“WORLDWIDE, BATTERIES ARE DAMAGED MORE COMMONLY BY BAD CHARGING TECHNIQUES THAN ALL OTHER CAUSES COMBINED TOGETHER”
Why Charge ???
The charger has three key functions
- Getting the charge into the battery (Charging)
- Optimising the charging rate (Stabilising)
- Knowing when to stop (Terminating)
The charging scheme is a combination of the charging and termination methods.
Once a battery is fully charged, the charging current has to be dissipated somehow. The result is the generation of heat and gasses both of which are bad for batteries. The essence of good charging is to be able to detect when the reconstitution of the active chemicals is complete and to stop the charging process before any damage is done while at all times maintaining the cell temperature within its safe limits.
If for any reason there is a risk of over charging the battery, this will result in a rise in temperature. Internal fault conditions within the battery or high ambient temperatures can also take a battery beyond its safe
operating temperature limits. Elevated temperatures hasten the death of batteries and monitoring the cell temperature is a good way of detecting signs of trouble from a variety of causes. The temperature signal, or a resettable fuse, can be used to turn off or disconnect the charger when danger signs appear to avoid damaging the battery. This simple additional safety precaution is particularly important for high power batteries where the consequences of failure can be both serious and expensive.
Basic Charging Methods
- Constant Voltage A constant voltage charger is basically a DC power supply which in its simplest form may consist of a step down transformer from the mains with a rectifier to provide the DC voltage to charge the battery as in car battery.
- Constant Current Constant current chargers vary the voltage they apply to the battery to maintain a constant current flow, switching off when the voltage reaches the level of a full charge. This design is usually used for nickel-cadmium and nickel-metal hydride cells or batteries.
- Taper Current This is charging from a crude unregulated constant voltage source. The current diminishes as the cell voltage (back emf) builds up. There is a serious danger of damaging the cells through overcharging.
- Pulsed charge Pulsed chargers feed the charge current to the battery in pulses. The charging rate (based on the average current) can be precisely controlled by varying the width of the pulses, typically about one second. During the charging process, short rest periods of 20 to 30 milliseconds, between pulses allow the chemical actions in the battery to stabilise by equalising the reaction throughout the bulk of the electrode before recommencing the charge. This enables the chemical reaction to keep pace with the rate of inputting the electrical energy. It is also claimed that this method can reduce unwanted chemical reactions at the electrode surface such as gas formation, crystal growth and passivation.
The optimum current profile depends on the cell chemistry and construction.
- Burp charging or Negative Pulse Charging: Used in conjunction with pulse charging, it applies a very short discharge pulse, typically 2 to 3 times the charging current for 5 milliseconds, during the charging rest period to depolarise the cell. These pulses dislodge any gas bubbles which have built up on the electrodes during fast charging, speeding up the stabilisation process and hence the overall charging process. The release and diffusion of the gas bubbles is known as “burping”.
- Trickle Charge: Trickle charging is designed to compensate for the self discharge of the battery. Continuous charge. Long term constant current charging for standby use. The charge rate varies according to the frequency of discharge. Not suitable for some battery chemistries, e.g. NiMH and Lithium, which are susceptible to damage from overcharging. In some applications the charger is designed to switch to trickle charging when the battery is fully charged.
- Float Charge: The battery and the load are permanently connected in parallel across the DC charging source and held at a constant voltage below the battery’s upper voltage limit. Used for emergency power back up systems. Mainly used with lead acid batteries.
- Random Charging: All of the above applications involve controlled charge of the battery, however there are many applications where the energy to charge the battery is only available, or is delivered, in some random, uncontrolled way. This applies to automotive applications where the energy depends on the engine speed which is continuously changing. More benign applications are in solar panel installations which can only be charged when the sun is shining. These all require special techniques to limit the charging current or voltage to levels which the battery can tolerate.
Batteries can be charged at different rates depending on the requirement. Typical rates are shown below:
- Slow Charge = Overnight or 14-16 hours charging at 0.1C rate
- Quick Charge = 3 to 6 Hours charging at 0.3C rate
- Fast Charge = Less than 1 hour charging at 1.0C rate
Slow charging can be carried out in relatively simple chargers and should not result in the battery overheating. When charging is complete batteries should be removed from the charger.
- Ni-Cads are generally the most robust type with respect to overcharging and can be left on trickle charge for very long periods since their recombination process tends to keep the voltage down to a safe level. The constant recombination keeps internal cell pressure high, so the seals gradually leak. It also keeps the cell temperature above ambient, and higher temperatures shorten life.
- Lead Acid batteries are slightly less robust but can tolerate a short duration trickle charge. Flooded batteries tend to use up their water, and LAs tend to die early from grid corrosion. Lead-acids should either be left sitting, or float-charged (held at a constant voltage well below the gassing point).
- NiMH cells on the other hand will be damaged by prolonged trickle charge.
- Lithium ion cells however cannot tolerate overcharging or overvoltage and the charge should be terminated immediately when the upper voltage limit is reached.
Fast / Quick Charging
As the charging rate increases, so do the dangers of overcharging or overheating the battery. Preventing the battery from overheating and terminating the charge when the battery reaches full charge become much more critical. Each cell chemistry has its own characteristic charging curve and battery chargers must be designed to detect the end of charge conditions for the specific chemistry involved. In addition, some form of Temperature Cut Off (TCO) or Thermal Fuse must be incorporated to prevent the battery from overheating during the charging process.
Fast charging and quick charging require more complex chargers. Since these chargers must be designed for specific cell chemistries, it is not normally possible to charge one cell type in a charger that was designed for another cell chemistry and damage is likely to occur. Universal chargers, able to charge all cell types, must have sensing devices to identify the cell type and apply the appropriate charging profile.
Note that for Automotive Batteries the charging time may be limited by the available power rather than the battery characteristics. Domestic 13 Amp ring main circuits can only deliver 3KW. Thus, assuming no efficiency loss in the charger, a ten hour charge will at maximum put 30 KWh of energy into the battery.
Charge Control Methods
Many different charging and termination schemes have been developed for different chemistries and different applications. The most common ones are summarised below.
Regular (slow) charge
- Semi constant current Simple and economical. Most popular. Low current therefore does not generate heat but is slow, 5 to 15 hours typical. Charge rate 0.1C. Suitable for Nicads
- Timer controlled charge system Simple and economical. More reliable than semi-constant current. Uses IC timer. Charges at 0.2C rate for a predetermined period followed by trickle charge of 0.05C. Suitable for Nicad and NiMH batteries.
Fast charge (1 to 2 hours)
- Negative delta V (NDV) Cut-off charge system
This is the most popular method for rapid charging for Nicads.
Batteries are charged at constant current of between 0.5 and 1.0 C rate. The battery voltage rises as charging progresses to a peak when fully charged then subsequently falls. This voltage drop, -delta V, is due to polarisation or oxygen build up inside the cell which starts to occur once the cell is fully charged. At this point the cell enters the overcharge danger zone and the temperature begins to rise rapidly since the chemical changes are complete and the excess electrical energy is converted into heat. The voltage drop occurs regardless of the discharge level or ambient temperature and it can therefore be detected and used to identify the peak and hence to cut off the charger when the battery has reached its full charge or switch to trickle charge.
This charging method is not suitable for LA batteries.
- dT/dt Charge system NiMH batteries do not demonstrate such a pronounced NDV voltage drop when they reach the end of the charging cycle as can be seen in the graph above and so the NDV cut off method is not reliable for ending the NiMH charge. Instead the charger senses the rate of increase of the cell temperature per unit time. When a predetermined rate is reached the rapid charge is stopped and the charge method is switched to trickle charge. This method is more expensive but avoids overcharge and gives longer life. Because extended trickle charging can damage a NiMH battery, the use of a timer to regulate the total charging time is recommended.
- Constant-current Constant-voltage (CC/CV) controlled charge system. Used for charging Lithium and some other batteries which may be vulnerable to damage if the upper voltage limit is exceeded. Special precautions are needed to maximise the charging rate and to ensure that the battery is fully charged while at the same time avoiding overcharging. For this reason it is recommended that the charging method switches to constant voltage before the cell voltage reaches its upper limit.
- Intelligent Charging System
Intelligent charging systems integrate the control systems within the charger with the electronics within the battery to allow much finer control over the charging process. The benefits are faster and safer charging and battery longer cycle life.
As a safety precaution with high capacity batteries a pre-charging stage is often used. The charging cycle is initiated with a low current. If there is no corresponding rise in the battery voltage it indicates that there is possibly a short circuit in the battery.Note
Most chargers provided with consumer electronics devices such as mobile phones and laptop computers simply provide a fixed voltage source. The required voltage and current profile for charging the battery is provided (or should be provided) from electronic circuits, either within the device itself or within the battery pack, rather than by the charger. This allows flexibility in the choice of chargers and also serves to protect the device from potential damage from the use of inappropriate chargers.
Chargers normally incorporate some form of voltage regulation to control the charging voltage applied to the battery. The choice of charger circuit technology is usually a price – performance trade off. Some examples follow:
- Switch Mode Regulator (Switcher) – Uses pulse width modulation to control the voltage.
- Series Regulator (Linear) – Less complex but more lossy – requiring a heat sink to dissipate the heat in the series, voltage dropping transistor which takes up the difference between the supply and the output voltage. Because there is no switching, it delivers pure DC and doesn’t need an output filter. It is suitable for low noise wireless and radio applications.With fewer components they are also smaller.
- Shunt Regulator – Shunt regulators are common in photovoltaic (PV) systems since they are relatively cheap to build and simple to design. The charging current is controlled by a switch or transistor connected in parallel with the photovoltaic panel and the storage battery. Overcharging of the battery is prevented by shorting (shunting) the PV output through the transistor when the voltage reaches a predetermined limit. If the battery voltage exceeds the PV supply voltage the shunt will also protect the PV panel from damage due to reverse voltage by discharging the battery through the shunt. Series regulators usually have better control and charge characteristics.
- Buck Regulator A switching regulator which incorporates a step down DC-DC converter. They have high efficiency and low heat losses. They can handle high output currents and generate less RF interference than a conventional switch mode regulator. A simple transformerless design with low switch stress and a small output filter.
- Pulsed Charger. Uses a series transistor which can also be switched. With low battery voltages the transistor remains on and conducts the source current directly to the battery. As the battery voltage approaches the desired regulation voltage the series transistor pulses the input current to maintain the desired voltage. Because it acts as a switch mode supply for part of the cycle it dissipates less heat and because it acts as a linear supply part of the time the output filters can be smaller. Pulse chargers usually need current limiting on the input source for safety reasons, adding to the cost.
- Universal Serial Bus (USB) Charger
The USB specification was developed by a group of computer and peripheral device manufacturers to replace a plethora of proprietary mechanical and electrical interconnection standards for transferring data between computers and external devices. It included a two wire data connection, a ground (earth) line and a 5 Volt power line provided by the host device (the computer) which was available to power the external devices. Power always flows from the host to the device, but data can flow in both directions. For this reason the USB host connector is mechanically different from the USB device connector and thus USB cables have different connectors at each end. This prevents any 5 Volt connection from an external USB source from being applied to the host computer and thus from possibly damaging the host machine.
Charger Power Sources
When specifying a charger it is also necessary to specify the source from which the charger derives its power, its availability and its voltage and power range. Efficiency losses in the charger should also be taken into account, particularly for high power chargers where the magnitude of the losses can be significant. Some examples are given below.
Easy to accommodate and manage.
- AC Mains
- Regulated DC Battery Supply
- Special Chargers
Many portable low power chargers for small electrical appliances such as computers and mobile phones are required to operate in international markets. They therefore have auto sensing of the mains voltage and in special cases the mains frequency with automatic switching to the appropriate input circuit.May be provided by special purpose installations such as mobile generating equipment for custom applications.Portable sources such as solar panels.
Opportunity charging is charging the battery whenever power is available or between partial discharges rather than waiting for the battery to be completely discharged. It is used with batteries in cycle service, and in applications when energy is available only intermittently. By avoiding complete discharge of the battery, cycle life can be increased.
Availability affects the battery specification as well as the charger.
Typical applications are:-
- Onboard vehicle chargers (Alternators, Regenerative braking)
- Inductive chargers (on vehicle route stopping points)
- Solar power
- Wind power
This is only applicable to specific cell chemistries. It is nor a charger technology in the normal sense of the word. Mechanical charging is used in some high power batteries such as Flow Batteries and Zinc Air batteries. Mechanical charging can be carried out in minutes. This is much quicker than the long charging time associated with the conventional reversible cell electrochemistry which could take several hours. Zinc air batteries have therefore been used to power electric buses to overcome the problem of excessive charging times.