LiFePO4 or LFP or Lithium Iron Phosphate battery is a type of lithium ion battery with long lifespan, good safety, stable performance, long life cycle, better temperature handling capabilities, low manufacturing cost, and environmentally friendly elements. Moreover, an LFP battery can be charged quickly. These features of Lithium Iron Phosphate batteries make them suitable for energy storage systems and EVs.
As international organisations tend to pay more heed towards green energy and clean future in the result of climate change and global warming, focus on such energy storage technologies is increasing. Users, technicians, and academics are interested in getting a complete guide on charging and discharging LFP batteries. In this article, we will discuss the structure of LFP batteries relating it to charging and discharging principles, charging curves of LFP battery related to preferred charging techniques, and an appropriate way of discharging a LFP battery.
Part 1: Structure of LFP Battery
LFP battery has olivine structure, making it thermally stable and letting the ions move in a single direction. Lithium Iron Phosphate battery has three main components: lithium, iron phosphate and graphite. Lithium metal in the battery plays a primary role in electrochemical reactions, making the charge transfer possible during charging and discharging.
Iron Phosphate makes up cathode of the battery as diagram shows. On the other hand, anode is made up of carbon or graphite. Both electrodes are separated from each other through a polymer separator, which allows passage of lithium ions and blocks mixing of two electrolytes.
Part 2: Charging and Discharging Mechanisms for LiFePO4 Batteries:
Functioning of any battery depends upon its charging and discharging mechanisms. For Lithium Iron Phosphate battery, lithium ions travel from Iron Phosphate cathode to graphite anode during charging phase. Upcoming lithium ions get stored at receiving end (anode). Movement of lithium ions takes place through electrolyte and separator. To balance the charge, created from movement of lithium ions, electrons travel from cathode towards anode through external circuit. Charge is stored at this stage to be used later on.
During discharging, movement of lithium ions and electrons gets reversed. Lithium ions are deintercalated from graphite anode and move back towards iron phosphate cathode. Flow of lithium ions happens through electrolyte and separator, just like that in charging phase. Meanwhile, electrons flow from anode to cathode through external connection or circuit with a load. Energy is released in this process and devices are powered.
Part 3: Various Charging Methods for Batteries:
While LFP batteries have several advantages like low self discharge, high durability, and high energy density, their shortcomings, like sensitivity to deep discharge and overcharging, urge users to know the ways to effectively charge and discharge a Lithium Iron Phosphate battery. So, if you are also looking for ways to charge and discharge an LFP battery while maintaining its capacity and ensuring its guaranteed life cycle, it is important to know about various charging methods or mechanisms and then correlate these with charge-discharge profiles of LFP battery to figure out best charging and discharging tips for lithium iron phosphate batteries.
There are three common modes or methods for charging any battery, namely:
- Constant Voltage (CV) Method
- Constant Current (CC) Method
- Constant Voltage Constant Current (CVCC) Method
CC Charging (Constant Current Charging):
As the name indicates, during constant current charging, current is kept constant. Current is set around 10% of maximum battery rating. For this method, charging time gets prolonged and battery might overheat if battery is left in charging mode for longer periods of time.
CV Charging (Constant Voltage Charging):
In CV charging, charger allows the full current to flow towards connected battery until a specific voltage is reached. After that, current is reduced to a minimum value which tickles into battery to keep its voltage at that specific level and compensate for self-discharge. This voltage is named as ‘float voltage’ and current is named as ‘trickle current’. When trickle current is pouring into the battery, it is going through trickle charging mode.
CCCV Charging (Constant Current, Constant Voltage Charging)
CCCV charging combines the methods and benefits of CC and CV charging. In this charging mechanism, battery is first charged in CC (constant Current) mode. After battery voltage reaches a pre-set value, charger enters CV mode; current is reduced, and voltage is set at a constant value. As battery keeps charging, current keeps reducing and attains a minimal value when battery gets fully charged.
If we look into benefits of CCCV charging, all applicable for LFP batteries also, it is quite safe because current is entering the battery in a controlled amount while battery is near full charge. Anyhow, initial current is not very small and thus battery is charged quickly, making the process efficient. As overcurrent and overcharging don’t take place during this mode, battery life is prolonged.
Part 4: Efficient Charging of Lithium Iron Phosphate Batteries
While using a LFP battery or planning to buy a LFP battery, it is important to find an answer to “How to charge and discharge lithium iron phosphate battery effectively” as it is important to enhance efficiency. Here, we will discuss charging techniques for LFP battery, while relating it to charging profile of the battery.
Charging profile of LFP battery summarizes battery charging mechanism.
- Initially, constant current (CC) charging mode is employed and a constant current of 0.3 to 1C is supplied until battery voltage rises to its pre-determined value around 4.2V. It must be noted that manufacturers usually recommend keeping charging current around 0.3C during CC charging mode. Anyhow, one can go through product catalogue to find current limit for one’s own battery.
- Once pre-determined voltage level is reached, battery enters CV charging mode. Here, current keeps reducing. Just saturation charge is provided while keeping the voltage constant. For LFP batteries, this phase does not take much time, making the battery charging process very efficient.
- Once battery is fully charged, and supply current (stage 2) achieves a cutoff value, the supply current is cut off entirely. At this stage, battery is ready for use.
- After supply current is removed, LFP battery will experience self-discharge. Self-discharge for these batteries is very low. That’s why these batteries are capable of maintaining their almost full capacity for 6-8 months without charging.
Note: Nonetheless, it is not recommended to store lithium iron phosphate battery with 100% SoC as it will affect its capacity.
- Stage 4 in above diagram shows that the battery voltage dropped during stage 3. To compensate for this voltage drop, a topping charge of a small amount can be given to boost the voltage back to its nominal value.
Detailed discussion about charging mechanism for a lithium iron phosphate battery shows that CCCV charging mechanism is preferred for the battery. In this charging mode, battery will charge safely without the danger of getting overcharged or overheated. Secondly, CCCV charging mode charges battery quickly, specially lithium batteries, so charging process of LiFePO4 batteries becomes efficient. Appropriate charging and choice of smart chargers make these batteries suitable for many applications including EVs, BESS, drones, and robots.
Part 5: Tips and Tricks Related to LFP Battery Charging
Above section details the charging process of the LiFePO4 battery. Nonetheless, here few common tips and tricks are mentioned which can enhance overall usage experience. This section will also answer some common queries related to battery charging process specifically centred around LFP batteries.
- Users must take caution while charging an LFP battery from solar panels. Constant voltage or constant current is to be maintained during charging phase of the battery as above curve depicts. On the other hand, solar panels have varying output. So appropriate electronics shall be employed to avoid damaging the battery.
- Lithium iron phosphate battery shall not be charged directly from generator either. It is because of supply requirements of battery. LFP battery demands constant voltage for charging. While generator supplies alternating current (AC) supply. As both are not compatible so battery can’t be attached to the generator directly.
- Battery size impacts capacity and thus charging current. Charging current can vary between 0.3 and 1C. If your battery has capacity of 50Ah, then you shall set charging current between 0.3 X 50 = 15A to 1 X 50 = 50A.
- Selected charge shall have appropriate electronics to terminate charging to avoid overcharging and switch charging modes when appropriate.
- If one has time, slow charging shall be preferred over fast charging as it will enhance battery life cycles.
- During charging, it must be ensured that battery doesn’t overheat. Battery Management Systems (BMS) have become an integral part of battery systems, so temperature could also be monitored and regulated.
Part 6: How to Discharge a LiFePO4 Battery?
In above section, while discussing charging and discharging mechanisms of LFP battery, it was mentioned that once battery gets charged, energy is stored in form of high potential electrons. These electrons move through outer circuit to power load. It makes it clear that attaching load is primary aspect of discharge phase. However, many important questions arise at this stage like how much load can be attached to a battery? What type of load is suitable for some kind of battery specifically LFP battery? What shall be the discharge rate? When shall the load be disconnected? Here, we will discuss step-by-step procedure to discharge a LFP battery appropriately. This procedure will answer mentioned questions also.
1. Determining Safe Discharge Rate
Amount of load that can be connected to the battery, depends upon capacity of battery. Typically, a battery can be discharged at discharge rates falling between 1C and 3C safely where C is the capacity (size) of battery. Battery discharging at 1C rate will be discharged fully in one hour, and the one discharging at 3C rate will be discharged fully in 1/3 hours. So, you can see capacity of your battery mentioned on its nameplate and connect appropriate amount of load.
2. Connecting the Load
Any kind of load, that is to be powered through battery, is connected between positive and negative terminals of battery. So, charge could flow, battery could discharge, and device/load could run. Connections shall be secure and polarity shall be right to avoid any damage to device, battery, and overall circuit.
3. Monitoring the Voltage
Though LFP batteries are robust, overdischarge affects them negatively in long run. For example, an LFP battery discharged to 50% DoD would have 5000 cycles, while the one with 80% DoD would have around 3000 cycles. So, they shall not be discharged below a certain limit. Voltage can be monitored easily because it drops as we increase DoD. Manufacturers mention voltage limit for each battery pack that shall be considered. As a rough estimate for LFP battery, it is around 2.5V per cell.
4. Disconnecting the Load
During usage, voltage of overall battery and each cell drops as discussed above. When voltage per cell reaches 2.5V (for LFP battery), then load shall be disconnected to avoid overheating and prolong battery life.
5. Storing the Battery or Preparing for Next Cycle
If you want to store the battery for a longer time period after the load is disconnected, then ensure not to store it with State-of-Charge (SoC) below 50%. Moreover, storage place shall be cool and dry.
If you want to recharge the battery to power the load again, follow charging steps and prepare it for next cycle.
Part 7: Conclusion
Charging and discharging are important operational aspects of Lithium Iron Phosphate or any kind of battery. To prolong battery life, ensure safety, and enhance efficiency, it is important to adopt appropriate charging mechanism and follow guidelines regarding the process. Moreover, step-by-step guide to discharge a battery shall be followed cautiously. Summarily, one shall avoid over charging and deep discharging the battery, battery must be kept clean, it must not be overheated during charge or discharge process, and its basic electrical and environmental parameters shall be kept under observation to avoid any damage.
Dr. Kevin Yang
Lead Electrical Engineer
Dr. Kevin Yang has 16 years of R&D experience in lithium battery power management systems (BMS) and 10 years in lithium ion battery engineer. Experienced in various lithium ion battery applications,he is capable of offering customized lithium battery solutions beyond cutomer expectation .
