Learn about evaluating battery chargers by analyzing their critical current characteristics. Discover charge rate, voltage, safety measures, and more.
A battery charger is an important component for charging various electronic devices and maintaining their battery life. This article focuses on different critical current characteristics to be considered when evaluating a battery charger. They include; the charge rate, charging capacity, charging voltage, charging method, safety measures, input voltage range, power factor correction, battery type compatibility, user interface, size and weight, ripple and noise, and efficiency.
Current analysis
Current is the flow of electric charge through a conductor. It can either flow as direct current (DC) where the flow of charge is in a single direction or as alternating current (AC) where the flow changes periodically.
The SI unit of current is the ampere denoted as (A). The current is derived from the expression below;
Q=I×T
I=QT
Example
If a total charge of 2 coulombs flows through a conductor in 1 second, calculate its current.
I=QT
=21
=2
Current is also expressed with a relationship between voltage and resistance through Ohm’s law which states that current flowing through a conductor is directly proportional to the voltage applied across it and inversely proportional to its resistance.
I=VR
Where; I=current
V=voltage
R=resistance
Charge rate
It is where energy is transferred in or out of a battery. It determines the rate at which the battery can be charged or discharged and how long the energy can be stored or delivered. The rate of charge is determined by battery chemistry, temperature, charging method, and the state of charge of the battery.
The charge rate is calculated using the formula below:
Charge rate A=CapacityAh×C-rate
The capacity of a battery refers to the amount of energy stored in the battery and is expressed in ampere-hours (Ah)
C-rate describes the discharge or charge rate relative to the battery capacity.
Consider an example shown below:
If you have a battery of 100 Ah and you want to charge it at a C-rate of 0.2, calculate the charge rate.
Charge rate A=CapacityAh×C-rate
=100 Ah×0.2
=20 A
Charging voltage
Is the voltage applied to an energy storage device during the charging process. Its SI unit is volts (V) and varies depending on the charging method and the type of battery being used. When charging a battery, applying too much voltage causes the battery to overheat and leads to damage, while applying a low voltage while charging takes a long time to charge and reduces the battery life.
Charging voltage is calculated by the following formula:
Charging voltage=Number of cells×Voltage per cell+Charging overvoltage
Charging overvoltage refers to the extra voltage applied to the battery during the charging process to make it fully charged.
Consider an example shown below:
If you are charging a 12-volt lead acid battery with six cells and 2.4 voltage per cell. Calculate its charging voltage.
Charging voltage=Number of cells×Voltage per cell+Charging overvoltage
=(6×2.4)+2.4
=16.8V
Constant current charging
Is a process where the constant current is applied to a battery or a storage device at the initial stage of charging. Varieties of chargers are designed to supply various constant currents at the initial stages of charging. At this point, the current is controlled to ensure it does not go beyond the battery’s maximum charging rate to avoid damaging the battery.
When the battery has reached a certain point of charge, the charger switches to a constant voltage mode while reducing gradually across the battery reducing current.
Figure 2. Constant charge current curve. Image used courtesy of Dan Wanunda
Battery charger current characteristics
This is an important aspect to be considered for you to meet the requirements for your charging device.
The charging current: This is the required amount of electrical current the charger delivers to the energy storage device during the charging process.
The maximum charging current: This refers to the total amount of current the charger delivers to the battery when charging. It usually depends on the type and capacity of the battery being charged.
Trickle charging current: Is a charging process used to maintain the level of battery current when not in use. It is therefore a low-level current the charger delivers to the battery during the process.
Float charging current: This is a type of charging process used to maintain the charge level of a battery.
Charge termination current: This is a point where the charger stops charging the battery when it has reached its maximum charging point. The charger will stop automatically and no current will flow to the battery.
Charging profile: Is a pattern during the experienced charging process where current and voltage can either remain constant or have a combination of both.
Charger controller: This is a device used to control the charging current to the battery to ensure it is efficiently and safely charged. The charge controller has overcharge protection, over-discharge protection, and temperature compensation to control charging performance.
Factors affecting current characteristics of a battery charger
We have various factors which affect the current characteristics of a battery charger hindering them to perform as expected.
State of charge
Each battery requires a current according to its state of charge, for example a battery that is fully discharged requires higher charging current than one that has some energy.
Battery capacity
The capacity of the battery varies on how charging will take place. A higher-capacity battery requires high charging current to charge it within the required time. A lower-capacity battery will take minimal time to be fully charged because of its small capacity.
Charger design
Charger design varies on their level of efficiency. A charger that has a higher efficiency rating will require less current to charge the battery while a charger that has a lower efficiency rating will require a high current to charge the same battery.
Input power supply
During the charging process, a higher voltage applied will lead to a high charging current while a lower charging voltage will lead to a low charging current.
Charging algorithm
Smart chargers have a more sophisticated charging algorithm that adjusts the charging current according to the battery’s characteristics and the charging conditions specified.
Charging mode
This depends on the type of battery being charged which will determine its rating.
Temperature
Charging a battery in cold places needs a high current charger to charge the battery and achieve the same as in warm places.
Battery Chemistry
The charger current has to be adjusted accordingly to how a charging device is since different batteries have different battery chemistry making them vary from one another.
Current measuring methods
Electrical current is measured using various methods to identify the best rating to apply when using a charger. It is calculated using Ohm’s law I=V/R.
Clamp-on current meter
A device known as a current clamp is used to measure the magnetic field produced by the current when flowing through a conductor.
Figure 3. Clamp-on current meter working principle. Image used courtesy of Dan Wanunda
Residual current measurement method
It measures the difference between the outgoing and incoming currents in a circuit. It is majorly applied in fault ground and cannot measure normal current.
Current transformer method
This method is used in both AC and DC measurements because of high voltage input. The transformer can measure the field produced by the current since it produces a voltage proportional to the current and is used to calculate the current.
Rogowski coil method
A coil is placed around a magnetic field produced by the current. The coil produces voltage used to calculate the current flowing through the conductor. This method has a higher frequency and is more accurate when calibration has been done.
Hall effect method
A hall effect sensor is applied to measure the magnetic field produced by flowing current. It produces voltage equivalent to the current and is used to calculate the current. This process is sensitive to temperatures because of the sensitivity and requires calibration to perform effectively.
Shunt resistor method
A load is placed in series with the shunt resistor to measure current as voltage drops from one point to another in the resistor.
Take away points on evaluating current characteristics in battery charger
Charging current varies depending on the type of battery, temperature, the charging method used, and the capacity.
Charging using high current leads to faster charging but may increase the risk of overheating and damaging the battery.
Charging using low current leads to low charging speed and affects the battery life.
The rate at which the charging current occurs has been limited by the charger and does not exceed the maximum charging rate to prevent undercharging or overcharging which will damage the battery.
Frequent monitoring should be done on charging current and other parameters like temperature, voltage, and charging time and ensure efficient and safe charging.
To achieve the charging process and ensure the battery is fully charged and minimize risks, sophisticated charging algorithms and control circuits have to be used.
The charger should be compatible with the battery being charged whether by type or size.
Nice article