Sierra Tools Battery-Operated Liquid Transfer Pump, Model# JB5684 https://www.amazon.com/dp/B000HEBR3I/ref=cm_sw_r_cp_api_i_0YcDCb9MZQRA4

Works really well for me. Just be careful not to get any big debris in the intake. Maybe super glue a piece of window screen to bottom of pvc pipe or something like that.