I don't have a surface book myself but have read in many places that the bases battery is used first then it switches over to the tablet portions battery the only issue is that let's say you use the tablet portion of the SB and it is now on 50 percent battery and you reconnect it to the keyboard Base it will not charge from the bases battery becuase that would actually be pointless in the biggest way which is you would be wasting battery power through electrical transfer. So the only way for it to charge the upper battery is if the SB is plugged in to the wall.I thought SB used the base battery first? Mine seems to be using both batteries together. It makes since to use the base battery first so that if you pull off the clipboard you have a full charge.
Which version of the surface book do you have I've heard of the i7 model of the surface book has some power limit throttling due to it using more power than the battery can supply at max load so that could be the reason so both of the batteries are used together to provide the demand of the system but I'm not too sure. I don't have a surface book. Maybe sharpcolorado can help or confirm the issue becuase i think he's one of the many people who have a surface book.It does, but not exclusively. The base battery will drain down to some partial level first, then the clipboard battery will join in. "Currently", my base is at 67% capacity, and my clipboard is at 97%. I'm not sure what the turnover point is, and I suspect it's a sharing of the load at that point.