So, the nights are drawing in again and it’s time to break out the trusty bike lights for my daily cycle to work. Last year I picked up a set of rather nice halogen headlamps:
which are fantastic - I just use the 10 watt one as that gives me plenty of light to see and be seen. Now, the trouble with this set up is that the battery is ridiculously heavy - not surprising, since it’s designed to power both lights for nearly two hours. All I need, however, is to power the 10W light for about 20 minutes, since that’s how long it takes me to get home. So, I find myself asking, surely it’s possible to rig up a much smaller (hence lighter) rechargable battery for the daily commute.
Just thinking out loud then; the 10W bulb is powered by (I think; will check this later) a 6v battery. Ergo, it must be drawing something around (10 / 6 =) 1.6 amps. So, I need a 6 volt battery that can put out 1.6 amps with a capacity of (1.6 amps x 0.3 hours = ) about 500 milliamphours. Or I could use a 12 volt 0.8 amp battery, or a 3 volt 3.2 amp etc. etc. Or switch down to the 5W bulb.
Is this safe/sensible? Any suggestions on which of these (if any) would work best?
I’ll dig out the multimeter tonight and have a play around, but I’d appreciate any suggestions.
If it is indeed 6V, you should be able to use four rechargeable AA batteries. Here is a data sheet for a 1800mAh AA battery. At 1.8A discharge rate, you have half an hour till it drops down to 1.2V. (But you might want to do some tests to see if 1.2v/cell is bright enough to get you home.)
If you do this, be sure to carry a spare white LED light in case something slows down your commute (flat tire, detour, etc).
Thanks for the reply, scr4. Just after I posted it occured to me that it might be easier to use a pack (a battery, if you will) of smaller batteries. The ones you link to look good. I’d just have to wire up a nice battery holder. And, of course, while I’m doing that, I might as well add a switch so I can turn the light on and off from the handlebars. In fact, how about adding some sort of switched resistor so that I can dim the light, or signal my anoyance by flashing it at motorists. Hmmmm…
Check out some model websites - aircraft especially use wuite a lot of esoteric batteries. You can get 1.2v 4/5 “C” size nickel/metal hydride cells that have 3.3Ah capacity, and they’re not too expensive. Wire them up as needed. Note that although their nominal voltage is lower than a non-rechargeable dry cell, they tend to drop less under load - so they may actually be giving you as much power as a 1.5v dry cell.
Find out the resistance of your light and do the math for how many volts you need to give you the wattage. A 12v battery might not cooperatively shove out 0.8 amps if your light has a resistance of less than 15 ohms.
I just linked to it as an example of a (hopefully) typical Ni-Mh battery specs. 1800mAh is actually a bit small now - you should be able to get 2000 or 2200 mAh batteries at any electronics store.
This isn’t worth the effort, IMO. The resistor itself will consume energy, and halogen bulbs (or any incandescent bulb) are inefficient when run at a lower-than-rated voltage.
If you need a low-power option, you could wire a white LED to your setup. But then you don’t have full redundancy. You might as well get a separate LED light as a backup light. The CatEye EL-400 is my favorite - extremely lightweight and surprisingly bright. The EL-120 is decent too, and somewhat cheaper. Or something like this if you want the lightest possible option.
Battery capacity (measured in mAh) is a function of current. This is due the battery’s internal resistance. At high current levels, battery capacity is significantly reduced. In other words, you can extract more total energy from a battery at low current than you can at high current.
White LEDs are much more efficient than Halogen. The issue, of course, is whether or not white LEDs have characteristics you desire (brightness, beam angle, color, etc.)
Which is why you should look at the data sheet - like the one I linked to - which shows discharge curves (run time vs. voltage) at many different current levels. You need to look at the discharge curve that corresponds closest to the current load of your device, and see how long it takes before the battery voltage drops to an unusable value. (And do separate tests or calculations to determine what exactly is an unusable voltage.)
LEDs aren’t as efficient as LED flashlight makers claim. It’s hard to find accurate cites, but my 3-watt LED headlight (L&M Vega, for which I paid over $150) seems comparable to a 6-watt halogen light. Definitely no match for a 10-watt halogen.
I also have the one you linked to, the CatEye EL-500. It is a 1-watt light. It is very bright, but only because all the light is focused into a pencil-thin beam of light. It’ll only illuminate a small patch of asphalt directly ahead of you.
primer_dimer, if you want to do some tinkering, I strongly recommend getting Willie Hunt’s LVR. It’s a microprocessor-controlled voltage regulator specifically designed for light bulbs. It allows you to run your halogen bulb at maximum efficiency.
