If you are still unsure on what all the different features of a charger actually mean, we have compiled a detailed list explaining each feature in more detail to help you choose the right battery charger.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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-delta V 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 -delta V. This technique can only be used in fast charging of NiCd and NiMH cells.
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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.
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What if the charge takes 4 cells and you only want to charge 1 or 2 cells?
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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.
Please have a look at the other sections of our Choosing Battery Charger Guide which are as follows:
