This is the Sigma BC 2006 cyclometer. The wheel sensor is wireless. The main unit, the wheel sensor, and the heart monitor strap each use a CR2032 battery. The power in the main unit shuts down automatically when not in use, but the wheel sensor broadcasts a digital signal 24 hours a day, 7 days a week. Battery life in the wheel sensor is disappointingly short. The batteries are relatively expensive. I wanted a way to control the battery drain from the sensor and provide a less expensive source of power.
Step 1: An Alternative to the CR2032 Battery
Pictured is the battery my digital camera uses. It is a CRV3 lithium photo battery rated at 3.0 volts. It actually produces a bit more than that. When it will no longer power my camera, it still has more than enough power and voltage left to power the wheel sensor on my SIgma cyclometer.
Step 2: The Original Battery Holder
This is the waterproof cover and battery holder for the wheel sensor. A metal tip built into the battery holder makes contact between the positive (+) edge of the battery and brass strip on the wheel sensor's circuit board. The exposed surface of the battery is the negative (-) terminal of the battery and it makes contact with a raised spring terminal on the wheel sensor circuit board.
Step 3: Battery Terminals on the Wheel Sensor Circuit Board
I had my battery modification to the wheel sensonr done before I decided to make it an Instructable. This graphic was done in Google Sketch Up with modifications in MS Paint. The yellow arrow shows the negative (-) battery terminal on the circuit board. I soldered a black wire to the center raised tip of this terminal. The red arrow points to the positive (+) terminal on the circuit board. I soldered a red wire to it near the arrowhead. If you look closely, you can see a darker arc at the other end of the brass strip to indicate the contact area where the metal tip on the battery cover rubs against this brass strip on the circuit board. I wanted to avoid that area with my soldering, lest I would need to undo my modification. I also waited until my warranty had expired before attempting this modification.
Step 4: Seal the Opening for Moisture
I considered drilling holes into the battery cover or the case of the wheel sensor for the wires. But, I decided to fill the opening with hot glue. When cooled it provides protection from damage due to movement of the wires and holds the wires in place. The hot glue also seals to protect against moisture. There is plenty of clearance for the magnet mounted on a wheel spoke. If I need to undo my modification I can dig out the hot glue fairly easily.
Step 5: Battery and Switch
I plan to make a holder for the battery. Currently the black and red wires are soldered to it and the battery is taped to the bicycle frame's top tube with plastic electrical tape. I added a sliding switch so I can disconnect the power to the wheel sensor when I am not riding. Later I will make a more permanent mount for the switch, too.
I could bypass the switch and run wires to power the main unit of the cyclometer, too. But, that battery lasts quite well.
On my inaugural ride with my used camera battery powering the wheel sensor my Sigma BC 2006 performed very nicely. I am pleased.