Charging capacitor with higher voltage power supply
That should work as far as the capacitor is concerned, but the PSU might not like it. It all depends on that; it might draw too much current from it. But what you could do is buy a resistor to put in series. You can use the following two formulas to work out the resistance and power rating they'd need to be:

P = I V

R = V / I

So if you want to use the 4v PSU and it has a 2A rating, the resistor you should use is:

R = 4v / 2A = 2 ohms (or higher)

And the resistor's power rating should be: P = 4v x 2A = 8W (or higher).

The 4v PSU sounds like the best choice, because less power would be wasted in the resistor while charging the capacitor. Also, since it has a high current rating, it should be faster than the other PSUs. [However, as the capacitor charges more fully, it will slow down charge rate to about a third. (4-2.5)/4 = 37% speed.]

If you had a 150v PSU, you would waste about 98% of power but it would maintain 99% of charging speed.

To help you decide on a PSU: for the fastest charging speed, you want a PSU with the highest current rating. (So a 24v, 2A PSU would charge at the same speed as a 4v, 2A PSU). But the closer the PSU's voltage is to 2.7v, the less power you will waste on the resistor. So if you used a 24v, 2A PSU and a 4v, 2A PSU with resistors as calculated above, they would charge it at the same rate. But the 24v PSU would consume  a lot more power doing so, wasting it through the resistor.







simulate this circuit &ndash; Schematic created using CircuitLab

You could use the following circuit to stop at a given voltage:







simulate this circuit

That should charge the capacitor up to about 2.3v. (A bit of safety margin is a good thing)