LiFePO4 Batteries Help Consumer Devices COME TO LIFE
LiFePO4 Batteries Help Consumer Devices COME TOLIFE
Lithium Iron Phosphate (LiFePO4) batteries were introduced a decadeago and, in recent years, have begun to be adopted for massproduction. LiFePO4, also known as LFP battery systems, areespecially suited for applications such as electric vehicles (EV),hybrid-electric vehicles (HEV), electric bicycles, and power toolsdue to their naturally low impedance, resistance to thermalrunaway, and ability to withstand a high number — about 2,000 —charge/discharge cycles.
One trend in applications that use LFP batteries is relatively highdischarge-continuous and pulse currents compared with applicationsusing conventional Li-Ion and Li-Polymer batteries. Additionally,applications that use LFPs typically involve multiple cells inseries and parallel.
Some LFP charge-management systems are developed withmicrocontrollers and an external power train to charge the batteryat a higher constant current or voltage. Other Li-Ioncharge-management systems are either modified with a voltagedivider or designed to charge at a higher reference voltage thanthe manufacturer's specifications recommend. Whatever the case,simple charge-management systems for low-power LFP batteries havenot yet been widely developed. We will show designers how toaffordably develop these types of systems, using LFPbattery-powered radio-controlled (R-C) toys and UPSs as examples.The LFP Battery Chemistry Depending on the manufacturer, nominaloperating voltages for LFP batteries range from 3.2 to 3.3 V. Mostmanufacturers require 3.6 V as a charge-reference voltage. LFPbatteries have a similar charge algorithm to Li-Ion batteries inthat they are charged with constant-current and constant-voltage(CC-CV) methods.
These methods begin with a set constant current. Most LFP batteriesallow a charge rate of more than 1 C for a short amount of time.Once a reference voltage is detected, the state machine moves tothe constant-voltage method. While a charge-management controllermaintains the battery voltage, the charge current decreases untilthe minimum set-current is reached. Some battery manufacturersprefer a fixed minimum current, such as 50 mA, while others use apercentage ratio of battery capacity, such as 0.1 or 0.05 C. Forproper operation, refer to the product datasheet or consult withthe battery manufacturer for the maximum charge-current rating.
Fig. 1 illustrates a CC-CV algorithm that was implemented by astate machine in the charge-management controller. When thebattery's presence is detected, the controller verifies the batterycondition by voltage. When this voltage is above thepreconditioning threshold, the battery moves to fast charge-currentmode. Depending upon the design, the fast charge current can beeither set by the resistor or a digital signal. When the referencevoltage is met, the charge current decreases with thebattery-voltage regulation until the charge is terminated. Whenerrors such as overtemperature or overvoltage conditions occur,batteries can short, prompting the system to output a signal usinga simple LED or LCD display to warn end users. If a rechargefeature is required, the charge-management system monitors thebattery voltage and only applies a charge when necessary to prolonga battery's life cycle.
Fig. 2 shows a typical charge profile of an 1,100-mAh-rated,single-cell LFP battery from a highly integrated, stand-alonelinear charge-management controller. Typically, the maximumcontinuous discharge current for LFP batteries ranges from 5 to 30C, and some batteries can produce more than 100 A of pulse current.Therefore, besides automotive and power-tool applications, R-C toysand UPSs can benefit from LFP batteries.
C-rate is the theoretical capacity of a battery as determined bythe amount of active materials in the battery. It is expressed asthe total quantity of electricity involved in the electrochemicalreaction in coulombs or ampere-hours. The ampere-hour capacity of abattery is directly associated with the quantity of electricityobtained from active materials.