You can either use the traditional external battery, or mount the meant-to-be internal one on the outside somewhere, then put it in the seatpost when you get a free frame from Specialised for your brand loyalty.
Honestly, having one system to troubleshoot is going to be more straightforward than two - imagine if your focus had a manual gearbox supplying power to one wheel with an auto box driving the other.
You're comparing a front derailler with TT friction shifter, literally the most simple shifting device I've ever used to a weird car gearbox... that's the worst attempt at making me poorer I've seen :P
You can either use the traditional external battery, or mount the meant-to-be internal one on the outside somewhere, then put it in the seatpost when you get a free frame from Specialised for your brand loyalty.
Honestly, having one system to troubleshoot is going to be more straightforward than two - imagine if your focus had a manual gearbox supplying power to one wheel with an auto box driving the other.