【Introduction】For now, this is probably the most common problem in life. At the turn of the century, the decreasing cost and improved performance of batteries, especially those based on lithium-ion, drove the steady growth of battery-powered energy storage and portable devices. In addition, supercapacitors are also increasingly used in various applications due to their unique properties. Lead-acid batteries are a 150-year-old technology that is still widely used in cars, wheelchairs, scooters, golf carts, and uninterruptible power supply (UPS) systems. Once the energy is depleted, these energy storage devices must be recharged. In 2019, the global shipment of charging ICs was 1.16 billion, and it is expected to grow to 1.72 billion in 2024, with an annual growth rate of 8.6%, which is quite healthy. Revenues were $518.1 billion and $735.4 billion, respectively, with a compound annual growth rate of 7.3%. This trend is shown in Figure 1 from OMDIA’s “Power IC Market Tracker – 2019”.
Figure 1. Global Charging IC Market
The need for more power, longer range, or runtime requires higher voltages used by energy storage devices. For example, lithium-ion battery stacks used in robots, drones, power tools, and many other things have grown from one or two cells to multiple (up to 12) cells. A 12-cell Li-Ion battery stack provides a maximum voltage of 50.4 V. A 12-cell battery lasts 12 times longer than a 1-cell battery at the same current rating. Alternatively, 12 cells can be connected in parallel for higher power, but this approach increases the current by a factor of 12. Higher currents result in more conduction losses, so parallel batteries are not suitable.
Industrial systems such as emergency lighting with battery backup, UPS backup power, HVAC, etc. use 24 VDC power, i.e. 24 V batteries are used to provide backup power to these systems. However, according to the IEC 61131-2 and IEC 60664-1 standards, the 24 VDC power supply can rise to 60 V peak under transient conditions.
In either case, the device requires a charger solution that can accommodate higher battery voltages and withstand higher input voltages during transient events.
There are many topologies for chargers. Linear chargers reduce the voltage difference between the power supply and the battery through a power switch. This type of charger is the least efficient because the power switch draws a lot of power when the voltage difference between the power supply and the battery is large. A boost charger boosts the voltage from the power supply to the battery voltage. This topology requires the supply voltage to be lower than the battery voltage. Buck chargers step down the voltage from the power supply and require the supply voltage to be higher than the battery voltage. A boost-buck charger can charge the battery with a supply voltage that is higher or lower than the battery voltage. This topology requires four power switches (the buck topology requires only two) and is generally not very efficient.
The synchronous rectifier buck charger has the highest efficiency and is the focus of this article. Figure 2 shows a generic synchronous rectifier buck charger circuit. Today, most buck chargers operate at relatively low voltages. Many chargers are only rated for 28 V input, some are 40 V. A 28 V rated charger can really only charge a 5S Li-Ion stack (max) if you allow for ±10% input voltage regulation and a 2 V buck charger drop. We’ll be looking at a new family of 60 V input charger ICs that support higher charging voltages – battery voltages up to 52 V (or a 12-cell Li-Ion stack), and can withstand 65 V input voltage transients.
Figure 2. Universal Synchronous Rectifier Buck Charger
The charger’s standby current should be low to save energy. Energy Star® specifies five stars for cell phone chargers and other small chargers with standby power consumption of 30 mW or less. One star applies to chargers with a standby power consumption of 300 mW or more, other stars apply to other chargers in between. Energy Star is designed to reduce the power consumption of personal chargers, which are mostly not unplugged when not in use. At any given time, there are more than 1 billion such chargers connected to the grid around the world.
Although lead-acid batteries, lithium-ion batteries, and supercapacitors are all energy storage devices, their charge/discharge characteristics differ significantly. We’ll examine these features and discuss charging solutions for each. A good battery charger provides good battery performance and durability, especially when charging under adverse conditions.
Lead acid battery charger
Lead-acid batteries are the oldest rechargeable batteries in existence, invented in 1859 by French physician Gaston Planté. One hundred and fifty years later, it is still widely used in cars, wheelchairs, scooters, e-bikes, golf carts and UPS systems.
Lead-acid batteries must be charged slowly. Typical charging time is 8 to 16 hours. Batteries must always be stored in a charged state, and regular full saturation charging is essential to prevent sulfation. A common practice is to charge the lead-acid battery to 70% in about 8 hours, and then take another 8 hours for the all-important top-up charge. Partial charging is fine if the lead-acid battery receives a full saturation charge from time to time to prevent sulfation. Leaving the battery on a float charge for extended periods of time will not cause damage.
Finding the ideal charging voltage limit is critical. High voltages (above 2.45 V/cell) produce good battery performance, but battery life is shortened due to grid corrosion on the positive plate. The low voltage limit can cause sulfation of the negative plate. Temperature also affects cell voltage, with a typical temperature coefficient of –5 mV/°C (0.028 V per cell per 10°F). A good charger must compensate for this temperature coefficient to avoid overcharging the battery at high temperatures or undercharging at low temperatures.
For example, the MAX17702 (see Figure 3) is a complete lead-acid battery charger controller designed to support an input voltage range of 4.5 V to 60 V. The device provides a high efficiency (over 97%), high voltage, synchronous buck solution for charging 12 V/24 V/48 V lead acid battery packs. Figures 4a and 4b show its charging cycle and charging efficiency.
Figure 3. High Voltage Lead Acid Battery Charger Controller
Figure 4a. MAX17702 lead-acid charge cycle
Figure 4b. MAX17702 charging efficiency
Lead-acid batteries have low energy density and are not suitable for portable devices. Portable devices require lithium batteries.
Li-ion battery charger
Lithium-ion batteries are generally accepted batteries for portable applications, heavy industry, electric drives, and satellites due to their light weight and high energy density.
Lithium-ion batteries require little maintenance. This battery has no memory effect and doesn’t need to be intentionally fully discharged to stay in good condition. But it requires protection circuitry, both inside the stack and the charger, to prevent short circuits, overcharge, thermal runaway, and overdischarge. Lithium-ion batteries may develop dendrites if kept below 1.5 V/cell for a week or more, compromising safety.
To prevent over-discharge, a built-in battery protection circuit puts the battery to sleep. This happens when the battery is stored in a discharged state and self-discharge brings the voltage down to the cut-off point. Conventional chargers treat such batteries as unusable, and the battery pack is usually discarded. Advanced Li-Ion chargers have a wake-up feature or “pre-charge” feature that can charge Li-Ion batteries that have gone to sleep due to overdischarge. In precharge mode, the charger applies a small charge current to safely boost the voltage between 2.2 V/cell and 2.9 V/cell, activates the protection circuit, and begins normal charging.
During normal charging, the Li-Ion charger operates in constant current constant voltage (CCCV) mode. The charging current is constant and the voltage stops rising when it reaches the set limit. When the voltage limit is reached, the battery saturates and the current drops until the battery cannot accept further charging, at which point charging is terminated. Each battery has its own low current threshold.
Lithium-ion batteries should always be kept cool while charging. Lithium-ion batteries cannot absorb excess charge. Therefore, it is important to monitor battery temperature and its charging voltage to ensure battery health and safety. A good charger must contain these features.
Figure 5 shows an example of an advanced Li-Ion battery charger. The MAX17703 is a high-efficiency, high-voltage, synchronous, step-down charger controller designed to support a wide input voltage range of 4.5 V to 60 V. The device provides a complete charging solution for up to 12-cell Li-Ion battery stacks.
Figure 5. Advanced High Voltage Li-Ion Battery Charger Circuit
The device provides accurate CCCV charge current/voltage at ±4% and ±1%, respectively. When the charging current is reduced to the end current threshold, the charger enters the supplementary charging state; after the end of the end timer period, the charger exits the charging state. When the output voltage falls below the charge threshold voltage, the charger initiates a charge cycle. It’s a great feature that keeps the battery fully charged when left on the cradle for extended periods of time without draining too much power, and is Energy Star compliant. The device can detect and precondition deeply discharged batteries, using the precharge feature to wake them up. For added protection, the device detects the battery temperature so that charging can only be performed within the specified temperature range. It also has an input short-circuit protection feature to prevent battery discharge if the input is accidentally shorted. Figure 6 shows the charge cycle for the MAX17703.
Figure 6. MAX17703 Li-Ion Battery Charge Cycle
Supercapacitors have some unique advantages over batteries and are therefore increasingly used in a variety of applications. Supercapacitors work on the principle of electrostatics without chemical reactions, avoiding the lifetime issues associated with chemical storage in batteries. Its high durability allows millions of charge/discharge cycles and a lifespan of up to 20 years, an order of magnitude longer than batteries. Its low impedance enables fast charging and discharging in seconds. Plus, it has a modest ability to hold a charge for a long time, all of which make supercapacitors ideal for applications that require rapid charge-discharge cycles. Ultracapacitors can also be used in parallel with batteries to support applications that require instantaneous peak power transfer during load transitions.
The fast charge-discharge cycle of supercapacitors requires the charger to be able to handle high currents, operating smoothly in constant current (CC) mode during charging, possibly starting at 0 V and then in constant voltage (CV) mode once the final output value is reached Work. In high-voltage applications, where many supercapacitors are connected in series, a charger is required to manage high input and output voltages.
The MAX17701 (see Figure 7) is a high-efficiency, high-voltage, synchronous, step-down supercapacitor charger controller designed for high-current charging and operates over a 4.5 V to 60 V input voltage range (VDCIN) . The output voltage is programmable from 1.25 V to (VDCIN–4 V). The device utilizes an external N-MOSFET to provide a logical OR function on the input supply side, preventing the supercapacitor from discharging back to the input. Figure 8 shows a simple but high current charging curve.
Figure 7. High Voltage, High Current Supercapacitor Charger
Figure 8. MAX17701 supercapacitor charging curve
The use of battery-powered energy storage and portable devices has grown steadily. The need for more power, longer range, or run time calls for higher voltages used by the battery stack. In industrial system applications using a 24 VDC power supply, 60 V peak can be seen under transient conditions. Traditional charger solutions are mostly limited to 28 V input. Thanks to the high-voltage synchronous buck charging topology, ADI’s new charger solution enables higher stack voltage and charging efficiency.
Lead-acid batteries, lithium-based batteries, and supercapacitors are all energy storage devices with very different charge/discharge characteristics that require dedicated chargers for optimal charging solutions. Advanced battery chargers also provide adequate protection for battery performance and durability, especially when charging under adverse conditions. These are also addressed in newer charger solutions.