Size matters in batteries. The bigger the electrode surface area more current can be drawn. A very simple example is that of ten 1.2 V pencil cells connected in series at home vs. a 12 V lead-acid battery. Which one can start a car? The lead-acid battery-although the voltage of both batteries is the same.
Not only does size matter, but the rate of electron transfer also matters in batteries. Poor "lead-acid" cell, despite its all negative environmental image, still has the fastest (I don't know the record) electron rate transfer to the electrodes, hence you can draw more current out of it.
So microbattery is not feasible for large sustainable currents. A cardiac pacemaker is also a very small power source but its out is also very low. You can search about electric power sources for space programs. They must be lightweight and reliable. If I remember correctly, each "kg" adds a million to the cost of launching.