We have been wanting to design, build and supply an open-design, relatively inexpensive but fully functioned charge regulator for solar PV, wind, pedal and small hydro systems for a while now.
The majority of off-grid renewable energy systems are based upon lead acid batteries. Lead-acid batteries need to be protected from over-charging by the renewable energy source and over-discharging from the loads. This can be one with a series regulator (in the case of solar PV) or a shunt regulator (for PV, hydro and wind).
The charge regulator pages here give the full design for a relatively simple but fully programmable and open-source charge regulator. Make one yourself or buy the kit.