LED Bicycle Light Powered by A Shimano Generator Hub and Batteries

My Latest Experiment November 2008

Two LED flashlights run by batteries and hub dynamo
This is the easiest project light I have built and the simplest to wire and construct.

The light is very visible to oncoming traffic.  The lights are always on when the bike is moving.  The Shimano generator efficiency is such that it is better to leave the lights on all the time (See Dynotest of bicycle generators ).

The effective light on the road is similar to 2.5 watt halogen bike lights.

The design  goal is to have a light that is always available, gives good light for city commuter riding at any speed or when stopped, and the batteries are recharged by energy from the front wheel hub generator.

LED lights have characteristics that are optimal for this type of light.  
LED lamps have high efficiency for converting a wide range of low power into visible light compared with incandescent lights which are  only efficient at a very narrow range of voltage and power.

As long as they are not overpowered, LED lamps are more reliable than other lights. 

These two small LED flashlights were chosen for several reasons.    See the flashlight below.

Note this Caveat:  The ability of the system to keep the batteries charged depends on the ratio of stopped time, slow riding, and faster riding, and night riding that one typically does.  With my use the batteries stay optimally charged.  See additional cautionary note below.

Shimano DH-3D71 hub generator
The hub generator I have used is Shimano Sport Dynamo DH-3d71 Model.  (I am not certain if there is a technical difference between what is meant by the two terms: "generator" and "dynamo".)

The shimano generator, and the tire driven generators I previously used, produce AC current.  If one wants to power LEDs or charge batteries then DC current is required so some strategy is needed to convert AC to DC.

I bought the generator hub at Jensen USA.  My reading suggests that wide range of bicycle generators are available.  I previously have used an inexpensive bottle generator sold at my local hardware store.  The wiring strategy for the two generators is identical.  The voltage and power output for the two generators is very similar.

Here is a link to a comprehensive test of a number of bicycle generators;
Dynotest of bicycle generators

The power available from standard generators sold for bikes is limited to 3 watts.  The test noted above shows that the power curve for these generators is fairly similar, developing quickly at low speeds.

Tires with rough tread will not work with the tire driven bottle generator.

For my rear light I have chosen to use an independent battery powered flashing red LED light.  The flashing light uses so little power that the dependability of the batteries is not a major problem.  Any good battery will last more than a year for my use.

I have recently converted to Sanyo Eneloop rechargeable batteries because they are less prone to loosing power over time than other NiMH batteries.
Small flashlight with 28 LED using three AAA bateries

The two small LED flashlights were chosen for several reasons.
Low cost: ~ $14.00 each in my local area.  Less on-line.

AAA batteries are small and provide plenty of power reserve for my range of riding conditions.  I use Sony eneloop rechargeable batteries because they keep a charge longer than other NiMH brands that I have tried.

The large number of LED lamps give an effective way to limit overpowering by the small 3 watt generator.  At optimal voltage and current these 56 LED would use a sum of just over 4 watts.  Even at 30 MPH the generator I have will not produce too much power for the 56 LEDs.

The water resistant case and built in switch are much more compact than any of the lights I have managed to construct on my own.

The design of the light lends to a very easy strategy of connecting to the generator.

Using two flashlights allows me to use each cycle of the AC generator.  The lights are alternately on with each change of cycle.  At walking speed and slow bike speed the frequency of the AC cycle with the hub generator is such that oncoming viewers will see a noticeable flicker of the lights, mostly because from that view one can see the side to side change from one light to the other.  From the rider's viewpoint, even at slow speed this flicker is not noticeable because the two lights are aimed at the same place on the ground.  I actually like the fact that the oncoming traffic will notice an attention getting flicker of light.

I have found this strategy of using separate lights and batteries for each phase of the AC current is more efficient at producing light than by running the AC current through a converter which then runs the entire system on DC.

View of the soldered wire connection

A view of the one solder connection that will be made on each flashlight.  The 16 gauge insulated wire is soldered to the positive post of the LED circuit board.  This is the point where the battery pack makes a spring loaded contact with the LED board.  A small hole is drilled through the case to allow routing the insulated wire.  The hole through the case must be carefully chosen so that it does not interfere with the circuit board or the threading of battery section of the case.

Note: do not overheat the circuit board when soldering.  Use care to keep the solder smooth so that the battery pack can make good contact.

The negative wire is screwed directly to the case (shown below).  The case has three places that unscrew.  One at the switch end.  A second threaded joint at the place shown in this photo.  And a third larger diameter threaded joint on the lamp end.  The case is aluminum.  I drilled and set the negative post screw so that it locks the large threaded joint near the lens and locks the internal threaded ring that supports the LED circuit board.  This assures that the positive wire does not accidentally become pinched if this large threaded section were to come unscrewed.  The case is aluminum, and the drill hole is slightly smaller than the screw.  The steel screw will self tap into the softer aluminum.  

A view of the negative wire attached directly to case, and the positive wire coming out of the case:
View of ground wire

My wiring plan has diodes added for two reasons:  

First, to make sure that when the battery switch is on, the current cannot short through the generator wiring.  

Second, to allow a voltage differential so that the voltage over the light is balanced with the expected 3.6 volts of the charged battery.  There is a small diode (1N914) that bypasses the switch so that if the battery voltage goes below 3.6 volts, some current will flow in reverse through the battery, thus charging the battery.  When the light switch is on, this small diode has no function.

This image shows the switch that is in the flashlight.  For the purpose of this plan I think of this switch as a battery switch, because the light is always on regardless of the position of this switch.
  End Cap Switch

A small hole is drilled beside the switch, appropriate for a 22 gauge insulated wire.  The hole goes through the end cap of the flashlight.  The wire is soldered to the diode.  The other end of the diode goes between the contact spring and the switch contact that touches the spring.   The direction of the current flow for the diode placement is such that only a reverse flow of current through the battery will pass through this diode.
  End cap view A

This view shows the small charging wire from the small diode, exiting the flashlight case:

End cap view B

Xray view of wiring plan
The wiring plan.  The top view is an x-ray view of one of the two lights.  The  lower view shows the wiring for both lights.  The AC input is reversed for the second light.

Diodes : IN 4001 for the full current across the light.  These diodes should be able to take 1 amp of current.  

A smaller diode: 1N914, bypasses the switch in the flashlight.  This diode allows charging the batteries and does not pass as much current.

Capacitors: 10uf or larger, electrolytic.  These serve a simple function and size is not critical.  

The switch shown is built into the flashlight.  If it is left open, the generator will still light the LED lamps, but the lamps go off when the bike is stopped.  If the battery charge is low, a small current will flow through the batteries in reverse, charging the batteries.  When the batteries get to about 3.6 volts, the current is all going through the LED lights.

There is no current or voltage limiting function added to the circuit.  I have tested it to make sure that the generator cannot supply too much power even at 30 MPH.  

Additional Cautionary Notes:
1. Other generators may have more power, so I suggest actually testing the current load at each positive connection for any other generator system.
wiring plan

2. When the battery switch is in the on position, the battery is supplying energy to each LED light for half of the run time, while the AC current is supplying energy to each light the other half of the time.  This will cause the battery to loose voltage over time.  As the battery gets low, the battery will send less energy to the LED lights and some of the AC energy will charge the battery, but since the AC is only energized half of the time, the battery voltage will still slowly drop below the nominal 3.6 volts of the battery pack.  

3. With the battery switch off, the lights are powered by the AC whenever the wheel is turning.  This is also when the batteries will most effectively charge and remain close to the nominal 3.6 volts of the battery pack.

4. In my use, I ride more time in the day than at night, and more time with the battery switch off.  I have not had a problem with the batteries staying charged.  

5. If a person rides all of the time with the battery switch on, I am not sure what voltage the batteries would eventually reach.

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