A Complete Guide To Battery Charging Fundamentals and Types

Even as new battery chemistries are being invented to power electric cars and store energy from renewable sources, the charging fundamentals remain the same, which are constant current, constant voltage, and auto cut-off. 

All three of the most commonly used charger types implement at least one of these fundamentals to push current into the battery cells and raise their voltage to the designed full level.

To understand this whole charging process, we’ll look at these three fundamentals in detail, as well as the three common charger types to see the benefits of each.

Battery Charging Parameter Fundamentals

All battery charging circuits implement at least one of these fundamentals.

Constant Current (CC)

As the name suggests, constant current charging involves charging the battery using a fixed current, which is maintained by varying the voltage. It is considered fast charging, which you might be familiar with when you plug your phone. CC runs until the battery reaches its predefined voltage value, after which CV kicks in.

Let’s consider a circuit that requires a 1A (1,000mA) current to recharge the battery, with 1.25V as the reference voltage from the LM317 voltage regulator.

A constant current electrical circuit

The circuit needs a resistor to regulate the current, and we can find its value using the formula R=V/I.

R=1.25V/1A, which is 1.25 ohms. But since there’s no 1.25-ohm resistor, we’ll use a 1.5-ohm resistor, which is the closest one.

If the power comes from a DC source, such as solar panels, you won’t need the step-down transformer, rectifier, or capacitors.

Constant Voltage (CV)

With a constant voltage circuit, the voltage output is kept constant to finalize the charging process, and this can be achieved using two resistors connected to the LM317 voltage regulator.

A constant voltage electrical circuit

Let’s say we want the output voltage to be 8.4V. The R1 value should be less than 1,000 ohms, such as 560 ohms.

Using the formula Vout = Vref (1+R2/R1), we can get the value of the second resistor.

8.4 = 1.25V (1+R2/560)

R2 = 3300 ohms (3.3 kilo ohms)

You can use any resistor combination that outputs around 8.4V in this setup.

Most battery charging systems combine CC and CV to form a hybrid system that runs CC first and then switches to CV when the voltage reaches the threshold value. CC defines the charging time, while CV influences the capacity utilization. The charging process is considered complete when the current levels off and full battery capacity is attained. 

Auto Cut-Off

Most batteries need auto-cut-off circuits to protect against overcharging, which can damage the cells over time.

An auto-cut-off electrical circuit for battery charging

In this circuit, you can adjust the power flowing into the voltage regulator’s ADJ pin using the variable resistor, which then varies the output voltage.

Once the battery is fully charged, the Zener diode produces a reverse voltage that flows into the BD139 transistor’s base. This turns on the switch (makes it conductive), which connects the ADJ pin to the ground, cuts off the voltage output from the regulator, turns on the red LED, and turns off the green LED.

Battery Charging Circuit Types

These three charging circuit types can implement either of the charging parameter fundamentals discussed above.

Linear Charger

Linear battery chargers are the simplest of the three types because they require fewer components, the key one being a linear regulator, such as the LM317. They are also cheap to make, take up little space, and generate little noise (radiated and conducted emissions) because they don’t implement switching and filtering functions.

However, this charger is inefficient, especially in high-power applications, because it dissipates a lot of power, which is released in the form of heat. The circuit must drop the AC voltage down to the battery voltage, which leads to high losses.

Reducing the charging current can lower these losses, but will increase the charging time. You can choose between the heat or extra charging time depending on the application.

The MAX1898 IC is more efficient than the LM317 at this linear charging task because it features a pre-qualification state that lowers the charging current for any battery voltage that is less than 2.5V.

A linear charging circuit using the MAX1898 IC

Therefore, the highest losses when using the circuit above occur when the load (battery) is above 2.5V and the input voltage is at the highest level.

Switch-Mode Charger

Switch-mode chargers are more efficient than their linear counterparts in high-power applications because their power dissipation factor is lower. Typically, this factor remains lower than 1W across the entire battery voltage range when charging, so it performs better across a broad range of input voltages.

On top of that, switch-mode circuits can be scaled easily to charge as many as four series cells concurrently at up to 4A.

However, this efficiency comes at a cost. Switch-mode charger circuits are larger and more complex than the linear types because they need multiple components.

For instance, when using the MAX1737 controller, you need an external buck converter, which comprises the following.

• Passive low-frequency bandpass LC filter

• Switch (usually MOSFET)

• Diode

These components require more PCB space and make the charger costlier.

A switch-mode charging circuit using the MAX1737 IC (note the external buck converter)

The switching action also generates a lot of EMI, while the filter’s inductor produces radiation noise. However, the MAX1737 has a 300kHz PWM oscillator to reduce this noise. The IC also features a safety timer to prevent overcharging and continuous battery monitoring (temperature and voltage) for safe charging.

Pulse Charger

Pulse chargers combine the benefits of switch-mode and linear chargers. They operate linearly when the battery voltage is low because their pass transistor remains on and conducts input current directly to charge the cells.

The charger also operates as efficiently as a switch-mode charger circuit because it pulses the input current as the battery voltage reaches the regulation point. This pulsing enables it to hit the desired charging current, which regulates the voltage at the desired limit. The non-linear operation at this region reduces the power losses, leading to lower heat generation.

Since the circuit doesn’t need an LC filter, it is less complex than switch-mode circuits and rivals the simplicity of the linear type.

A pulse charging circuit using the MAX1736

However, this circuit has some special considerations, such as requiring a reasonably accurate current-limited voltage source as the power input. This level of current limiting for pulse charging chips like the MAX1736 is expensive and not universally available.

Summary of the Different Charger Types

Future Trends in Battery Charging Technology

• Soft Switching: Future switch-mode chargers will switch to soft switching to reduce EMI and energy losses, resulting in efficiency and performance enhancement.

• Switching To Active AC-DC Rectification: Diodes bring forward voltage losses in the circuit, which can be reduced by replacing them with active semiconductor switches or bridgeless topologies.

• Leveraging SiC and GaN To Accelerate Switching: Silicon Carbide and Gallium Nitride provide faster switching and lower losses, which can enable the shrinking of heatsinks and passive magnetic components, resulting in smaller chargers.

• Implementing Advanced Cooling Mechanisms: Battery chargers will always emit losses in the form of heat no matter their efficiency. Advanced passive, liquid-based, or fan-driven cooling systems will help to evacuate this heat faster so that the components can function more efficiently and last longer.

• Leveraging Integrated Systems To Simplify Designs: Integrated systems are more compact, cheaper, and less complex than building circuits with discrete components. Future chargers might have buck converters and other parts built into the chip to achieve these benefits.

• Using Denser Circuit Designs To Reduce Space: Real estate on the PCB is a precious commodity. Also, customers want more compact electronics, including chargers. Therefore, future charging technologies will have high-density, multilayer designs that bring forth more efficiency in a compact package.

Conclusion

As you can see, linear, switch-mode, and pulse chargers implement the fundamentals above, albeit in different ways. However, the end result is the same.

You can use different battery charging ICs for these tasks or even buy complete chargers that are ready to use for different functions, such as solar charging, or to handle specific battery chemistries, such as lithium-ion. The latter can help you reduce the time to market for your product, but either option can work.