How To Design a Charger Based on the Battery Chemistry

Battery charging seems simple from a general overview because you only have to push current from the power source into the cells. But this is easier said than done because of one thing: you have to charge the battery correctly, especially during fast charging, to avoid fires/explosions and to extend its service life.

Therefore, automatic battery charging circuits have to be optimized to handle specific battery chemistries. In this automatic battery charging circuit guide, we’ll analyze these chargers based on the three popular battery chemistries, which are lead acid, nickel, and lithium-based batteries. Let’s get right into it!

Lead Acid

These battery chemistries require a three-stage charging process christened IUoU to provide optimal performance and have a long lifespan.

1. I (constant current) for bulk charging: This phase charges the battery to about 80% full charge using a constant current of 10–25% of the battery’s Ah rating, depending on the battery’s manufacturer. Charging at more than 25% in this phase has a higher probability of damaging the battery.

2. Uo (constant voltage) for absorption charging: Also known as the equalization stage, absorption charging charges the remaining 20% using a constant voltage. The charger maintains the target charging voltage (14.1–14.8V DC) while decreasing the current,

3. U (constant voltage) for trickle charging: This floating stage reduces the charging current to about 1% of the battery’s Ah rating, while the voltage is maintained at around 13–14V DC. Lead-acid batteries can be kept on float mode indefinitely (with no auto cut-off) to increase the battery life because it eliminates self-discharging and the possibility of irreversible damage.

The IUoU lead-acid battery charging curve

If the current does not drop as expected in step 2, the charging system should shut down or switch to stage 3. This issue can occur due to sulfation.

It is possible to reverse sulfation if the lead-acid battery is not completely ruined using pulse charging. The high current pulses between the terminals can break down the sulfate crystals coating the plates.

But this process doesn’t use the pulse charger discussed above. It requires short, high-current pulses, so the desulfation circuit should have mechanisms for controlling variables like pulse width and frequency to match the required conditions. Some of the possible solutions include using:

• Pulse width modulation

• High-voltage pulse circuits

• Transformer and bridge rectifier circuits

• High amplitude pulse current (using 555 boost circuits)

Nickel Cadmium (NiCd) and Nickel Metal Hydride (NiMH)

NiMH batteries are gradually replacing their NiCd counterparts because they have a lower memory effect. However, both have similar charging requirements (a constant current source), and this charger can handle as many cells as you like, provided they are connected in series.

Lithium Polymer and Lithium Ion

Lithium polymer and lithium-ion batteries also have similar properties, and they require careful, controlled charging using these steps to avoid damage. 

The lithium-ion battery charging curve

1. Trickle Charging

Battery management systems only implement this step if the battery voltage is below the lowest threshold (usually 2.1V). When a lithium-ion or polymer battery hits these lows, its internal protection IC disconnects the battery by opening its FETs.

In such a case, the charger’s chip charges the battery pack’s capacitors using a small current (around 50mA), which triggers the protection IC in the battery pack to close its FETs and reconnect the battery.

The process lasts a few seconds and the charger’s IC should have a timer to stop charging if the battery doesn’t come back online. This usually indicates the battery is damaged.

2. Pre-Charging

Pre-charging kicks in once the battery reconnects, pushing 10% of the battery’s capacity in mAh as current into the pack. This gradually raises the voltage level until it gets to the threshold for CC charging.

3. Constant Current

CC or fast charging begins when the voltage per cell reaches about 3V. At this level, the battery can handle higher currents of about 50–300% of its capacity.

4. Constant Voltage

The threshold for CV in lithium batteries is 4.1–4.5V per cell, and the charging IC implements CV charging because the external battery voltage exceeds the internal battery voltage. This occurrence is due to issues like PCB, internal cell, and equivalent series resistance from the FET to the battery cells.

To ensure safe operation, the IC doesn’t allow the battery voltage to exceed the maximum floating voltage, making CV activation a safety measure.

5. Charge Termination

Once the current flowing into the battery drops below the set threshold in the CV charging phase, the IC terminates the charging cycle, and the battery is considered fully charged. This threshold could be 10% of the battery’s capacity in mAh.

Lithium Ion Battery Charging Topologies

There are several alternative topologies for charging lithium-ion batteries, but the most popular ones are NVDC and HPB.

Narrow Voltage DC Charging

NVDC chargers enhance the system’s efficiency by reducing its voltage range. They achieve this feat by replacing the regular battery charger with a system charger, which incorporates a buck converter. This setup optimizes DC-DC conversion, eliminates power path switches, and reduces power dissipation, space consumption on the PCB, and overall costs.

The charger operates as a buck converter both when charging the battery and when the battery is supplementing the AC adapter to power the device. This second option is handy when the system’s load exceeds the adapter’s rating.

The lithium-ion battery charging curve using NVDC

However, it has its disadvantages, which include:

• High bus currents during low system voltages. These increase conduction losses in the board’s traces.

• Cost, size, and power dissipation can be higher due to the high current-rated inductors and FETs used in the circuit.

Hybrid Power Boost Charging

This topology also serves the system with that extra juice from the battery when the load exceeds the adapter’s rating. However, it implements this differently by reverse-boosting the battery’s power.

The buck converter operates normally in this charger while the adapter brings in the external power. When this is insufficient, the converter operates in reverse, enabling the battery to assist the adapter.

This setup has one primary disadvantage, which is a lower light load efficiency.

How To Pick a Charger Depending on the Battery Capacity and CC Rating

The charging current is critical when picking a battery charger because it determines the charging time, which should be as short as possible.

For instance, when charging a 50 Ah lead acid battery, the bulk charging phase should supply 10% of this capacity to the load, which is 5A. However, circuits have losses, so you should consider something like 8A. Using this charging current to full charge reduces the charging time to:

50Ah/8A = 6.25 hours (without accounting for losses)

If using a 5A charger, this time will increase to 10 hours.

But if the battery’s specifications indicate it can handle 25% of its capacity during bulk charging, you should pick a 12.5A charger, or 15A to account for losses. This will reduce the charging time to 3–4 hours.

Safety Precautions for Batteries and Their Charging Circuits

• Thermal Management: All battery charging circuits experience some level of losses, which manifest as heat. Therefore, they should have sufficient thermal management mechanisms to keep the board and components cool.

• Overcharge Protection: Save for lead acid batteries, the other types must have overcharge protection circuits to protect the battery chemistry from experiencing permanent damage or even exposing users to safety hazards like fires and explosions.

• Over-Discharge Protection: Over-discharge protection is equally critical because it protects against fully discharging the cells, which can damage them permanently by decomposing the electrolyte, destroying the reversibility of the positive and negative materials, etc.

• Handling Procedures: Each battery has specific handling procedures laid out by the manufacturer, such as the current and voltage specifications for charging. When designing the battery management system, keep these factors in mind.

Emerging Battery Charging Technologies

Wireless Charging Methods

Wireless charging has become popular because it is flexible, mess-free, and convenient, especially in cars. Some of the techniques used to enable this charging technology include:

• Electromagnetic induction

• Magnetic resonance

• Electric field coupling

• Radio reception

Ultra-Fast Charging Techniques

Ultra-fast charging incorporates techniques like adaptive charging, supercapacitors, and DC charging to make the process even faster.

Adaptive Charging

This technique involves adjusting the charging conditions (current, voltage, and temperature thresholds) to optimize the charging process and make it as quick as possible.

DC Charging

DC fast charging is common in EV charging stations, where the technology can fill lithium car batteries to 80% in less than 20 minutes. 

Supercapacitors

Also known as ultracapacitors, supercapacitors are ideal for short charge and discharge cycles. The technique requires high current and voltage levels, but stores energy to cover short ranges of about 10 km, making it ideal for in-city EV driving.

New Battery Chemistries

Lithium-ion batteries are far ahead of the rest due to their high charge densities, low self-discharge rates, and lightweight structures. But newer chemistries like sodium ion, solid state, lithium-sulfur, etc., might provide better performance and safety in the future and will require different charging technologies.

Conclusion

Battery charging is a simple concept in theory, but implementing it requires careful consideration of the battery’s chemistry to ensure safety, maximum efficiency, and a long lifespan.

Whether you choose linear, switch-mode, or pulse charger circuits, the only things that need tweaking are the current and voltage levels in CC and CV charging in the BMS to match different thresholds from 0–100%.

These circuits require integrated circuits and various components, which we can provide at reasonable prices for your project. Check out our products or contact us for more information to get started.