This thesis deals with a design, construciton and testing of a switch-mode power supply (SMPS) which is working as a LiFEPO4 battery charger with output current up to 100~A and output voltage up to 14,6~V. The output voltage and current can be regulated by the operator from zero to maximum value. For this SMPS Half-bridge asymmetrical forward converter with two transformers and common output inductor topology is chosen. The control circuits are run by the IC SG3525. Cascaded regulation of output voltage and current is implemented by two discrete operational amplifiers. Undervoltage protection of the control circuits and independent overcurrent protection on the primary side is also implemented.

Battery Management System (BMS) is an essential part in battery powered applications where large battery packs are in use. BMS ensures protection, controlling, supervision and accurate state estimation of battery pack to provide efficient energy management. However the particular application determines the accuracy and requirements of BMS where it has to implement; in electric vehicles (EVs) accuracy cannot be compromised. The software part of BMS estimates the states of the battery pack and takes the best possible decision. In EVs one of the key tasks of BMS’s software part is to provide the actual state of charge (SOC), which represents a crucial parameter to be determined, especially in lithium iron phosphate (LiFePO4) batteries, due to the presence of the high hysteresis behavior in the open circuit voltage than other kind of lithium batteries. This hysteresis phenomena appears with two different voltage curves during the charging and discharging process. The value of the voltage that the battery is going to assume during the off-loading operation depends on several factors, such as temperature, loop direction and ageing. In this research work, hybrid method is implemented in which advantages of several methods are achieved by implementing one technique combined with another. In this work SOC is calculated from coulomb counting method and in order to correct the error of SOC, an hysteresis model is developed and used due to presence of hysteresis effect in LiFePO4 batteries. An hysteresis model of the open circuit voltage (OCV) for a LiFePO4 cell is developed and implemented in MATLAB/Simulink© in order to reproduce the voltage response of the battery when no current from the cell is required (no load condition). Then the difference of estimated voltage and measured voltage is taken in order to correct the error of SOC calculated from coulomb counting or current integration method. To develop the hysteresis model which can reproduce the same voltage behavior, lot of experiments have been carried out practically in order to see the hysteresis voltage response and to see that how voltage curve change with the variation of temperature, ageing and loop direction. At the end model is validated with different driving profiles at different ambient temperatures.

This master’s thesis deals with the design and fabrication of DC arc welder supplied from its own rechargeable battery. Battery cells’ type is LiFePO4 which can provide high currents even at relatively low capacity. The BMS circuits are designed within the battery. Major power part of the welder is a step-down converter with synchronously switching low transistors at the position of free-wheel diode. The converter operates at 100 kHz frequency. The current of the battery welder can be regulated in the range from 0 to 120 A.

This master thesis is focused on proposition and following implementation of DC arc welder. This paper is based on semester’s paper, where the first version was realized. The welder is powered by a battery pack with LiFePO4 type cells. Battery cells are protected by BMS circuits. The driving electronics controls the duty cycle step-down (buck) converter in a closed current or power regulation loop. The power regulator has been designed and validated by simulations. Switching power transistors, low side and high side, are implemented by optically isolated gate drivers. Charging the battery is solved by a switching flyback converter. For the charger and the welder is sheet metal construction made of aluminum sheet.

Charging station; PV panel; accumulator; battery; lithium; LiFePO4; Arduino