Practical Magnetic Tomography for Lead Batteries

A variety of economic factors currently motivate the development of electrochemical energy
storage. The effective use of renewable energy requires short term storage, for which
electrochemical cells may be used. Electrified transport is also driving development; stored
energy limits the range of electric vehicles. In hybrid vehicles, improved dynamic charge
acceptance will help to optimise powertrain efficiency.
A non-invasive measurement of current distribution within a cell is a useful aid to
understanding its operation and optimising its design. Here, the coupling between the cell
current and the resulting magnetic field is exploited by taking measurements of magnetic flux
density outside the cell and inferring the current distribution within. This technique may be
termed magnetic tomography or magnetotomography.
In this thesis, a practical system is implemented in order to observe the current distribution
within a single lead acid cell. An existing method of constraining and solving the inverse
problem is adapted for use in conjunction with 3D finite element software, to make it suitable
for modelling the complex geometry of a commercial electrode. Some tolerance of unknown
material conductance is built into the solver method. An array of sensors is used to obtain a
set of magnetic field measurements simultaneously, allowing temporally- and spatially-
resolved current distribution images.
Solutions from the magnetic tomography system are verified against data from an array of
ferrous cores, submerged in the electrolyte. Measurements are taken while the cell is
operated at a current of approximately 0.625 C. The current distribution is found to be very
uniform throughout most of the testing, although fatigue of the cell plates does lead to a non-
uniform distribution. The magnetic tomography system is tested on both uniform and non-
uniform distributions. Mean absolute errors of approximately 5 – 7 % are achieved. The effect
of model errors on solution accuracy is investigated.