Automatic Battery Charging Circuit

In this blog, we are going to learn about the battery charging circuits and how they work, before everything let’s look at a brief explanation about the battery chargers.

What is a battery charging circuit?

What is a battery charging circuit

A battery charger circuit is a device used to put energy into a secondary cell or rechargeable battery by forcing an electric current through it. The charging protocol is determined by the size and type of the charged battery. Some battery types can be recharged by connecting to a constant voltage or constant current source; simple chargers of this type require manual disconnection at the end of the charge cycle or may have a timer to cut off the charging current at a set time. Other battery types are unable to withstand prolonged high-rate overcharging; the charger may include temperature or voltage sensing circuits as well as a microprocessor controller to adjust the charging current and cut off at the end of the charge cycle. A trickle charger provides a relatively small amount of current, only enough to counteract the self-discharge of a battery that is idle for a long time. Slow battery chargers can take several hours to charge; high-rate chargers can restore most capacity in minutes or less than an hour, but they usually require battery monitoring to avoid overcharging. For public use, electric vehicles require high-rate chargers; the installation of such chargers, as well as distribution support for them, is a concern in the proposed adoption of electric vehicles.

What are the 3 stages of battery charging?

Three-stage battery chargers are commonly referred to as smart chargers. They are high-quality chargers and are popular for charging lithium batteries. Ideally, however, all battery types should be charged with three-stage chargers, this three-stage charging process keeps the battery healthy.

Before getting into three-stage battery charger circuits, we must understand more about multi-stage battery chargers and why they are used.

Lithium batteries have 3 stages of charging, usually divided into these three stages:

  1. Constant Current Pre-charge Mode
  2. Constant Current Regulation Mode
  3. Constant Voltage Regulation Mode

Sounds similar to a Lead-acid battery? Something different. That’s why we need to buy a new charger for lithium batteries. Moreover, what exactly is “quick charge” and how can it make your battery be charged faster?

What are the 3 stages of battery charging?

The battery charging circuit process consists of 3 stages. There are explained in detail below.

1. Constant Current (CC) Charging

CC charging is a simple method that uses a small constant current to charge the battery during the whole charging process. CC charging stops when a predefined value is reached. This method is widely used for charging NiCd or NiMH batteries, as well as Li-ion batteries. The charging current rate is the most important factor, and it can significantly influence the battery’s behavior. For this reason, the main challenge of CC charging is setting a suitable charging current value that will satisfy both charging time and capacity utilization. A high charging current provides a quick charge but also significantly affects the battery’s aging process. A low charging current provides high capacity utilization but also produces a very slow charge, which is inconvenient for EV applications.

2. Constant-Voltage (CV) Charging

Another method is CV charging, which regulates a predefined constant voltage to charge batteries. Its main advantage is that it circumvents overvoltages and irreversible side reactions, thus prolonging battery life. Since the voltage is constant, the charging current decreases as the battery charges. A high current value is required to provide a constant terminal voltage at an early stage of the charging process. A high charging current from 15 percent to 80 percent SOC provides fast charging, but the high current stresses the battery and can cause battery lattice collapse and pole breaking. The most difficult aspect of CV charging is determining an appropriate voltage value that balances charging speed, electrolyte decomposition, and capacity utilization. In general, the CV charging method is effective for quick charging, but it depletes the capacity of the battery. The negative effect is caused by an increased charging current at a low battery SOC (at the beginning of the charging process), where the current value is significantly higher than the nominal battery current. The high battery current causes the battery lattice frame to collapse and contributes to the pulverization of the active battery pole substance.

3. Constant-Current-Constant-Voltage (CC-CV) Charging

A CC-CV charging method is a hybrid approach that combines the two previously mentioned charging methods. It uses CC charging in the first charging stage, and when the voltage reaches the maximum safe threshold value, the charging process shifts to the CV charging method. The charging process is complete when the current levels off or when full battery capacity is reached. The charging time is mainly defined by the constant current value (CC
mode), while the capacity utilization is predominantly influenced by the constant voltage value (CV mode).

Fundamentals Charging Parameters

Li-ion batteries follow a relatively common charging profile, described in greater detail below. Note that if a charger IC provides configurability, the designer may be able to set their thresholds for these phases. These configurable thresholds are highly beneficial, considering that most battery manufacturers specify certain thresholds for different maximum charge current levels. Configurability can provide an added layer of safety by protecting the battery from over-voltage and over-temperature conditions and overloads, which could permanently damage the battery or degrade its capacity.

Fundamentals charging parameters
1. Trickle charge

Generally, the trickle charge phase is only used when the battery voltage is below a very low level (about 2.1V). In this state, the battery pack’s internal protection IC may have previously disconnected the battery due to it being deeply discharged, or if an over-current event occurred. The charger IC sources a small current (typically 50mA) to charge the capacitance of the battery pack, which triggers the protection IC to reconnect the battery by closing its FETs. Although trickle charging usually lasts for a matter of seconds, the charger IC should integrate a timer that stops charging if the battery pack is not reconnected within a certain amount of time, as this indicates that the battery has been damaged.

2. Pre-charge

Once the battery pack has been re-connected or is in a discharged state, pre-charging begins. During pre-charge, the charger starts to safely charge the depleted battery with a low current level that is typically C / 10 (where C is the capacity (in mAh)). As a result of pre-charge, the battery voltage slowly rises. The purpose of pre-charge is to safely charge the battery at a low current. This prevents damage to the cell until its voltage reaches a higher level.

3. Constant current (CC) charge

Constant current (CC) charge is also considered fast charging, which is described in greater detail below. CC charging starts after
pre-charge, once the battery has reached about 3V per cell. In the CC charge phase, it is safe for the battery to handle higher charge currents between 0.5C and 3C. CC charging continues until the battery voltage has reached the “full” or floating voltage level, at which point, the constant voltage phase begins.

4. Constant voltage (CV) charge

The constant voltage (CV) threshold for Lithium cells is usually between 4.1V and 4.5V per cell. The charger IC monitors the battery voltage during CC charging. Once the battery reaches the CV threshold, the charger

transitions from CC to CV regulation. CV charging is implemented because the external battery pack voltage seen by the charger IC exceeds the actual battery cell voltage in the pack. This is due to the internal cell resistance, PCB resistance, and the equivalent series resistance (ESR) from the protection of FET and cell. To guarantee safe operation, the charger IC must not allow the battery voltage to exceed its maximum floating voltage.

5. Charge termination

The charger IC determines when to terminate the charge cycle based on the current going into the battery dropping below a set threshold (about C / 10) during the CV phase. At this point, the battery is considered fully charged and charging is completed. If charge termination is disabled in the charger IC, the charge current will naturally decay to 0mA, but this is rarely done in practice. This is because the amount of charge going into the battery exponentially decreases during CV charging (since the cell voltage is increasing like a large capacitor), and it would take a significantly longer time to recharge the battery with a very little increase in capacity.

Why are CC and CV important?

CC-CV charging was initially used to charge lead-acid batteries and, later, to charge Li-ion batteries. Li-ion batteries require a much longer CC mode. The CC-CV charging method is more efficient than either the CC or CV methods individually, and as such, it is used as the reference for comparison with the latest charging methods.

The main challenge with CC-CV charging is defining suitable constant values for each model. The suitable current value will provide a balance between charging performance and battery safety. Having a current that is too high or too low can cause negative effects as previously discussed.

Auto-cut off circuits

Auto Cut-off – It continuously detects battery charging voltage and once the battery reaches the full charge level, cuts off the charging voltage. We will see the XH-M602 Automatic battery charging cut-off circuit.

this image shows Auto-cut off circuits
The XH-M602 automatic cut-off battery charging circuit works by measuring the voltage on the battery terminal and by breaking the circuits. The XH-M602 module has a transformer and a relay, which are used to break the circuit. In this circuit, we can set the voltage by using the buttons on the board for breaking the charger when the voltage reaches the set value.

Components and Supplies

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