During charge and discharge of a lithium battery, intercalation of lithium ions into the electrodes can cause their noticeable expansion, compressing the soft separator between them. To assess the role of these effects on the battery performance, it is necessary to know the response of the battery separator under compressive loading. Here we develop a model for predicting the elastic response of a commercial separator immersed in fluid to compression at different strain rates. We show that the response of the separator is determined by combination of viscoelastic behavior of the polymer skeleton and poroelastic behavior, due to the flow of the fluid in the pores. Poroelastic behavior causes effective stiffening of the separator, which increases with the strain rate. For a sample of ca. 1 cm in diameter these effects become pronounced at strain rates greater than or similar to 10(-3) s(-1) and have to be taken into account in coupled mechano-electrochemical models for lithium-ion batteries. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.