I was always told it would deplete the smaller battery first then the bigger battery. I didn't buy this so I tested it for myself. They always were drained proportionally. If a 1000mAh battery was drained by 200mAh, a 2000mAh battery in parallel would be drained 400mAh or both by 20%. Now I will say that I do always use packs from the same manufacturer, same brand of cells, etc. That might make a difference, not sure.