I been making these headlights for wheelchairs (manual type) and the lights themselves come from these T6 keychain flashlights which take a single 16340 battery. I been connecting them with 4.2v battery packs and they been working great. I am currently waiting on more of the 4.2v packs to arrive from china but for now I have some 8.4v battery and charger combos. My question is if I run the 2 lights in series (instead of parallel) with the 8.4v packs will it divide the voltage in half and give 4.2v to each light? Will it be reliable for the long term? I been making these setups non profit and all the parts cost around $25 The 8.4v packs & chargers are a lot more common and easier to get, so I would like to use them if possible.
I’m not sure what driver boards are used in the lights. The bulbs are Cree T6 leds and the lights have 3 modes -high-low-strobe. Would strobe mode create voltage fluctuations between the lights since they wouldn’t be in sync?
As long as the current drawn in each LED is the same (ie, 2 identical units), absolutely, it’ll split roughly down the middle.
The only time you’d have an issue is if one light would want to draw more than the other, then current would be limited by the smaller one (if both are regulated).
If unregulated, then you’d consider the “resistance” of each. Eg, take a ~30W 1157 bulb and ~2W 194 bulb, both rated for 12V, and put them in series. The 194 would then probably see something like 10W across it and the 1157 maybe only 2V. (Hotwire-bulb resistance is nonlinear.) The 194 would be almost fully lit, the 1157 dimly glowing.
But 2 identical “4.2V” LED lights, in series, connected to 8.4V, would roughly get 4.2V each and light up the same as if each were lit from a single 4.2V cell individually.
Be warned, if the lights get into different modes, one will end up with a higher voltage across it than the other. If the voltage difference is enough, that might push it over the maximum voltage limit of the light’s driver board. The ATtiny13A microcontroller many drivers use is rated for up to 5.5V in normal operation, with an absolute limit of 6V, for example.
I also have no idea what will happen if the lights are using some form of PWM regulation, or indeed if they get into strobe mode. There’s a substantial risk that the current variations will cause the lights to interfere with each other in all manner of ways, some of which could cause damage.
As an alternative, you could try a step-down or buck regulator board to bring the 8.4V battery pack down to 4.2V and then wire your lights in parallel. There are lots of cheap boards on EBay if you want to experiment with that approach. You’ll lose at least 10% of the power in conversion losses, though, so the series approach is more efficient as long as the lights play nicely with each other.
