5v hydro power generator produces almost 12v
You MUST provide a link to specifications and descriptions of your device.
You have previous said that you are using this alternator

There are many sources of that alternator or ones that appear identical or very similar - there will be fewer manufacturers as suppliers buy them and name or rebrand them. The specifications given by suppliers who care may be accurate but it is common top find claimed specifications which are wrong or even copied from some other source with no care whether they are relevant.

These ones mostly claim 12V output.

Here are many similar ones for sale on "Alibaba".

In the case of your alternator, the brief specification given was always suspect. They claim 5V output but without a regulator of some sort the output depends on load and water volume / pressure and more. Even with a regulator, Vout can not reach Vrated for a given load if water volume is too low.

In your case the "proper thing to do" (and which your supervisors will probably want you to show if they are doing a good job) is to "characterise" your alternator for various loads and flow rates.  Example only


Add a load which should be "about right" at maximum output


As your supplier supplied specifications are too scrambled to be sure of power output you need to guess. Others like it say 10 Watts max so say 5W to start. Assume a 12V output.  Rload at 5W and 12V = Volts^2 / Power  = 12^2/5 = 28.8 Ohms.
So start by trying a 27R or 33R resistor rated at 5W to 10W dissipation.


Measure voltage across load and flow water through the turbine.
Measure flow rate by measuring time to fill a calibrated container.


You will want pressure across turbine in due course but for now flow is enough.


Find a flow rate where Vout does not increase much as flow increases.


If Vout is very low use a larger R.
If Vout rises to 12v+ at very low rates try a smaller R.  

You may find that it is a better match to 5V across a wide flow range when loaded. 


Now record V and power and I for various flow rates from say 2 x rate where power levels off down to a "trickle".
Repeat for a sensible range of load resistors.


You now know the basic alternator characteristics.



You can charge a 6V lead acid battery directly with this alternator PROVIDED that you manage the battery so it does not over (or under) charge.

If output is DC you MAY need a series diode (there may be one inside the alternator.)

If output is AC you will need a bridge rectifier and possibly a smoothing capacitor depending on other issues.