On my R5 with the BG-R10 grip, the battery charge is similar on both batteries which indicates that it draws from both batteries. That's starting with both batteries at 100%. Then shoot some pictures and both batteries will indicate approximately the same lowered capacity. I just started charging two Canon batteries and they were both about 70% which indicates both were drawing at the same time. Now when you are charging the batteries in the grip, it charges one at a time as indicated by led lights. Then when both are charged, the led's go out. As a side note you cannot charge non-Canon batteries in the grip. There must be only Canon batteries in the grip for it to charge.