Thanks Eric and on1aag!
on1aag:
Yes, you're right, the pn junction acts like a diode and allows current in one direction. That's ok if I make that direction the battery incoming current. My main concern is about battery supplying power when it's deeply discharged.
However, MOSFETs datasheets don't have characteristics for the reverse current operation, even the datasheet that Eric mentioned doesn't have any information about that.
That would be usefull at least knowing the PN juntion forward voltage @ Inom.