The lights on my bike were designed for 3x AAA for the headlight and 2x AA for the taillight. I don't like replacing AAA batteries and I did not want to buy a new rechargeable headlight and taillight so I rewired the existing lights to uses a single rechargeable RC battery(that I store in a top bag on the rear rack of my bicycle).

Step 1: Tools and Parts List

Tools needed:

1) soldering iron

2) wire cutters

3) heat gun (to shrink heat-shrink tubing)

Parts needed:

1) RC battery (LiPo)

2) RC battery charger

3) DC-to-DC Converters (1 adjusted to 4.5V and the other to 3.0V)

4) cutoff switch

5) 2-conductor wire and 2-pin (polarized) connectors

6) heat shrink tubing

Step 2: The RC Battery (and Charger)

The battery I chose is a 4500 mAh RC battery from a quadcopter project. This is way overkill for this project and I only used it because I already had it. You may want to choose a smaller/cheaper RC battery (1000 mAh should be plenty .. unless you're using some very high powered offroad headlights).

NOTE: if you have any friends in the RC hobby, you can ask them for their old/weak batteries. An RC battery that no longer has enough capacity to run a quadcopter, still has plenty of life left in it to run bike lights.

Step 3: DC-to-DC Converters

A cheap and readily-available source of DC-to-DC converters are car cell-phone chargers. I often find them at a thrift stores for $.50 or less. The output voltage of the DC-to-DC converters needs to be adjusted to 4.5V for 3x AAA replacement and to 3.0V for 2x AAA replacement. I have another instructable that covers modifying the output voltage of (cell phone charger) DC-to-DC converters.

https://www.instructables.com/id/Modifying-the-outp...

You can also find adjustable DC-to-DC converters on eBay. An example of one of these is shown in the second picture. These converters are simple to adjust (by just turning an adjustment screw on a potentiometer) and they are not very expensive (you should be able to buy 2 of them for < $5).

Step 4: Main Cutoff Switch

I re-purposed an inline light switch to disconnect the battery when it's not in use.

NOTE: The orange connectors I used are non-standard connectors. I used them because I have a cheap supply of them from a local surplus electronics shop. You'll need to find another source of a more standard connector ( I have no suggestions but I'm sure you'll find a creative solution)

Step 5: Taillight Mod

Adjust the DC-to-DC converter to 3V (you can use either an eBay-sourced converter or a hacked cell-phone charger https://www.instructables.com/id/Modifying-the-outp...) and create a short pigtail with the DC-to-DC converter located inline (the exact length will depend on how you want to mount it on your bike). Pop open the taillight and remove the 2 screw holding in the circuit board. Solder the 3V end of the pigtail to the circuit board (make sure you get the polarity correct). You'll also need to drill a hole in the back end of the taillight to route the 3V wire back to the DC-to-DC converter and the battery.

NOTE: The converter output voltage level does not need to be exact .. experiment and see what works best

NOTE: I used heatshrink tubing to cover/protect the inline converter and I used a little hot glue to seal both ends of the heatshrink tube.

Step 6: Headlight Mod

Adjust the DC-to-DC converter to 4.5V (you can use either an eBay-sourced converter or a hacked cell-phone charger https://www.instructables.com/id/Modifying-the-out... and create a long pigtail with the DC-to-DC converter located inline (the exact length will depend on how you want to mount it on your bike). Pop open the headlight and remove the battery holder. Solder the 4.5V end of the pigtail to the battery holder (make sure you get the polarity correct). You'll also need to drill a hole in the back end of the headlight to route the 4.5V wire back to the DC-to-DC converter and the battery.

NOTE: The converter output voltage level does not need to be exact .. experiment and see what works best

NOTE: I used heatshrink tubing to cover/protect the inline converter and I used a little hot glue to seal both ends of the heatshrink tube.

Step 7: Putting It All Together

After testing all the parts separately, connect them together and test everything again. The battery and converters should be kept out of the weather; I keep mine in a top bag on the rear rack of my bike.