Since Li-Ion batteries have been commercialized since the early 1990’s. Its capacity has been constantly increasing from 1,000 mAh to 3,000 mAh through improvements of the active materials and package technology for the 18650 cell. Li-Ion batteries are widely used in laptops, digital still cameras and cellular phones. However, existing Li-Ion batteries with Lithium Cobalt Oxide and Lithium Manganese Oxide as the cathode materials suffer from a low discharge rate, safety concerns, and short cycle life. When the cell temperature increases to 175oC with 4.3 V charge voltage, the battery enters into a thermal runaway mode and could cause explosion. To solve these problems, Dr. John Goodenough and his team at the University of Texas patented the LiFePO4 as a potential cathode in 1996.[1] It is a very stable material due to the covalent P-O bonding, which stabilizes the fully-charged cathode. Scientists have developed the LiFePO4 battery using Lithium Iron Phosphate as a cathode to alleviate safety concerns in the battery for electric vehicles and power tool applications.
Battery safety has been greatly improved by using LiFePO4 as the cathode. The P-O chemical bond is extremely strong, and the material is thermodynamically stable. LiFePO4's olivine crystal structure decides its crystal lattice deformation smaller in the sufficient electric discharge process. Its material structure is stable and safe, and also its cycle life is extremely long. These characteristics also can make LiFePO4 withstand oxidation and acidic environment. So, the battery has more electrolyte choice, and the battery performance can be optimized.
Figure 1 shows the LiFePO4 discharge characteristics under different discharge currents. Unlike the LiMnO2 cathode-based Li-Ion battery, its discharge curve is fairly flat and its voltage is almost constant. It has a very strong discharge capability with higher discharge rates, and is suitable for power tools and electric vehicle applications. The LiFePO4 cycle life could be longer than 1,000 cycles at room temperature (25oC / 77oF), and can reach 1,000 cycles even at 60oC /140oF compared with the average life of 300-500 cycles for Li-Ion battery. However, the main drawback is the energy density, which is only about 50 percent of the Li-Ion battery.
Figure 2 shows the LiFePO4 battery charging profile, which is similar to the traditional Li-Ion battery with Lithium Cobalt Oxide as the cathode material. It uses constant current (CC) and constant voltage (CV) and consists of three charging phases: pre-charge; fast-charge CC; and CV.
In the pre-charge phase, the battery is charged at a low rate for testing, if the battery is internally shorted when the cell voltage is below 0.5 V.
Fast-charge current is applied to charge the battery quickly. Its charging rate can be up to 10C rate, which is much higher than the traditional LI-Ion battery without additional degradation. The charger enters to the CV mode when the battery reaches a voltage regulation limit (typical of 3.6 V/cell). During the CV mode, the charge current exponentially drops to a pre-defined termination level where the battery is fully charged and the charging is terminated. Since the LiFePO4 battery has much lower internal resistance, its charging time is much shorter than the Li-Ion battery.
While the LiFePO4 is much safer than the Li-Ion battery, a fast charge safety timer is usually required to prevent charging a dead battery for an excessively long period. The LiFePO4 battery can be overcharged to 4 V without safety issues, even though it is specified to charge to 3.6 V. However, the energy stored in the battery between 3.6 V and 4 V is very limited. From the discharge curve in Figure 1, the voltage drop is very fast at the beginning of the discharge period. This demonstrates that the battery does not store much energy at higher voltages.
Most of the battery energy is stored near the battery voltage between 3.0 V and 3.4 V for 1C-5C discharge rates. It does not give much benefit to charge the battery higher than 3.6 V though it does not degrade the battery. The voltage difference between rechargeable voltage threshold and battery charge voltage should be around 200 mV, since it takes a few seconds to drop the battery voltage from 3.6 V to 3.5 V. Although the LiFePO4 battery has excellent and stable high temperatures, it is still preferable to monitor its temperature to improve safety.
Figure 2. LiFePO4 battery charge profile.
High Efficiency LiFePO4 Battery Charger
The LiFePO4 battery is used in high-discharge current and high-temperature applications such as power tools, electric vehicles, e-bikes, and uninterruptible power supply (UPS) applications. It should not be used for cellular phones and other portable devices due to its energy density limitations.
Figure 3 shows the standalone synchronous switching 6S LiFePO4 battery charger with dynamic power management functions. The synchronous switching buck-based battery charger is composed of MOSFETs Q4 and Q5, inductor L, output filter capacitors C7 and C8. Synchronous switching is used to efficiently charge the battery where over 95 percent efficiency can be achieved.
Figure 3. The typical application circuit for charging a LiFePO4 battery.
A power source selector is achieved through MOSFETs Q1, Q2, and Q3. When the adapter is not available, MOSFET Q3 is turned on and the battery is used to power the system. However, the adapter can power the system and charge the battery simultaneously when the adapter is available. There is a total of three control regulation loops: charge current, charge voltage, and input current regulation loops.
The charge current sense resistor (R2) is used to sense the charge current to make sure that the charge current does exceed the programmed charge current set by ISET1 pin. The resistor dividers (R11 and R12) are used to monitor the battery charge voltage so that the charge voltage does not exceed the programmed charge voltage. More importantly, the input current sense resistor (R1) is used to monitor the input adapter current. This is done so that it can fully use the adapter power while not crashing the adapter when the adapter powers the system and charges the battery simultaneously.
The charge current is automatically reduced when the input current reaches the input current limit set by the ACSET pin, which continuously increases the system load current. This is a dynamic power management. Additionally, the charger monitors the battery temperature through the TS pin. The safety timer can be programmed through the TTC pin for different charge current and battery applications. The charge status indicators (STAT1 and STAT2) give the end users charge information, if the battery is in charging, fully charged, or in the fault conditions.
Figure 4 shows the tested LiFePO4 battery charge waveforms with a 4S 18650 LiFePO4 battery from A123. The battery charge efficiency is shown in Figure 5 under different input voltages, number of cells, and load currents. It shows that over 96 percent efficiency can be achieved for 5S and 6S applications.
Figure 4. The LiFePO4 battery charge waveforms.
Figure 5. Battery charge efficiency under different adapter voltages and the number of cells.
Conclusion
LiFePO4 battery has many advantages such as being safer with a longer life cycle, higher discharge rate capability, and potential lower cost. It is the best for power tools, electric vehicles, e-bikes, and UPS applications. The LiFePO4 charge and discharge characteristics are unique and requires a dedicated battery charge controller such as the bq24630 for optimizing the charge and system performance.
References
A. K. Padhi, K. S. Nanjundaswamy, and J. B. Goodenough, “Phospho-Olivines as Positive-Electrode Material for Rechargeable Lithium Batteries,” J. Electrochem. Soc., Vol. 144, No. 4, April 1997, The Electrochemical Society, Inc.: http://www.phostechlithium.com/documents/LiFePO4_Goodenough_ECS_1997.pdf
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Jinrong Qian is a Battery Charge Management Manager and Distinguished Member of the Technical Staff for Battery Management Solutions at Texas Instruments. He has published a of peer-reviewed power electronics transactions and power management papers, and holds 21 US patents. Dr. Qian earned a BSEE from Zhejiang University, and a Ph.D. from Virginia Polytechnic Institute and State University.
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