  The increasing deployment of renewable energies requires three fundamental
changes to the electric grid: more transmission lines, a flexibilisation of the
demand and grid scale energy storage. Liquid metal batteries (LMBs) are
considered these days as a promising means of stationary energy storage. Built
as a stable density stratification of two liquid metals separated by a liquid
salt, LMBs have three main advantages: a low price, a long life-time and
extremely high current densities. In order to be cheap, LMBs have to be built
large. However, battery currents in the order of kilo-amperes may lead to
magnetohydrodynamic (MHD) instabilities, which - in the worst case - may
short-circuit the thin electrolyte layer. The metal pad roll instability, as
known from aluminium reduction cells, is considered as one of the most
dangerous phenomena for LMBs. We develop a numerical model, combining fluid-
and electrodynamics with the volume-of-fluid method, to simulate this
instability in cylindrical LMBs. We explain the instability mechanism similar
to that in aluminium reduction cells and give some first results, including
growth rates and oscillation periods of the instability.
