OK guys. A lot of times, the sign of the current is ignored or even the sign of power. A 16 MW power plant is actually -16 MW. A solar cell with a Jsc of 100 ma/sqcm is actually -100 mA/sqcm. These make the equations work. Power generated is negative and power dissipated is positive.
That out of the way, there are two types of power supplies. 2 quadrant or 4 quadrant. These quadrants are the same in math with the X axis being V and the Y axis being I or J (current density).
So you have 4 quadrants:
V+, I+
V-, I-
and
V+,I-
V-, I+
The first two would be what's possible with an alkaline battery.
The latter is possible with re-chargeable battieries. It's possible, but not recomended to actually reverse the polarity of a NiCad battery.
if we were charging a battery in a car we might have a meter that shows -25 A to +25A. This is actually backwards from convention. When the alternator is delivering power, the current is positive, BUT this gage is relative to the battery charging or discharging. 10 A is 10 A being delivered by the alternator into the battery.
So, what I am trying to say is that the real number's sign is dependent on the meter's label.
When charging is involved, all quadrants are POSSIBLE, but V- and I+ is not recommended.
Purchased power supplies are normally 2 quadrant. They won't tolerate a battery discharging into them. A 4-quadrant power supply and/or an electronic load will.
Take a look here:
https://www.youtube.com/watch?v=Gg6eHcfnsYs
Again, the signs are flipped. In reality, the graph should be flipped around the x-axis, then a negative current and positive voltage would be generated power SIGN(V*I)=-1. The X-axis is voltage.
One parameter of a solar cell is short-circuit current. This is when the voltage is zero and the current is negative.
But remember what I said, watch the signs. If this was a solar array charging a battery and the ammeter indicating like a car, you may well have zero voltage and positive current.
While I'm here, there is conventional current and electron flow because Ben Franklin got the signs wrong. Conventional current flows from positive to negative of a battery. Electrons go the other way. Electrons don't flow, but rather collide with each other like a chain reaction. Picture a row of bumping cars standing still and apart. Then a car rams them. There is an "appearance" of movement.