How to estimate the efficiency of a battery?

Hello guys,

Currently I am working on a project that requires estimating the efficiency of a battery (lithium-ion in this case). I am using the generic battery block that is part of the Simulink library. Is it possible to estimate this efficiency using the block, and if so what would be the best way to do it?. The battery is feeding a load that varies with time and when the load is low a source charges the battery.

Thanks