Cell Phone Battery Powered Bike Lights




EDIT: Ok, well I've just been informed by sladek that this has the potential to blow private bits off. So, be EXTREMELY careful as I cannot delete this instructable. I guess it shows that I have no real training in electronics past my college electrical engineering 101 gen ed.

"sladek says:
Cellphone batteries are lithium based, and with lithium batteries it's dangerous to drop them below their minimum voltage. A full charged 3.7V battery is roughly 4.2V, once it disipates down to 3.7V, it's minimum safe voltage, the battery can swell and explode, without the need for ignition."

I would suggest not even trying this with a phone battery. Replace with a different battery.

I've been a fan of this site for a while, but this is my first instructable and it is very simple. Hopefully it will open the door to other ideas using cell phone batteries to power things.

I know the bike light underglow idea has been done before, but when it came to the batteries, that was a problem for me. It had to charge up a LED light, and I wanted something small, lightweight, and rechargeable, since I was using it for my bike. I didn't want to spend money on D cells (don't have a recharger for them) or for big 6v batteries (no recharger and too big), like in maxwell and shammallamaman's instructables, where I got the initial idea.

I had a couple cell phones I didn't use any more, so I decided to do the green thing and make them useful again. Most cell phone batteries are around 3.7 V. The ones I had were 3.6 and 3.7 V, so it worked out perfect.

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Step 1: Parts List


-2 cell phone batteries
-12 V LED light kit - I bought it from Walmart auto section, but you can make one easy. Wire up 2 3 mm LEDs and stick them in a clear plastic tube, one on each end.
-100 ohm resistor
-tissue or foam
-rubber bands
-cassette tape case or Altoids case - Altoids case would probably work better. All I had was the cassette case and the solder melted through.
-zip ties
-hot glue (optional)

-Soldering iron
-Wire strippers (optional, can use scissors carefully)
-Hot glue gun (optional)
-Super glue

Step 2: Step 1

All of this is straight forward if you have worked with electronics. I put the batteries in series, soldered in the resistor, then the switch, then the LED.

I want to point out that you have to have an entry and exit for the switch and LEDs and arrange everything before you solder. May seem obvious, but it will save you time from having to go back and cut and resolder everything. Trust me, I know :(.

Step 3: Step 2

This is the tricky part. I couldn't solder the batteries onto anything. They could only touch the metal. This is because to recharge, I had to put the batteries back into the cell phone, where the plug in is. That's the reason for the tissue paper and wood.

Put the wood in the cassette case to make the batteries fit in tight so they don't move around. I also used a ton of solder so if it did move around, it would still make contact.

Once the batteries were in place, I put the tissue paper in to keep the batteries from moving around.

Step 4: Fin

Everything is in place now. Super glue the soldered parts (except for the batteries of course). Use the rubber bands to keep it closed and slap it on your bike with the zip ties.

Next bike project...spinners? body kit? flame paint job? NOS? The sky's the limit.

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    11 Discussions


    3 years ago

    Cell phone batteries have built in charging circuitry to prevent over charging and discharging.

    doo da do

    7 years ago on Introduction

    I like the recycle thing, I think hair ties that are about 1.5in in diameter would work and last a lot longer than the bands. Just a suggestion. Doodado

    doo da do

    7 years ago on Introduction

    Those hair ties would work good, they look about 1.5 inch in diameter and last
    Longer than the bands. Just a suggestion. Doodado


    8 years ago on Step 3

    How did you wire the batteries to charge them/ power the leds, cant find anything on how to do that


    8 years ago on Introduction

    Now what would be even cooler (especially for commuters like me) would be some way at the end of the day I could just plug my cell phone to my light system via the USB port, and power the lights right off of my phone. I have to charge the phone every night anyway!


    9 years ago on Introduction

    Actually Sladek is only partially right. It is not a good idea to drop lithium batteries below their minimum voltage nor charge them at too high a voltage. However it is common for cellphone batteries to have current regulation of some sort built onto them. Take one apart and see for yourself there is circuitry there to regulate discharge and it will cut power once it drops too low. The sensor the cellphone reads is just an additional safety measure to watch the temperature.  I have used cell phone batteries in many projects as a power source. I have seriously abused them and charged them with chargers never meant for that purpose. I have never had one explode, swell, or leak. I think the dangers of exploding lithium batteries has been over hyped.  The only real danger comes from dead shorting a fully charged cell. That will lead to extreme heat and a possible fire/explosion if the batteries on-board circuit doesn't burn out first. 


    10 years ago on Introduction

    Cellphone batteries are lithium based, and with lithium batteries it's dangerous to drop them below their minimum voltage. A full charged 3.7V battery is roughly 4.2V, once it disipates down to 3.7V, it's minimum safe voltage, the battery can swell and explode, without the need for ignition. The way all lithium polimer gadgets overcome this is by using a cut-off circuit that kills the power at minimum voltage. Overcharging about 4.2V is just as dangerous. Just my 2 cents

    very nice, this fixes the problem i was having, i was using a litlle kid's electric toy car's battery, but those are expensive


    11 years ago on Step 2

    oops, I had it setup with the switch before the resistor, but it should work either way.