Motorola has a device that is powered by stomach acid. Given that a battery consumes the electrolyte as it generates a charge, how much power could be generated using a device such as this? Let's assume the electrodes are large enough that the electrolyte is the limiting factor in this reaction. Would the body just generate more acid to compensate? Would there be any physiological side-effects from using such a battery?
Well a battery works on the principle of dissolving one of the electrodes with the electrolyte. This means that you would have a solution of the metal and electrolyte in your stomach. If consumed in large quantities or in quick succession, then you most probably would notice some physiological effects, however, I would assume they'd be very minor given the mass of the tablet in question. Other than under those circumstances, I would expect the physiological effect to be non-existant. Given this has been approved by the FDA, you can be pretty confident that it's safe.
The electricity generate depends mainly from the electrodes. This is the same misconception of lemons battery. In fact the energy of every battery comes from the metals and not from the acid use as electrolyte, as you can't generate energy from lemon juice but from the different potential between copper and zinc coin so you can't generate energy from a stomach acid but with it. However $HCl$ is not used normally for battery so I think you could not produce a big amount of energy even if you ingest very big electrodes.