Summary： In part two of this article, our focus is on methods of charge termination, and methods of charge control when charging batteries.
.
Charging a battery is not a big challenge in the area of power electronics. Ensuring that batteries are not damaged and have an elongated lifespan is the most challenging thing. Also, reducing the charging period for the battery is another challenge. Therefore, battery chargers are designed to solve the issue of safety and time. This can only be achieved through current charge termination, the introduction of quick charging systems, and using the right charging system for the right battery cell as per the manufacturer’s specifications. This article is a great informer when it comes to battery charge control, charger types, and sources of charging power. Let us dive deeper into the content of this article.
Methods of Charge Termination
We are going to summarize the charge termination ways for different types of batteries in a simple table below.
Table 1: Methods of Charge Termination
From the table,
TCO stands for Temperature Cut Off
I min stands for minimum current
Delta TCO stands for temperature rise above ambient.
There exist so many charging and charge termination schemes and datasheets for many different types of cells that work depending on cell application and chemistry. Here we shall be summarizing the most common ones.
Controlled Charging
Controlled charging can be classified into regular charge and Fast charge methods.
This method can be classified into two methods namely the semi-constant current method and the timer-controlled method.
The method is very simple and cheap (economical), most popular, utilizes very low current, low heat generation, has a charge rate of 0.1C, and a charge time range of between 5 and 15 hours. It is suitable for Nicad’s battery charging and termination.
Timer-controlled Charge Control Method
The system is also simple and very economical with higher reliability than the semi-constant current method. It makes use of the timer made of IC. The rate of charging is 0.2C for a specified period then shifts to a trickle charge of about 0.05C charge rate. The system works suitably when there is an introduction of temperature cutoff methods. Once the process of charging starts, we should allow the process to run to completion without interfering with it. The charging control method is recommended for NiMH and Nicad types of batteries.
Fast Charge Control Methods
The fast charge control method involves all the charging control methods where charging occurs within a period that ranges between 1 and 2 hours. Let us have a look at such methods below.
NDV cut-off Charge System
NDV stands for Negative Delta Voltage and this has been proven to be the fastest method for charging Nicads rapidly.
The graph below is a summary of what happens within such cells during the charging process.
Figure 1: NiMH and NiCad Charging Characteristics
In this method, a current rating of between 0.5 to 1.0C is used to constantly charge the battery. This will progressively raise the battery voltage as the charging goes on to reach the peak when completely charged then it falls subsequently. The -delta V voltage drop is caused by oxygen building up or polarization within the cell which is an effect that comes in when the battery cell is charged fully. The method is suitable for SLA batteries.
Voltage-Controlled Charging System
This involves fast charging where it charges batteries at a rate ranging between 0.5 to 1.0C. the charger goes off immediately after the battery reaches the predetermined set voltage. The system works better when combined with temperature sensors to protect the battery against overheating.
dT/dt Charge System
Some batteries such as the NiMN do not demonstrate NDV voltage drop characteristics when the charging gets to the end therefore, the NDV method cannot be used to end the charging of such batteries. For this type of battery the charger senses cell temperature increase rate per charging time and anytime the predetermined rate is attained, the charging stops, and the tickle charge method is introduced. The method is way too expensive but has the advantage of increasing the battery life. It is recommended that a timer is introduced during the charging of the battery to protect it against the damaging of the trickle charging associated with trickle charging.
Constant Current Constant Voltage Controlled charging system
This is best suited for lithium and any other types of batteries which are prone to damage any time the upper limit charging is exceeded. The specified constant current charging rate is the manufacturer charging rate that a given battery can be allowed into the battery without damaging it. It is therefore advisable to switch to constant voltage charging anytime the cell approaches full charge. Therefore, charging for lithium-ion type of batteries must have the ability to control both the battery voltage and the charging current.
The graph below shows the lithium-ion battery charge characteristics.
Figure 2: Lithium-Ion Battery Charging Characteristics
To maintain a particular constant current charging rate, the charging voltage together with the cell voltage should increase in unison to help overcome the effects of back EMF during the charging process. As the charge approaches completion, the current reduces to the trickle charge. When a minimum current point that is predetermined is met, the cut-off will occur.
V-Taper Controlled Charge System
It has a similar functioning as the voltage-controlled charging system. On reaching a predetermined voltage level, the rapid charge current reduces systematically through the reduction of the supply voltage before switching to the method of trickle charge. Used in SLA batteries.
Intelligent Charging System
This is where the charger is integrated with an intelligent electronic system to allow a much better charging process. They are faster and very safe and ensure batteries have a better life cycle.
