This is the kind of experiment in which you need to make several more kinds of measurement to determine exactly what is going on. For example, did you measure the current at a given voltage as you changed the area of the electrodes? Alternatively, did you try to move the electrodes closer together and watch the voltage? Why this is necessary, is that electrolytes conduct currents both ways between the electrodes.

(The other path is called battery leakage. In general, the voltages you measure should be a strong function of the different electrode types, while the current that the battery delivers is related to both the separation of the electrodes (declining as the electrodes are separated,) the area of the electrodes (proportional to the area of the wetted electrode surface), and the resistance of the measurement circuit.

The decreasing voltage you measured as the electrolyte concentration is increased may be caused by the increased leakage in the battery itself, or by possible changes to the electrode which only occur for high concentrations of the electrolyte.

If you have the time, I suggest you design a few additional experiments based on your ideas of what is happening to try to find out for yourself what is going on.

It's a current-thing, from the sound of it. The more electrolytes there are will make the solution a better conductor, and that means that at a given voltage, more current will flow through it than a less electrolytic solution.

There is, however, a limited amount of charge separation possible, and the charge separation is what creates the voltage.

So, in putting more strain on your battery (or whatever power source you are using), the overall voltage is lessened because the power is increased.

The more concentration of chemicals you have, the bigger current flowing through the electrodes. The relationship for current and voltages is:

V = I x R

V is for voltage,
I is for current and
R means the resistance that the current finds in its way. If you keep R constant, when the current increases, the voltage has to drop in order to maintain the relationship among them. And this is always the case: more current, less voltage, and vise versa, less current, more voltage. We keep R constant, as it is.

When R is big, the current will not flow as easy as when R diminishes. When you have short circuits, the current grows a lot, this means that R is very low in value, and V drops completely.

The voltage produced in a cell is usually only dependent on the types of metal and electrolyte, not the concentration of the electrolyte. However,if you have a higher concentration electrolyte, generally the cell can produce more current, which is a different electrical measurement from voltage.

With some measuring equipment, it may be hard to separate these two. One way to think of the difference is that current tells how much electricity is flowing, and voltage tells what kind of barrier the electricity can get over.

The more current, the more electricity is flowing, and the higher the voltage, the higher the barrier it can overcome. Just as an example, have you ever touched a doorknob and got a nasty shock? That shock was several thousand volts, but had very, very little current so it doesn't hurt you much at all. On the other hand, a battery can supply lots of current, but has a small voltage, so you can't shock yourself with it because your skin is a barrier.

A practical outcome of all this can be seen in a car. Those batteries are only 12V; you can get the same voltage from eight AA batteries, but the current the car battery can supply is huge compared to those little AA batteries. Any ideas why?