charging an auto battery for a gate opener?
You need to "float charge" the battery with a voltage of between 13.8 and 14.1V.

The output of mini hydro is likely to vary with the speed of the water.

You will need to either electronically regulate the generated output of the mini-hydro to 14V, or possibly (not a good idea) use a constant head of water to drive the mini hydro, such that it always outputs 14V.

You should also look at a caravan battery, rather than a car battery.