Further
Information
If you are still unsure on what all the
differnt features of a charger actually mean, we have compiled
a detailed list explaining each feature in more detail to
aid you in choosing the right charger.
Worldwide
use possible: This means that the charger can be
used around the world; this is because they can run of an
input voltage of 100V – 240V AC. In the UK, standard
mains voltage is 240V, however in different parts of the
world 100V is used. Chargers marked with a star can run
from either of these two voltages without the need for adaptors.
Some of the chargers come supplied with a primary plug set
for the different regions, others, plug sets are additional.
Trickle
charge function: Many modern chargers use a high
charging current in order to charge up cells quickly. This
is fine whilst the cells are being charged, however, once
full charge is complete, maintaining this high charge
current will damage the cell due to overcharging. The trickle
charge function reduces the charge current to one that is
very low, all this does is maintain the charge in the battery,
it does not charge it any further.
Short
Circuit Protection: This is a function which protects
the charger from a cell which has become short-circuited.
If this occurs, the current flowing through the battery
will be very high and will both damage the cell further
but also overload the charger and cause it to break.
Reverse
Connection (Polarity)
protection: When you connect a battery to a device
you need to connect it the right way round, i.e. match up
the + and – on the battery with the + and –
marked in the device otherwise the device and battery will
be damaged. This is the same when placing batteries into
a charger, if they are placed the wrong way round it will
be damaged through the charging process as well damaging
the charger itself. Some chargers can detect this and the
charging process will not commence until the batteries are
inserted correctly.
Temperature
control: During the charging process cells will
heat up due to the chemical reactions going on inside the
cell, this is normal, however if the temperature of the
cell becomes too hot then it is prone to damage.
Temperature controlled chargers either have a cooling fan
to maintain cell temperature and/or have a sensor which
will stop the charge process if the cells get too hot.
Microprocessor
Controlled: Microprocessor controlled chargers
are intelligent chargers; they use microprocessors to detect
the state of charge of the batteries plugged into them.
It enables them to detect when a battery is fully charged
and so can halt the charge process when complete to protect
the batteries from damage due to overcharging. Depending
on how advanced the charger is depends on what else the
microprocessor controls and automates. The additional features
a microprocessor can bring are shown in the table
above in the main table and the ‘Additional Features’
column.
Refresh
Function: The refresh function repeatedly pulse
charges and discharges the battery in order to shake up
the electrolyte in the battery to bring it back to a state
where it can perform as a new cell. A cell may need to be
refreshed for a number of reasons; extended storage, extended
discharge or careless cycling. The refresh function cannot
‘refresh’ every battery; all rechargeable batteries
have a finite life and if they are damaged beyond a certain
point then they cannot be brought ‘back to life’
as it were. Many chargers which come with this facility
will, after completion, accept or reject the cell depending
on whether the cell is faulty beyond repair or not; this
means that time is not wasted by charging a damaged cell.
Faulty
Cell Detection: Rechargeable cells as with primary
non-rechargeable have a finite service life and reach a
point where the cells are dead. At this point the cells
cannot be recharged to their full or even partial capacity
and will not function correctly if at all in their application.
The time it takes for a cell to reach the end of its life
depends on the type of use; a general rule of thumb is that
NiCd and NiMh cells will last between 500 to 1000 charge/recharge
cycles.
When
a cell reaches the end of its life, it is pointless attempting
to recharge it, it wastes both time and power in trying
to recharge a dead cell and you will only know that the
cell is dead after recharging it, inserting it into your
device then finding it fails. This can be a major problem
if it is critical that after charging that cells you rely
on them to power your device effectively and for the required
time. Most intelligent chargers have a function that checks
the ‘state of life’ of a cell to determine if
it is faulty/dead and therefore can no longer be used. If
the charger determines a cell is faulty it will indicate
the fact and won’t charge the faulty cell; this helps
you identify when a cell is dead before having to wait for
your device to fail.
-deltaV
Charging: Reads ‘minus delta V charging’
or protection is a method of charging which is considered
to be one of the best methods to charge NiCd and NiMh cells.
As the cells are charged at constant current, the cell
voltage rises through the charging process up to a peak
voltage; at this point, the cell voltage cannot increase
any further and subsequently begins to fall. This voltage
drop is due to polarisation, or oxygen builds up inside
the cell which starts to occur once the cell is fully charged.
This is the danger zone where overcharging can damage the
cell due to a rapid rise in temperature because the cell
cannot be charged further so the electrical energy is converted
directly into heat as opposed to chemical changes which
occur during charging. So in order to avoid cell damage
due to over-charging, many chargers use this voltage drop
as the indication that the cell is fully charged. The small
voltage drop is known as ‘-‘ negative, i.e.
falling/drop; 'delta', i.e. small change; ‘V’
voltage; collectively known as –deltaV. This technique
can only be used in fast charging of NiCd and NiMh cells.
Individual
Supervision of cells: Most consumer chargers have
the capacity to charge more than one cell at a time, there
are two issues which are of concern.
1.
What if the charge takes 4 cells and you only want to charge
1 or 2 cells?
2. What if the cells put in are not at the same state of
charge when put into the charger?
E.g. take a charger that can take between 1-4 AA cells;
older/basic chargers need either 2 or 4 cells
for the charging process to commence as they need them to
complete a circuit so the charger can
deliver its charge current to the cells. The intelligent
chargers do not need multiples of cells to be
inserted for the charge process to begin because charge
current is applied to individual charge
terminals and not a group of them.
With
regards to the second point, standard chargers have a specific
charge current which is applied to each cell in the charger
either for the time of the charge or until it is manually
turned off. This can cause several problems including possibility
of not charging a cell enough or overcharging the cells;
you could also be putting cells into a device which are
at different states of charge because the charge was only
based on time spent in the charger. To counter these problems
intelligent chargers work on each cell individually through
the individual charging
terminals. The state of charge of the individual cells is
monitored throughout the charge process; when the cell is
fully charged the charger will either stop the charge or
reduce the charge current to a trickle charge to avoid overcharging
of cells. This process works on the individual charge terminals
so the charger controls its charge current depending on
the state of charge of the cell in that terminal.
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