Electricity field question - need help!

Dec 2019

Apologies if anything is confusing, I'm translating it from my language. You can ignore the specific numbers, they don't mean much to this specific question.
This is some sort of a millikan experiment. There's an oil drop inside a constant field between two infinite plates. I know that:
1. There is the gravitational force Fg
2. There is the electric force from the electric field, Fe (the oil drop has a charge)
3. There's an additional friction force defined as f = a*v. a is a certain constant, v is the current velocity of the oil drop. This force goes against the direction of the velocity.

Due to these three forces, I'm told that the oil drop moves at a constant velocity. Meaning the sum of forces is 0.
There are a bunch of force-related question that I solved, but the last one is the one I struggle with - "Are there any energy conversions during the oil drop's movement?"

On one hand, the voltage doesn't change, meaning the field is constant, so the electric energy is constant. The velocity is constant too, so the kinetic energy should be constant as well, and so does the "friction" they defined me (v*a).
On the other hand, there's the gravitational energy that depends on the height (mgh) and as the drop goes up, the height changes. So would the energy change as well? What does it convert to? How do I explain that?

Thanks in advance to anyone who can help!! :)