  A state-space model for Li-ion battery packs with parallel connected cells is
introduced. The key feature of this model is an explicit solution to
Kirchhoff's laws for parallel connected packs, which expresses the branch
currents directly in terms of the model's states, applied current and cell
resistances. This avoids the need to solve these equations numerically. To
illustrate the potential of the proposed model for pack-level control and
estimation, a state-estimator is introduced for the nonlinear parallel pack
model. By exploiting the system structure seen in the solution to Kirchhoff's
laws, algebraic conditions for the observer gains are obtained that guarantee
convergence of the estimator's error. Error convergence is demonstrated through
an argument based upon Aizerman's conjecture. It is hoped that the insight
brought by this model formulation will allow the wealth of results developed
for series connected packs to be applied to those with parallel connections.
