We think of a body of water as being flat, but it is not. Gravity is also in the dynamic and the surface on a large body of water follows the gravity field (which might not be uniform).
Again on our body of water, Brownian Motion will sometimes throw a water molecule toward the surface, breaking surface tension, and be lost. Conversely, a nearby water molecule may rejoin the pool. And, at a high temperature (boiling) all of the molecules will want to leave the pool. We can look at this on a more precise level by examining "partial pressure", sometimes equated with "vapor pressure". If you place a liquid or solid substance in a sealed, evacuated space, this evacuated space will eventually become filled with evaporated substance at a pressure such that there is equilibrium between departing molecules and arriving molecules. This is the vapor pressure of that substance at a specific temperature. When a liquid boils, its partial pressure equals the surrounding atmospheric pressure and the molecules leave the liquid state.
In our battery, there will be similar thresholds. I've never seen any charts giving the minimum voltage differential below which one cannot strip or force an electron into the battery chemistry. You could crudely measure this with a battery, variable voltage source, and an ammeter.
In the charging schemes that I've seen, charging is regulated by current control. There is a maximum safe voltage to be applied in order to avoid secondary breakdown mechanisms. Cell voltage can be used to sense the endpoint of the charge.
