The battery charging system looks simple when seen in a general view since we have to deliver current from the power supply to the cells. That is easier said than done due to one point: you have to fill the battery correctly, especially during fast charging, to prevent fires/explosions and to extend its service life.
So automatic battery charging circuits have to be optimized for handling certain battery chemistry. In that automatic battery charging circuit details, we will analyze these chargers according to 3 famous battery chemistries, which are lead-acid, nickel, and lithium-based batteries. Let’s get right into it!
What is Lead Acid battery
This battery chemistry needed a three-phase charging process christened IUoU to provide the best performance and have a long lifespan.
I (constant current) for bulk charging:
This phase charges the battery to about 80 percent fully charged with a constant current of about 10–25 percent of the battery’s Ah rating, according to the battery’s manufacturer. Charging at more than 25% in this phase has a higher chance of damaging the battery.
Uo (constant voltage) for absorption charging:
It’s also called the equalization phase, absorbing the remaining 20 percent with constant voltage. The charger maintains target charging voltage (14.1–14.8 V DC) while decreasing the current.
U (constant voltage) for trickle charging
This floating stage reduces charging current to one percent of the battery’s Ah rating while the voltage is maintained at around 13–14V DC. Lead-acid batteries can be kept in float mode indefinitely (with no auto cut-off) to increase the battery life since it reduces 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 fault can result 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 affect the sulfate crystals coating the plates.
But this process doesn’t use the pulse charger discussed above. It needs short, high-current pulses, so the desulfation circuit should have a system for controlling variables like pulse width and frequency to match the proper conditions. Some of the possible solutions are
- 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 batteries because they have a low memory effect. However, both have the same charging requirements (a constant current source), and this charger can manage as many cells as you like, provided they are connected in series.
Lithium Polymer and Lithium Ion
- Lithium polymer and lithium ion batteries come with same features and needed prper regullated charging with these points to prevent damage.
The lithium-ion battery charging curve
Trickle Charging
Battery management system: just apply this point if the battery voltage is less than the lowest threshold (usually 2.1 V). When a lithium-ion or polymer battery hits these lows, its internal protection IC disconnects the battery by opening its FETs.
In these conditions, the charger chip charges the battery pack capacitor with a small current (around 50 mA), which triggers the protection IC in the battery pack to close its FETs and reconnect the battery.
The process works for 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 shows the battery is damaged.
Pre-Charging
Pre-charging started one-time battery reconnection, pushing 10 percent battery capacity in mAh as current into the pack. This slowly raises the voltage level until it gets to the threshold for CC charging.
Constant Current
CC or fast charging starts when voltage per cell is about 3V. At this level, the battery can manage higher currents of about 50–300% of its capacity.
Constant Voltage
The threshold for CV in a lithium battery is 4.1–4.5 V per cell, and the charging IC implements CV charging since the external battery voltage crosses the internal battery voltage. This occurrence is due to faults like PCB, internal cell, and equivalent series resistance from the FET to the battery cells.
For safe working, the IC doesn’t allow the battery voltage to exceed the maximum floating voltage, making CV activation a safety measure.
Charge Termination
When the current moves to a battery-less value set threshold of the CV charging phase, the IC terminates the charging cycle and the battery is fully charged. This threshold could be 10% of the battery’s capacity in mAh.
Lithium Ion Battery Charging Methods
- there are different methods for charging lithium-ion batteries, but the most common ones are NVDC and HPB.
Narrow Voltage DC Charging
NVDC chargers provide high system efficiency with a minimised voltage range. They get this with the replacement of a regular battery charger through a system charger that comes with a buck converter. This setup configures DC-DC conversion, minimises power path switches, and controls power dissipation, space usage on the PCB, and overall costs.
The charger functions as a buck converter both when charging the battery and when the battery is supplementing the AC adapter to power the device. This 2nd option is easy when the system’s load exceeds the adapter’s rating.
The lithium-ion battery charging curve using NVDC
However, it has its disadvantages, which comeswith
- 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 process is used as a system with extra supply from the battery when the load is higher than the adapting rating. But it applies in a different way with reverse-boosting the battery’s power.
The buck converter functions normally in this charger and adapter that gets external power. If there are not enough converter functions in reverse, help the battery support the adapter.
This setup has one main disadvantage, which is a lower light load efficiency.
How To Pick a Charger Depending on the Battery Capacity and CC Rating
Charging current is important when getting a battery charger since it defines charging time, which needs to be as short as possible.
Such as when charging a 50 Ah lead-acid battery, the bulk charging phase should supply 10% of this capacity to the load, which is 5 A. However, circuits have losses, so you should consider something like 8A.
Using this charging current to fully 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 show it can handle 25% of its capacity during bulk charging, you should use a 12.5A charger or 15A to account for losses. This will minimize the charging time to 3–4 hours.
Safety Precautions for Batteries and Their Charging Circuits
Thermal Management:
All battery charging circuits face losses, which are shown in heat generated. So they must have a proper thermal management system for keeping the board and components cool.
Overcharge Protection:
Save for lead-acid batteries, other types should have overcharge protection circuits for prtection battery chemistry from facing permanent damage and also protect users from safety hazards like fires and explosions.
Over-Discharge Protection:
Over-discharge protection is important to protect from fuly discharging cells that affect permanent decomposition of electrolyte, destroying the reversibility of the positive and negative materials, etc.
Handling Process:
Each battery comes with a certain handling process given by the manufacturer, like current and voltage features for charging. If designing a battery management system, note these points:
Emerging Battery Charging methods
Wireless Charging Methods
Wireless charging is important since it is flexible, mess-free, and convenient, mostly in cars. Some methods used for helping with these charging methods are
Electromagnetic induction
Magnetic resonance
Electric field coupling
Radio reception
Ultra-Fast Charging Techniques
Ultra-fast charging incorporates methods such as adaptive charging, supercapacitors, and DC charging to make the process even faster.
Adaptive Charging
This method involves adjusting charging conditions (current, voltage, and temperature thresholds) to optimize the charging process and make it fast.
DC Charging
DC fast charging is a common factor for EV charging stations, where technology fills lithium car batteries to 80% in less than 20 minutes.
Supercapacitors
Also called ultracapacitors, supercapacitors are best for short charge and discharge cycles. The method requires high current and voltage levels but stores energy to manage short ranges of about 10 km, making it ideal for in-city EV driving.
New Battery Chemistries
Lithium-ion batteries are better due to high charge density, low self-discharge rate, and lightweight structures. But newer chemistries like sodium-ion, solid-state, lithium-sulfur, etc., might offer good performance and safety in the future and will need different charging technologies.
Conclusion
Battery charging is a simple factor in theory, but implementing it needs proper 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%.
