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F.A.Q.
- Battery
Chemistries
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Q.
What is a lithium polymer battery?
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| A. A Li-ion
polymer cells offers the same energy density per volume as a
Li-ion cell, with the added advantage that the housing need
not be rigid. This type of cell uses a plastic film like electrolyte
(polymer) which also acts as the separator. This film in itself
does not conduct electricity and is therefore an insulator,
but it does allow the exchange of ions. Having no liquid or
gel electrolyte also means that these cells are safer to handle
and do not present risks of fire. This technology is becoming
more diffused nowadays. Potentially, these cells should cost
less than Li-Ion equivalents due to the substances used in their
manufacture. |
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Q.
Which is the one main disadvantage of Li-polymer over Li-Ion?
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| A. Impedance.
The internal resistance of the cell is at least double that
of Li-Ion making it unsuitable for some applications where high
currents need to be drawn. In order to overcome this difficulty,
in the case of GSM mobile phones for instance, a hybrid battery,
which contains some gel electrolyte has been developed. |
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Q.
What is an Li-Ion battery made of?
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| A. Within
a stainless steel or aluminium package, there is a 3 layered
coil consisting of the positive electrode, the negative electrode
and the insulating material. The positive electrode is usually
made of lithium cobalt oxide (or manganese) as its main chemical.
The negative electrode is made of speciality carbon. The cell
profile can be prismatic or cylindrical. The more compressed
the arrangement is, the higher the capacity or energy density.
Other features in the construction of these cells include several
safety features. |
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Q.
What is the difference between an Ni-MH and a Li-Ion battery?
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| A. The
Li-Ion battery offers a higher energy density and has a terminal
voltage which is 3 times higher (3.4V~3.6V typically). Both
technologies can be classified as environmentally safe. |
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Q.
How long does a Li-ion cell last?
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| A. The
life cycle varies from 500 to 1000 cycles. However, not all
cells may be able to last so long. Much depends on the treatment
they receive during their lifetime. |
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Q.
Are Li-Ion cells expensive.
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| A. Li-Ion
cells are the most expensive. However, the voltage is three
times as much as that of Ni-MH and they may be smaller in size.
Research is being made by designers to increase the cell capacities
per unit volume/weight. |
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Q.
Which is one main big advantage of using a Li-Ion battery?
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| A. Li-ion
batteries have very flat voltage discharge characteristics due
to the use of speciality carbon as one of their main constituents.
This means that they will perform in a very stable way during
use. |
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Q.
Which is the ideal charging current for Li-Ion cells?
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| A. A charging
current in the region of 0.7C ~ 1.0C is recommended provided
that the battery voltage is higher than 2.8V (threshold) |
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Q.
If a Li-Ion battery is not used for a long time, does it deteriorate?
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| A. Yes.
A battery can be considered as a living object because its action
continually depends on chemical reactions. If not used for a
long time, the chemicals in the cell degrade and therefore the
battery performance is drastically reduced. Manufacturers use
state of the art techniques to hermetically seal the cells,
however leakage can also occur over a long period of time. |
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Q.
What is the maximum charging voltage for a Li-Ion cell.
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Q.
What is memory effect?
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| A. Memory
effect is a physical phenomena mostly attributed to Ni-Cd technology,
whereby the battery, Due to improper charging, is unable to
deliver its full capacity. In such cases, the best thing to
do is to fully discharge the cells and recharge them using them
using the manufacturer designed charger. Fortunately, Ni-MH
and Li-Ion cells exhibit little or no memory effect. |
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Q.
How is a Li-Ion cell wired up.
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| A. Li-Ion
cells are not so straightforward as their counterparts. Their
operation needs electronic control. The charging voltage must
not exceed 4.3V(over-charge). The minimum cell voltage must
never go below 23V (over-discharge). They also need different
charging currents, depending on the cell voltage and temperature.
They need to have thermal and short circuit protection both
during charge and discharge. All these functions are controlled
by a special PCB, which is normally connected and housed in
the same battery package. |
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Q.
Which is the typical operating temperature for Li-Ion cells?
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| A. Typically
from 0C to 45C during charge and discharge. Beyond these values,
performance degrades considerably. |
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Q.
Which is the typical fast time charge?
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| A. 2 ~
4 hours depending on the quality of charger and battery condition. |
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Q.
Does a Li-Ion intelligent charger incorporate further features?
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| A. A good
charger adjusts the charging current according to the actual
cell voltage. It also incorporates devices for sensing temperature
gradients and voltage rise over time. A timer is also included
to avoid prolonged charging |
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Q.
Is a trickle charge recommended for Ni-Mh batteries.
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| A. A trickle
charge is not recommended at all. Actually, it should only be
used after the battery has received a rapid charge. The trickle
charge (around 0.05C) will continue to top up the battery in
this case, but care must be taken not to over charge them. |
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Q.
Can parallel charging of Ni-Mh cells be made
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| A.Yes,
but with caution. The charging device must have design features
for this job. |
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Q.
What is rapid charge for an Ni-Mh battery?
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| A.We refer
to rapid charge as one of around 0.8C ~ 1C max. Beyond this
value, the cell temperature may rise causing the safety vent
to open leading to leakage of the materials from inside the
cell. |
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Q.
Is the charging of Ni-Mh straightforward?
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| A. A good
charger incorporates several of the features which are also
found in Li-Ion chargers. However, these chargers are not compatible
with both types of cells unless specifically designed for this
job. The chargers may also include V detection delay for more
accurate sensing of this parameter. |
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Q.
How is a Ni-MH battery pack protected?
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| A. It is
good design practice to incorporate a polyswitch (positive temperature
coefficient) and an NTC (negative temperature coefficient) device
inside the battery pack. |
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Q.
Can a Li-Ion pack substitute a Ni-Mh pack?
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| A. Li-Ion
cells are rapidly becoming more diffused, especially since their
costs are decreasing. The characteristics of these cells, permit
packs of higher performance in less bulk and weight. For portable
applications, this can be a major advantage. |
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Q.
Will Ni-Mh or Li-Ions be replacing Ni-Cd?
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| A. In future
maybe, but not right now. Ni-Cd still offers a much better cost
performance. These cells are the fastest to re-charge and the
best load current deliverers. Peak currents of 20C can be achieved,
making them ideal for power tools, say. Their main disadvantage
is of course, the memory effect. They are also environmentally
hazardous. (Some countries are already restricting their usage). |
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Q.
Do Ni-Cd cells have further disadvantages?
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| A. They
have lower energy densities than their counterparts and have
a higher rate of self discharge. |
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Q.
Is it true that normal alkaline batteries can be re-charged?
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| A. It is
possible to re-charge alkaline batteries if these have not been
completely used up (typically 50%). The number of cycles will
normally be very limited . This process involves the emission
of Hydrogen which can be dangerous. Because of this, re-charging
of alkaline batteries is not recommended. |
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