Consumer Charger Features

The world of chargers can be a confusing place; with so many different chargers to choose from it can be very difficult to make the right decision on a charger that will suit both your charging needs and your pocket. As with everything in life, you get what you pay for; as quality and functionality increases, so does the price, therefore it is important you choose a charger which suits your needs and ensure you are not paying for features you do not need. This section outlines the key features that need to be taken into account when deciding on the right charger.

The final input voltage that may be available is to run from a 12V DC input; if a charger has this input voltage then it can be run from a cigarette lighter socket in your car or any device which can deliver a constand 12V DC supply.

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. These various features are outlined in the following section.

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.

Charging Method (-delta V detection)
Intelligent chargers often offer a number of different methods for detecting the end of charge of a cell, with consumer NiCd and NiMh chargers, minus delta V or negative delta V is probably the most commonly used and considered one of the best methods.

The charger deliveres a current to the cell during charging, this increases the voltage inside the cell bringing it back to its fully charged state. During this process, the voltage of the cell will reach a peak, at this point, if current is still applied to the cell the voltage will begin to fall. This voltage drop is due to polarisation, oxegen builds up inside the cell causing a rapid rise in temperature and potential damage. The reason for this rise in temperature is because the cell cannot be charged further the electrical energy from the charge current is converted directly to heat as opposed to chemical reactions which occur during charging.

This is where - delta V comes in, charges with this control detect this drop in voltage from the peak and subsiquently halt the charge process or reduce the charge current to a low trickle charge. The reason behind the name is that the drop in voltage refers to the '-' negative; delta indicates a small change; V indicates the voltage hence, - delta V. The sensitivity of this control depends on the charge current used, generally the higher the charge current (i.e. the faster the charge) the more sensitive the detection is.

Temperature control
During the charging process cells will heat up due to the chemical reactions going on inside the cell, this is normal and does not necessarily mean the cell is being overcharged, however if the temperature of the cell becomes too hot then damage could be caused. 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. This is particularly common in fast chargers.

Safety Timer
A charger which has a safetly timer means just that, the circuit inside the charger will contail a timer which halts the charge process after a given time to give some protection against overcharging. This timer may be basic and be a default time regardless of the type of cell inserted or it may vary detecting the type of cell inserted and adjusted accordingly.

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. When this trickle charge current begins depends on the charger, it may be after a given time or it may be after the end of charge voltage has been reached.

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 up to 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. Certain 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, chargers which need multiple cells to begin charging 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; if the cells you put into the charger are at different states of charge it would be possible that whilst one cell is charged fine, another may be overcharged and possibly damaged.

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.

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. Certain 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.

Refresh Function
This kind of function is generally found on high end chargers which focus on battery management and maintanence of rechargeable batteries.

A cell may need to be refreshed for a number of reasons; extended storage, extended discharge or careless cycling. 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. 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.

Discharge Function
Much has been written about the so called 'Memory Effect' suffered by NiCd and NiMh batteries if you consistently discharge and recharge to an intermediate level. The amount of effect this has is really somewhat of an unknown quantity but it is known that occasionally putting the battery through a full dishcarge recharge cycle is benificial for the battery. Some chargers offer a feature which fully discharges the battery before proceeding to charge, this may be a manual process or automatic depending on the charger.

Automatic Cell Diagnosis
This is an advanced feature found on some of the high end chargers which provide oprimum conditioning of the rechargeable cells. With Automatic Cell Diagnosis, the charger will analyse the state of charge and condition of the inserted cell, from this it will then automaticaly select the appropriate action of charging, discharging, or refreshing to get the most out of the battery.