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No single battery offers all the answers,
rather, each chemistry is based on a number of compromises.
Figure 1 compares the pros and cons of the six most common
batteries: the sealed NiCd, NiMH, SLA, Li-Ion and Li-Polymer.
| Battery
Type |
NiCd |
NiMH |
SLA |
Li-ion |
Li-Polymer |
Energy density
(Wh/Kg)
|
50 |
75 |
30 |
100 |
175 |
| Cycle life
(typical) |
1500 |
500 |
200-300 |
300-500 |
150 |
| Fast-charge
time |
1-1/2h |
2-3h |
8-15h |
3-6h |
8-15h |
| Self-discharge |
medium |
high |
high |
very low |
very low |
| Cell voltage
(nom.) |
1.25V |
1.25V |
2V |
3.6V |
1.5V |
| Load current |
very high |
medium |
low |
high |
low |
| Exercise req.
(days) |
/30 |
/90 |
/180 |
N/A |
N/A |
| Battery Cost |
low |
medium |
very low |
very high |
high |
estimated, ref., $)
|
50.00 |
80.00 |
25.00 |
100.00 |
90.00 |
| Cost per cycle
($) |
0.04 |
0.16 |
0.10 |
0.25 |
0.60 |
| In common
use since |
1950 |
1970 |
1970 |
1990 |
1990 |
Figure 1: Pros and
cons of the most commonly used rechargeable batteries
'Energy density' is measured in
watt-hours per kilogram (Wh/kg).
'Cycle life' indicates the typical number of charge-discharge
cycles before the capacity decreases from the nominal 100%
to 80% (65% for the reusable alkaline).
'Fast-charge time' is the time required to fully charge
an empty battery.
'Self-discharge' indicates the self-discharge rate
when the battery is not in use. "Moderate" refers
to 1-2% capacity-loss per day.
'Cell voltage' multiplied by the number of cells provides
the battery terminal voltage
'Load current' is the maximum recommended current the
battery can provide. "High" refers to a discharge
rate of 1C*; "very high" is a current higher than
1C*.
'Exercise requirement' indicates the frequency the
battery needs exercising to achieve maximum service life.
'Battery cost' is the estimated commercial price of
a commonly available battery. 'Cost-per-cycle' indicates
the operating cost derived by taking the average price of
a commercial battery and dividing it by the cycle count.
'In commercial use since' is the approximate year when
the battery became commercially available.
* C-rate is a unit by which charge
and discharge times are scaled. If discharged at 1C, a 1000mAh
battery provides a current of 1000mA; if discharged at 0.5C,
the current is 500mA.
It is interesting to observe 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 exercise requirements. For applications where
high energy density is critical, regular exercise is impractical
and cost is secondary, the NiMH is considered the best choice.
Not without problems, NiMH batteries have a cycle life one-third
that of NiCds. Furthermore, field use has revealed that the
NiMH also needs some level of exercise to maximize service
life, but to a lesser extent than the NiCd. In comparison,
the SLA needs little or no maintenance but has a low energy
density.
Among rechargeable batteries, the NiCd
has been around the longest since 1950. It is also one of
the best understood chemistries and has become a standard
against which other batteries are compared. This study compares
the different chemistries relative to the NiCd.
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