Maybe simply too much current demand while the battery is still too cold ... pity there are no more details re. the Boeing tests (NTSB report page 33, end of paragraph 1.7).
On the other hand, post# 1222 in the Other thread, re. E/E cooling, mentions air drawn from the cabin (?) ; however the FDR data has things like Cooling_Sensor_Aft_Temp_EE, about 20 degree F. How does it work really?