CHOICE OF BATTERY

A few years ago, the Nickel Cadmium (NiCd) was the only suitable rechargeable battery for applications such as portable radios, cellular phones, laptop computers and video cameras. In an effort to satisfy the demand for increased run time, new battery chemistries have emerged that provide twice the energy densities compared to the NiCd.

Will the new generation of batteries eventually replace the classic NiCd? The answer is no — at least not for now. Every invention that solves one problem creates new ones. By altering well-proven chemistries and packing more energy into a cell, characteristics such as load current, ease of charge and cycle-life are often adversely affected and the operating costs frequently increased.

Research has brought about a number of different battery chemistries, each offering distinct advantages but none providing a fully satisfactory solution. Today’s most common battery chemistries are:,

Nickel Cadium (NiCd) — mature and well understood, the NiCd is used where long life, high discharge rate and economical price are important. Main applications are portable radios, emergency medical equipment, professional video cameras and power tools.

Nickel-Metal Hydride (NiMH) — provides incremental improvements in capacity over the NiCd at the expense of reduced cycle life and lower load current. Applications include cellular phones and laptop computers.

Sealed Led Acid (SLA) — most economical for larger power applications where weight is of lesser concern. The SLA is the preferred choice for medical equipment, wheelchairs, UPS systems and emergency lighting.

Lithium Ion (Li-ion) — fragile technology requiring protector circuit, the Li-ion is used where very high energy density is needed and cost is secondary. Applications include notebook computers, video cameras, new generation cellular phones and advanced military communications devices.

Lithium Polymer (Li-ion) — a potentially lower cost version of the Li-ion under development. When commercially available, the Li-polymer will offer high energy density and low self-discharge) but will suit only low-power applications.

Reusable Alkaline — replaces disposable household batteries; suitable for low-power, low-cost applications. Its limited cycle life is compensated by low self-discharge, making this battery ideal for portable entertainment devices and flashlights.

The table below 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 typical ratings of commercially available batteries at the time this was written.

	NiCd	NiMH	SLA	Li-Ion	Li-Polymer	Reusable Alkaline
Energy Density (Wh/Kh)	40-60	60-80	30	100	150-200	80 (initial)
Cyclelife (capacity decrease from 100% to 80%)	1500 1	500 2	200-500 2	500-1000 2	100-150 2	10 2
Fastcharge time	1-1 h	2-4 h	18-16h	3-4h	8-15	2-4h
Overcharge tolerance	moderate	low	high	very low	N/A	miderate
Self-discharge per month(room temperature)	20% 2	30%3	5%	10% 4	N/A 4	0.3%
Cell voltage (nominall)	1.25v5	1.25v5	2v	3.6v	2.7v 6	1.5v
Load current	2c	0.5-1c	0.2c 1	1c or less	0.2c	0.2c
Operating temperature 8	-40 to 60 C	-20 to 60 C	-20 to 60 C	-20 to 60 C	N/A	0 to 65 C
Maintenance requirements (to obtain max service life)	30 days	90 days	3-6 months 9	not req.	not req.	not req.
Typical Battery cost (US$ reference only)	$50.00	$70.00	$25.00	100.00	($90.00)	$5.00
Cost per cycle (US$) 10	(7.5v) $0.04	(7.5v) $0.14	(6v) $0.14	(7.2v) $0.10-0.20	(8.1v) ($0.60)	(9v) $0.50
In commercial use since	1950	1990	1970	1991	N/A	1992

1 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.
2 Cycle life is based on the depth of discharge. Shallow discharge provides more cycles than deep discharges.
3 The discharge is highest after the first 24h, then tapers off. The NiCd discharges 10% in the first 24h, then drops to about 10% every 30 days thereafter. Self-discharge increases with higher temperature.
4 Internal protection circuits typically consume 3% per month.
5 1.25V is the official cell voltage rating, 1.2V is now also commonly used. There is no difference between the cells.
6 2.5-3.0 volts depending on positive electrode used.
7 Capable of pulsed load currents of up to 1C.
8 Applies to discharge only; charging temperature range is more confined.
9 Maintenance may be in form of topping charge.


Observation: It is interesting to note that the NiCd has the shortest charge time, delivers the highest load current and offers the lowest cost-per-cycle, but is most demanding on maintenance requirements.

Within a designated battery chemistry, several cell types are available, allowing for tailoring to a specific application. Some cells are built for maximum capacity, others are made to deliver high current under a rugged discharge/charge regime and yet another type is designed to operate at high or low temperatures. The performance characteristics of the cells may also vary between manufacturers. The NiCd system currently has the widest range of cell types and sizes to chose from.

To obtain the desired battery voltage, several cells are connected in series. Parallel connection is also used to get higher ampere-hour (Ah) ratings, however, selecting a larger cell is the preferred choice. Packs with fewer cells in series perform better than those that have 12 or more. Like a chain, the more links that are used, the larger the odds of one breaking. Put in perspective, a battery with 20 small AA NiCd cells driving an apparatus requiring 25 volts has a larger chance of premature failure than a 6-cell pack of rugged C cells powering a 7.5V device. Cell matching also becomes an important issue, especially if 12 and more cells are connected in series.

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