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.
