Battery Charging Method

INTRODUCTION

A battery is a device that converts chemical energy contained within its active materials directly into electric energy by means of an electrochemical redox reaction. This types of reaction involves the transfer of electrons from one materials to another via an electric circuit. An electric battery is a source of electric power consisting of one or more electrochemical cells with external connections for powering electrical devices. Electric batteries are rechargeable and can be charged by various methods as follows:

1. Constant Current Method

In this charging method the batteries are charged at a constant current. The charging current is set by introducing some resistance in the circuit. The current is limited to a value which keeps the battery voltage and temperature to low levels. Normally current at state of charging of (20%) and the charging is terminated when the state of charges comes to (80%).Generally there us a timer to switch off he charger to prevent excessive gassing and water loss and reduce positive  grid corrosion. This methods is suitable for Nickel-cadmium and Nickle-metal hydride (Ni-MH) types of batteries. The battery must be disconnected or a timer function used once charged.

Drawbacks

• This methods has its own drawbacks because state of charge of the battery in not taken into account.
• Initially the charging rate may be high but when battery I charged up to some extent the charging rate will be less.
• This method of is unsuitable for sealed or low maintenance flooded lead – acid batteries.
• Charger time is relatively long with the disadvantages that the battery may overheat if over-charged leading to premature battery replacement.

2. Constant Voltage Method

In this method a constant voltage is applied through battery. The charging current reduces with time until battery voltages arrives its default value. Constant voltage allows the full current of the charger to into the battery until the power supply reaches its pre-set voltage. The current then taper down to a minimum value once that voltage level reached. The lead acid cells for car and backup power system typically use constant voltage chargers. In addition Li-ions cells often use constant voltage system although these use more complex with added circuitry to protect battery and for the user safety.

Drawbacks

• The disadvantages of this method is the existence of huge increment current in the beginning     charging current which makes this techniques hazardous for charging batteries.
• This types of charging generally takes a long time due to the low current levels in the latter   charging periods.

3. Constant-current Constant-voltage (CC-CV) method

This strategy uses both CC and CV techniques as well. In the first stage the battery is charged using CC strategy until the voltage reaches its cut off voltage, then the charging techniques transferred to operate like constant voltage method until the battery reaches its full charge.

Used for charging Lithium and some other batteries which may be vulnerable to damage if the upper voltage limit is exceeded. Constant current charging rate is the maximum charging rate which the battery can tolerate without damaging the battery. The charging mode switches to constant voltage before the cell voltage reaches its upper limit.  The charger limit the amount of current to a pre-set level until the battery reaches a pre-set voltage level. The current reduces as the battery become fully charged. Normally battery is charged up to 80% state of charge (SOC) by CC mode and above 80 % SOC battery is charged by CV mode.

Advantages

• It doesn’t push too much current through the battery and doesn’t put too much voltage across the terminals.
• It can charge the battery at a steady rate quickly to 80% SOC , safe power force upon a battery and robustness.

Disadvantages

• It has a limitation to quickly charge the battery at end of charge, multiple output and input devices and increasing power on the battery during constant current phase.

Fig: CC-CV strategy waveform 

4. Two-Step Current Charging Method

This strategy has two modes. In this mode, a constant current is pushed through the battery until its terminal voltage reaches the bulk voltage. In the second mode, a pulsating current is used to maintain the battery voltage to its floating value. Charging starts at SOC (20%) and the charging is over when the SOC of battery comes to (80%). It is seen that the battery voltage increment with the charging time of the rated battery voltage, at that point pulse current are applied to skip battery over charging.

Fig: Two-step Current Charging Method

5. Pulse charging strategy

This strategy has two modes. The first mode inject high current into the battery. While in the second mode, no current is injected to the battery, which is called the rest mode. This mode is adopted to improve the performance of the battery electrolyte. The charging process started with an average charging SOC of 20%  and charging is terminated at 80 %.

Pulsed chargers feed the current to the battery in pulses. The charging rate can be precisely controlled by varying the width of the pulses, typically about one second. During the charging process, short rest period of 20 to 30 milliseconds, between the pulses allow the chemical action in the battery to stabilise  by equalising the reaction throughout the bulk of the electrode before recommencing the charge. This enable the chemicals reaction to keep pace with the rate of inputting the electrical energy. This method can reduce unwanted chemical reaction at the electrode surface such as gas formation, crystal growth and passivation. Pulse charging of Li-ion battery is proven empirically and experimentally as a promising charging technology, which offers fast and efficient charge performance. The pulse charge have been claimed to be a fast and efficient charging algorithm for lithium-ion batteries.

Advantages

• longer battery life span
  
• maximal battery material utilization
•  low heating, and low degradation of battery material

Fig: Pulse Charging waveform

6. Multistage current charging

Multi-stage current charging has different levels at which the charging system may charge the battery. Multistage current charging (MSCC) algorithm was developed to charge the battery, where various methods were proposed to determine the optimal charging currents in each charging stage of the battery. When the battery is charged at a single level for an extended period of time and reaches max voltage, the next lower stage of current is applied until the battery achieves max voltage again; and continues to do this until the lowest current stage is achieved. This method of charging the battery is technically safer than the pulse charging in the sense that it won’t push the voltage over the voltage limit and instead steps to a new current value.

Advantages

•  It is simple to implement
•  Charges the battery safely
• Operate on discrete charging system

Disadvantages

• This charging technique does not charge the battery quickly to maximum voltage
• Lacks the ability to maintain the battery at maximum voltage during the end charging phase

Fig: Multistage Constant Current waveform

7. Burp charging

Burp charging is also called reflex 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 depolarises the cell. The pulses dislodge any gas bubble which have built up on the electrodes during fast charging, speeding up the stabilization process and hence the overall charging process. The release and diffusion of the gas bubble is known as burping. Controversial claim have been made for the improvements in both the charge rate and the battery life time as well as for the removal of dendrites made possible by this technique. The least that can be said is that it doesn’t damage the battery.

Fig: Burp Charging waveform

8. IUI charging

IUI charging is a new method of charging which allow fast charging of standard lead-acid batteries. IUI charging is specific to only specific lead-acid batteries. The charging method works by charging the battery at a constant rate until the cell voltage achieves a configured value. This is usually a voltage at which gassing takes place.

Initially the battery is charged at a constant (I) rate until the cell voltage reaches a pre-set value-normally a voltage near to that at which gassing occur. The first part of the charging cycle is known as the bulk charge phase. When the pre-set voltage has been reached, the charger switches into the constant voltage (U) phase and the current drawn by the battery will gradually drop until it reaches another pre-set level. This second part of the cycle completes the normal charging of the battery slowly diminish rate. Finally the charger switches again into the constant current mode (I) and the voltage continue to rise up to a new higher pre-set limit when the charger is switched off. This last phase is used to equalise the charge on the individual cells in the battery to maximize battery life.

Fig: IUI charging waveform

 9. Trickle charge

Trickle battery charging accommodates the self- discharge of the battery. Trickle charge is a continue charging method. It provides a long-term current which is constant for standby use. There is a variation of the charge rate which is according to the frequency of the discharge.

This method is suitable only to the specific types of batteries and can cause damage due to overcharging to lithium-ion batteries and NiMH batteries. In some scenarios, the charger may changes over to trickle charging when the battery is fully charged. Trickle charging will not keep a battery charged if current is being drawn by a load.

    

Fig: Trickle charging profile 

10. Float charging

Float charging is most commonly used for backup and emergency power application where the discharge of the battery is infrequent. During the float charging the charger, battery, and load are connected in parallel. The charger operates off the normal power supply which provides current to the load during operation.

 Fig: Float charging Profile