# Bicycle headlight from old mobile phone. What type of light? **URL:** \ **Category:** Factual Questions\ **Created:** [May 12, 2004, 1:31am UTC](https://boards.straightdope.com/t/bicycle-headlight-from-old-mobile-phone-what-type-of-light/244715 "2004-05-12T01:31:38Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![antechinus](https://avatars.discourse-cdn.com/v4/letter/a/fbc32d/32.png) [@antechinus](https://boards.straightdope.com/u/antechinus)\ **Post date:** [May 12, 2004, 1:31am UTC](https://boards.straightdope.com/t/bicycle-headlight-from-old-mobile-phone-what-type-of-light/244715/1 "2004-05-12T01:31:38Z")
My idea is to convert my old mobile phone into a headlight for my bicycle. This will make recharging easy since I have three of the same old phones, giving me a couple of spare batteries and rechargers. The batteries are a 7.2 v Li ion. I intend to pull the guts out of the phone and use the battery connecters to provide the power to a light that I will fit into the body of the phone. Then rig up a bracket to mount the light on the handlebar. What is the best light to use for this sort of battery? Bright LED’s, halogen, something else? Is it dangerous - can I inadvertantly blow up the battery? Do I need any special circuitry? I can recharge every day, so the power does not have to last long, but the brighter the light the better. ---
**Author:** ![mike1dog](https://avatars.discourse-cdn.com/v4/letter/m/ba9def/32.png) [@mike1dog](https://boards.straightdope.com/u/mike1dog)\ **Post date:** [May 12, 2004, 1:38am UTC](https://boards.straightdope.com/t/bicycle-headlight-from-old-mobile-phone-what-type-of-light/244715/2 "2004-05-12T01:38:45Z")
Seems like a lot of work, since a headlight wouldn’t cost more than twenty bucks. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [May 12, 2004, 2:16am UTC](https://boards.straightdope.com/t/bicycle-headlight-from-old-mobile-phone-what-type-of-light/244715/3 "2004-05-12T02:16:10Z")
> [@antechinus](#): > > What is the best light to use for this sort of battery? Bright LED’s, halogen, something else? > Is it dangerous - can I inadvertantly blow up the battery? > Do I need any special circuitry? With regards to LEDs, yes, yes, sortof. LEDs have no internal resistance. Put that across a LiIon batt pack and the pack will heat up and cause problems. If the cells burst open, emergency room type problems. LEDs need a resistor in series to handle this. (Some come with one builtin.) But no Radio Shack LED is going to do you any good. Bright LED packs are still something of a specialty item. I know how to do that sort of stuff and it still doesn’t seem worth it… ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [May 12, 2004, 2:59am UTC](https://boards.straightdope.com/t/bicycle-headlight-from-old-mobile-phone-what-type-of-light/244715/4 "2004-05-12T02:59:49Z")
You need to design and build a circuit that prevents overcharging and over-discharging. The phone has one built in, but it’s part of the circuit board. This would be a difficult project - if you value your time at all, it’s much better to buy a high-end commercial light. Also, there isn’t much energy in a cell phone battery. The battery in mine is 3.6V 630mAh. That’s less energy than a single rechargeable AA battery. ---
**Author:** ![antechinus](https://avatars.discourse-cdn.com/v4/letter/a/fbc32d/32.png) [@antechinus](https://boards.straightdope.com/u/antechinus)\ **Post date:** [May 12, 2004, 3:21am UTC](https://boards.straightdope.com/t/bicycle-headlight-from-old-mobile-phone-what-type-of-light/244715/5 "2004-05-12T03:21:22Z")
> [@mike1dog](#): > > Seems like a lot of work, since a headlight wouldn’t cost more than twenty bucks. This basic rechargable [2.4 watt Halogen bulb](http://www.deanwoods.com.au/store/prod37.htm) is $69AUD - expensive enough. A good [10-watt wide-angle halogen bulb](http://www.deanwoods.com.au/store/prod461.htm) light is expensive - $185. This [30 watt bulb](http://www.deanwoods.com.au/store/prod464.htm) is very bloody expensive - $419. I want a very bright light and dont feel like spending lots if I can do it myself. From **ftg’s** post, it looks like a halogen would be best. If I use a 6 volt 15 watt halogen light would I have to do anything to avoid annoying things like explosions? ---
**Author:** ![antechinus](https://avatars.discourse-cdn.com/v4/letter/a/fbc32d/32.png) [@antechinus](https://boards.straightdope.com/u/antechinus)\ **Post date:** [May 12, 2004, 3:26am UTC](https://boards.straightdope.com/t/bicycle-headlight-from-old-mobile-phone-what-type-of-light/244715/6 "2004-05-12T03:26:17Z")
> [@scr4](#): > > You need to design and build a circuit that prevents overcharging and over-discharging. The phone has one built in, but it’s part of the circuit board. This would be a difficult project - if you value your time at all, it’s much better to buy a high-end commercial light. > > Also, there isn’t much energy in a cell phone battery. The battery in mine is 3.6V 630mAh. That’s less energy than a single rechargeable AA battery. OK you have convinced me. I forgot about the charging circuit. Thanks. Just seems like such a waste of good batteries and powerpacks. Oh well, I’d better start saving. ---
**Author:** ![mike1dog](https://avatars.discourse-cdn.com/v4/letter/m/ba9def/32.png) [@mike1dog](https://boards.straightdope.com/u/mike1dog)\ **Post date:** [May 14, 2004, 6:00am UTC](https://boards.straightdope.com/t/bicycle-headlight-from-old-mobile-phone-what-type-of-light/244715/7 "2004-05-14T06:00:43Z")
I’ve actually used one of [these.](http://www.rei.com/online/store/ProductDisplay?storeId=8000&catalogId=40000008000&productId=47678095&parent_category_rn=4500821) Unless you’re really stuck on a halogen bulb these are adequate unless you want to blind oncoming traffic. ---
**Author:** ![alaricthegoth](https://avatars.discourse-cdn.com/v4/letter/a/4bbf92/32.png) [@alaricthegoth](https://boards.straightdope.com/u/alaricthegoth)\ **Post date:** [May 14, 2004, 6:25am UTC](https://boards.straightdope.com/t/bicycle-headlight-from-old-mobile-phone-what-type-of-light/244715/8 "2004-05-14T06:25:22Z")
I just got one of those led jobs, and its great–especially for the weight. I once had a critical performance demand that I filled with a rechjargable 12 volt battery from radio shack ($25.00) meant to back up home alarm systems. I strapped that to my luggage rack and ran wires to automobile halogen driving lights from aftermarket store (you could just as well simply buy a raw headlamp and fabricate a mounting to comport with your bike) serious visibility, HEAVY battery… I like my 4 AA cell led better.