As part of the German research project SkIES - Scalable Integration of Energy Sharing, this study addresses questions on how the flexibility provided by a shared battery in an energy community can be coordinated and examines its effects on the integration of local renewables, cost, and optimal battery sizing. The study uses a simulation framework drawing on a pool of synthetic and heterogeneous load profiles, representing household with and without rooftop PV and behind-the-meter storage, alongside shared PV and wind assets. Three scenarios representing increasing coordination are introduced, a rule-based baseline, pooling of residual household flexibility after individual optimisation, and full centralised dispatch minimising residual grid demand. Each is evaluated under flat and dynamic spot-price tariffs, with battery capacity systematically varied. Comparing costs, curtailment, and self-sufficiency by source reveals that coordination primarily changes which asset supplies flexibility. Central optimisation reallocates household batteries' discharge from directly covering their own owner's load to a pool-facing role, curtails renewable generation at roughly a third of the baseline's rate, and lowers cost, though substantial cost reductions are realised only when coordination is paired with a dynamic tariff. The cost-optimal battery size is set almost entirely by the tariff and barely shifts with coordination mechanism or community size.