Modify Bicycle Headlight

Hello,

I have an 1964 bicycle with a tank light. I'm converting the bulbs to LED. The original setup uses 2 D cell batteries (1.5v) in parallel and has two filament E10 flashlight bulbs.

I was only able to find and purchase 3v e10 LED flashlight bulbs. This is OK, as I assume the 3v will be brighter anyway. Since I have two D cell batteries in parallel, each bulb is only getting 1.5 volts. I believe I could remedy this by converting my D cells to AA adapters, but I can only find parallel adapters which give 1.5v total, or 3 AA adapters which give 4.5v total. Would 4.5v burn out a 3v LED? My assumption is yes. I haven't been able to find an adapter that has 2 AA batteries in series to provide 3 volts.

Could I purchase a 3 volt D cell to AA adapter and solder a wire between the terminals where a AA should be, thus giving me two AA batteries in series, which should be 3v? Is there a better alternative? I have included a drawing of my setup below.

Thanks,

Welcome here, VettemanSixtyOne! :wave:

Is there a better alternative?

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Yes, several I can think of:

* See if you can find a Li-ion cell in 32600 size (or maybe 32650 if battery holders in your lamp allow), using LiFePO4​ (Lithium Iron Phosphate) chemistry. In use this chemistry outputs around 3.2V. In that case leave the 2nd battery holder empty, and paired with ~3V specced LED bulb(s) you should be fine. Charging would be a -bit- of an issue though: not many chargers support LiFePO4​ chemistry, and even fewer will have space to insert a 32600 (or -650) sized cell. With spacers you could use smaller cells, at the cost of runtime.
* Find regular Li-ion cell in one of these sizes, and pair with LED bulbs that are specced for ~4V input voltage.
* If space allows: insert a voltage boost circuit into the lamp(s) circuitry. Output voltage is normally adjustable, so you can pick what voltage & what LED bulbs to use. Suitable modules are cheap & easy to find, although usually input voltage ranges start at around 3V. But it would surprise me if you can't find a module that takes down to ~1V input. A flashlight driver meant to use AA, C or D cell may also be used for this purpose.
* Get your LED light fix elsewhere, leave this lamp in original condition, buy a stack of spare bulbs, and enjoy the glow of good old incandescents. :candle:

Welcome to the forums, VettemanSixtyOne!

That plan would work fine. Or you could put a dummy cell in. AA cells give you the most bang for your buck anyway.

The 3V bulbs won't be brighter just because they are 3V. They might be quite a bit brighter because the efficacy of LEDs is something like 10 times that of incans. So you could get 5 times the light at half the current! It will depend on the design choices made by the manufacturer.

RetroTechie's suggestions are great if you use the lamp a lot and go through a lot of batteries. If you do, then rechargeables can save you a lot of money and save a lot of waste in the long run. I would add to those suggestions that a 26650 size is also an option, and may be easier to find charger for. It's a good bit longer (3.5mm or a bit over 1/8") than a D cell though, so might not fit. I don't believe you will find a 26600 rechargeable, thought there are 26600 non-rechargable lithium batteries available. In case you don't know, the first two digits are the diameter in millimeters, and the second two are the length in millimeters. A D cell is 33.2 x 61.5mm.

If you do find LiFePo4 cells that fit, you might be tempted to put one in each side of the battery holder. I certainly would, as this would double your run time. If you want to do this, you should only do it if both cells are fully charged. If you parallel two cells with significantly different charge states, you will get a massive dump of charge from one cell to the other. This can damage the cells, the battery holder, or both.

Good luck, and keep us posted!

Thanks for all the good info, and for the explanation. I have no knowledge the different types of batteries by number and didn't know about the different charge rates being hazardous to the life of the battery, so thanks you for that.

Is there a technial term for a dummy AA battery that I could search for? The battery holder mounts inside the tank of the bicycle and the area the battaries mount inside has zero extra room in any direction for a larger size battery. The two D cells fit very, very tightly in there now.

Thanks again, I'll try to get two AA to D cell series adapters and use one AA dummy in each (if I can find the dummy AA, otherwise I'll solder a wire). This will be great because now there should be no chance of the LEDs melting the clear plastic lens on the light.

VettemanSixtyOne said:

I was only able to find and purchase 3v e10 LED flashlight bulbs. This is OK, as I assume the 3v will be brighter anyway.

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They might put out more light, but you may find the beam pattern thoroughly wrecked in the process, the way so many of those E10 LED drop-ins are made.

i would not even bother, e10 led almost never focuses correctly in bulb reflectors, i have tried them at least in half a dozen of vintage lights, none worked right. it may be brighter than a bulb, but inside the reflector it is next to useless for any distance farther than 2-3 meters max.
nowadays you can get much much better light, for 20-30 bucks. that will be infinitely better performer than your old modded inc bulb.
there is a way to mod old lights so they perform as good as modern lights, but it involves a lot more work, and parts, than just new bulbs and batteries. i'm into this hobby for over 20 years, moded, build well over 100 lights, based on that i would not recommend washing time, and get disappointed.