BUCK BOOST CONVERTER BASED BATTERY CHARGE BALANCING CIRCUIT

Abstract

A large number of direct current applications make use of rechargeable DC batteries made up of a string of series connected cells. Charge imbalance of cells in such battery systems is typical in series connected battery strings. Usually, the individual cells in a battery have somewhat different capacities and may be at different levels of state of charge (SOC). Imbalanced cells can directly affect the operation of the battery pack and life of individual cells. It is imperative to use circuits that balance cells or maintain a check on the degree of imbalance during the charging or discharging of battery. Starting with the general description of battery operated systems and the common challenges faced during battery operation, the problem of battery imbalance is explained in this thesis. The reasons for imbalance in batteries, the need for charge balancing and popular charge balancing techniques used to tackle this problem are further elaborated. The design, operation and control of a DC-DC converter based active balancing circuit are discussed in detail. The balancer circuit discussed here transfers the energy from fully charged battery cell to the weakest charged battery in a fashion similar to buck-boost operation. This operation maintains batteries at the same charge and voltage level. The design methodology and simulation results that act as proof of concept, are presented for a system consisting of two lead acid batteries connected in series. The working of the circuit is further demonstrated by building a hardware prototype and testing it. A microcontroller based control is implemented to provide PWM signals to the switches in the circuit. The hardware results match with the expected theoretical predictions and qualitatively verify the working of the circuit.