  We consider point particles in a table made of two circular cavities
connected by two rectangular channels, forming a closed loop under periodic
boundary conditions. In the first channel, a bounce--back mechanism acts when
the number of particles flowing in one direction exceeds a given threshold $T$.
In that case, the particles invert their horizontal velocity, as if colliding
with vertical walls. The second channel is divided in two halves parallel to
the first, but located in the opposite sides of the cavities. In the second
channel, motion is free. We show that, suitably tuning the sizes of cavities,
of the channels and of $T$, non--equilibrium phase transitions take place in
the $N\rightarrow \infty$ limit. This induces a stationary current in the
circuit, thus modeling a kind of battery, although our model is deterministic,
conservative, and time reversal invariant.
