🔦 Bicycle Battery Box and Headlamp

Nighttime bicycle rides in the summer are ideal:

• 🧊 cool temperatures
• 🚘 little traffic
• 🦝 nocturnal wildlife

But it is not easy to see in the dark, so I bring a headlamp and okay, fine, this works. With my current headlamp I can see and be seen just fine. But there are some limitations. I see wonderfully directly ahead of me in the narrow cone of light provided, but at the edges of this light cone I see practically nothing at all, as though I am looking through a paper towel roll. The beam pattern is optimized for riding in a straight line, with little regard for what lies in the periphery. I would like a greater field of view, so I will need a lamp with a wide beam pattern.

🔦 Headlamp

I start by selecting the headlamp. This is the most important component, and everything will revolve around it. My selection criteria is simple. The light must have these features:

• 🔆 Ultra-bright
• ↔️ Ultra-wide beam pattern
• 🛠️ Mountable on handlebars
• ❌ No integrated battery housing
• 🌧️ Weatherproof
• 💲 Inexpensive

I could not find a single bicycle headlamp that met these criteria. To get mostly everything I wanted, I had to look at headlamps marketed as ‘motorcycle headlamp’ and other such non-bike categories. Many things in life are like this for me.

I had to settle for the OPL5 Motorcycle LED Headlamp:

Notice the sharp beam cutoff at the top. This is what allows me to have a very bright headlamp while also not blinding oncoming traffic. Also, the beam gradient at the bottom transitions from very bright to very dim, in a manner that maximizes usable light. It concentrates maximum light near the upper cutoff where it will disperse across a large surface area at long distances, while minimizing output in the near and mid field, where the illuminated surface area should be relatively small.

✅ This will do.

🧰 Case

I selected a cheap weatherproof case from Harbor Freight Tools. It will be large enough to accommodate many sizes of batteries, since battery packs of the nature I seek are almost always composed of a bunch of even smaller batteries, typically the 18650 type batteries. The dimensions of this battery type are also its namesake: 18 mm dia. × 65 mm l.

I mention all of this because the height dimension of the battery packs are almost always around the same height of the 18650 battery. Battery packs are assembled with the batteries standing up on end, making their height never more than 30 or so mm taller than the length of the battery, to account for the BMS, terminals, plastic enclosure, etc.

The internal dimensions of this case are 206 mm L × 95 mm × 143 mm.

✅ This will work.

🔋 Battery

Since the headlamp consumes 48 W, I need a battery that can sustain this load for a couple of hours without totally draining the battery. I can see how long a given battery will endure.

• $t$ = run time (hr)
• $E_\text{battery}$ = battery energy (Wh)
• $P_\text{load}$ = LED headlamp power (W)

$$ t = \frac{12.8 \text{ V} \cdot 16 \text{ Ah}}{48 \text{ W}} = 4.27 \text{ hr} $$

I selected a cheap, unremarkable 12 V battery with a capacity of 16Ah.

✅ This will work. It is a comfortable, if not excessive, amount of energy for the headlamp.

🔌 Switches and Ports

I splurged on a nice weatherproof SPST switch and mounting bracket.

The port that will feed power to the headlamp power is some kind of ‘SAE’ connector, which as far as I can tell do not actually comply with any official standard. They are used for a lot of things in weird cheap Chinese electronics, and I could have selected a better connector. I selected this mostly because it matches the connector on my battery charger and it is cheap and ubiquitous. It will be adequate.

🔩 Parts List

🛠️ Assembly

Now for final assembly. This is all very simple and I ran into no issues.

• Applied double-sided adhesive tape to fasten the battery enclosure to the box
• Soldered on spade connectors to wires between switch and battery
• Installed inline fuse
• Sealed the power port and switch mounting holes with silicone
• Removed the battery enclosure lid since the battery terminals were ≈ 1 mm proud of the battery box lid. The battery box would still close, but it was straining the plastic and the weather sealing would be compromised.

Here is a wiring diagram of the circuit.

🚴 Test Drive

All that remains is the test drive.

🎉 It works!

✅ Done.