typically 50% higher than nickel-cadmium.
• Performance degrades if stored at elevated temperatures - nickel-metal-hydride should be stored in a cool place at 40% state-of-charge.
• High maintenance - nickel-metal hydride requires regular full discharge to prevent crystalline formation. nickel-cadmium should be exercised once a month, nickel-metal-hydride once in every 3 months.

Lithium-ion - fastest growing battery system.

Lithium-ion is a low maintenance battery, an advantage that most other chemistries cannot claim. There is no memory and no scheduled cycling is required to prolong the battery's life. In addition, the self-discharge is less than half compared to nickel-cadmium, making lithium-ion well suited for modern fuel gauge applications. Lithium-ion cells cause little harm when disposed.

Despite its overall advantages, lithium-ion has its drawbacks. It is fragile and requires a protection circuit to maintain safe operation. Built into each pack, the protection circuit limits the peak voltage of each cell during charge and prevents the cell voltage from dropping too low on discharge. In addition, the cell temperature is monitored to prevent temperature extremes. The maximum charge and discharge current on most packs are is limited to between 1C and 2C. With these precautions in place, the possibility of metallic lithium plating occurring due to overcharge is virtually eliminated.

Aging is a concern with most lithium-ion batteries and many manufacturers remain silent about this issue. Some capacity deterioration is noticeable after one year, whether the battery is in use or not. The battery frequently fails after two or three years. It should be noted that other chemistries also have age-related degenerative effects. This is especially true for nickel-metal-hydride if exposed to high ambient temperatures. At the same time, lithium-ion packs are known to have served for five years in some applications.

Manufacturers are constantly improving lithium-ion. New and enhanced chemical combinations are introduced every six months or so. With such rapid progress, it is difficult to assess how well the revised battery will age.

Storage in a cool place slows the aging process of lithium-ion (and other chemistries). Manufacturers recommend storage temperatures of 15°C (59°F). In addition, the battery should be partially charged during storage. The manufacturer recommends a 40% charge.

Advantages

• High energy density - potential for yet higher capacities.
• Does not need prolonged priming when new. One regular charge is all that's needed.
• Relatively low self-discharge - self-discharge is less than half that of nickel-based batteries.
• Low Maintenance - no periodic discharge is needed; there is no memory.
• Specialty cells can provide very high current to applications such as power tools.

Limitations

• Requires protection circuit to maintain voltage and current within safe limits.
• Subject to aging, even if not in use - storage in a cool place at 40% charge reduces the aging effect.
• Expensive to manufacture - about 40 percent higher in cost than nickel-cadmium.
• Not fully mature - metals and chemicals are changing on a continuing basis.

Reusable Alkaline

The idea of recharging alkaline batteries is not new. Although not endorsed by manufacturers, ordinary alkaline batteries have been recharged in households for many years. Recharging these batteries is only effective, however, if the cells have been discharged to less than 50% of their total capacity. The number of recharges depends solely on the depth of discharge and is limited to a few cycles at best. With each recharge, the amount of capacity the cell can hold is reduced. There is a cautionary advisory. Charging ordinary alkaline batteries may generate hydrogen gas, which can lead to explosion. It is not prudent to charge ordinary alkaline unsupervised.

The reusable alkaline is designed for repeated recharge. Also, there is a loss of charge acceptance with each recharge. The longevity of the reusable alkaline is a direct function of the depth of discharge; the deeper the discharge, the fewer cycles the battery can endure.


Advantages

• Inexpensive - can be used as a direct replacement for non-rechargeable (primary) cells.
• More economical than non-rechargeables - allows several recharges.
• Low self-discharge - can be stored as a standby battery for up to 10 years.
• Environmentally friendly - no toxic metals used, fewer batteries are discarded.
• Maintenance free - no need for cycling; no memory.

Limitations

• Limited current handling - suited for light-duty applications like portable home entertainment, flashlights.
• Limited cycle life - for best results, recharge before the battery gets too low.

Charging
Simple Guidelines:

• Avoid high temperature during charging. Discontinue the use of chargers that cook batteries.
• A charger for nickel-metal-hydride can also accommodate nickel-cadmium, but not the other way around. A charger designed for nickel-cadmium would overcharge the nickel-metal-hydride battery.
• Nickel-based batteries prefer fast-charge. Lingering slow charges cause crystalline formation (memory).
• Nickel- and lithium-based batteries require different charge algorithms. The two chemistries can normally not be interchanged in the same charger.
• If not used immediately, remove the battery from the charger and apply a topping-charge before use. Do not leave nickel-based battery in the charger for more than a few days, even if on trickle charge.