You don’t need the relay or the voltage regulator, been there and tried that. Ni-MH batteries will regulate the voltage and give any excess current a place to go in the form of recharging the batteries. I use 4 AA solder tab Ni-MH batteries for 4.8 volts with the dynamo off and 5.2-volts with the dynamo engaged and making power at speed. The batteries place a high impedance load on the dynamo effectively regulating the system voltage to 5.2-volts with the dynamo engaged. 6-volts will work just as well. The regulated voltage with the dynamo engaged should be about 6.3-volts with 5 1.2-volt Ni-MH batteries. The reason I use 5-volts is the ability to run and recharge a GPS unit and a Cellphone from a USB port I have wired up to the batteries. USB ports have +5-volts and ground to power USB devices and it's common to find GPS units and phones that use that power for recharging as well. There are 4 pins in a USB connector. Pin-1 is +5-volts and is the red wire in the cable. Pin-4 is ground and is the black colored wire in the cable. I just dropped by a dollar store and purchased a USB extension cable and cut it stripping back the casing exposing the 4 wires. The other 2 wires are for data and may be cut even with the casing as these wires are not required or necessary and you don’t want them shorting to the power wires. I built my own LED headlight and taillight. The Headlight is a Luxeon 1-watt LED and Fraen 10X20 elliptical lens and holder. I came up with 4.9-ohms resistance to regulate current to 350mA by paralleling resistors and watching a current meter between the fully charged batteries and LED as I rode around with the dynamo engaged so I could not possibly overdrive it with too much current. My schematic is a little outdated. I have moved the 470mF capacitor from the rectifier board to the headlight LED leads. This was done so the LED could be driven directly from the Dynamo in the event of damage or loss of the batteries. This will decrease the life of the LED, as the full available current from the dynamo will flow through it. It should still last in excess of 5,000 hours. This would give plenty of time to replace the batteries when possible instead of trying to ride at night with no lights. Here is my schematic.
When S1 and S2 are closed the LED's are using 380mA with 120mA used to recharge the batteries. This is the recommended slow recharging rate for Ni-MH batteries and is about right when riding in city stop and go traffic. If riding long periods without stops I disengage my hub dynamo by opening S2 for 15 minutes of every hour to prevent overcharging the batteries. For bottle and BB type dynamos you would of course mechanically disengage it but S2 should be retained and opened when the bicycle is parked to prevent slow discharge across the bridge rectifier.
When powering USB devices I have S1 open and never use USB devices when also powering the LED lights. It is also easy to overcharge the batteries in the USB device so be extremely careful to watch the power meter in the device when the dynamo is engaged.