| 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 a battery pack does not automatically
guarantee high energy density. 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.
Compromises also 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.
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.
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 mW |
100 to 2001
6V pack |
200 to 3001
6V pack |
<1001
12V pack |
150 to 2501
7.2V pack |
200 to 3001
7.2V pack |
200 to 20001
6V pack |
| Cycle Life (to 80% of initial capacity) |
15002 |
300 to 5002,3 |
200 to
3002 |
500 to 10003 |
300 to
500 |
503
(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) |
1.25V6 |
1.25V6 |
2V |
3.6V |
3.6V |
1.5V |
Load Current
- peak
- best result |
20C
1C |
5C
0.5C or lower |
5C7
0.2C |
>2C
1C or lower |
>2C
1C or lower |
0.5C
0.2C or lower |
| Operating Temperature (discharge only) |
-40 to
60? |
-20 to
60? |
-20 to
60? |
-20 to
60? |
0 to
60? |
0 to
65? |
| 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) |
$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 |
1991 |
1999 |
1992 |
| |
|
