That's extremely inefficient. If it's a 6v bulb, you'd waste 5 times as much electricity as you use, and if its a 2v one, you'd waste 17 times as much as you use. It's much better to use a buck converter. There are loads of cheap lights on Ebay, Amazon and everywhere else that have buck converters in them so that they can work off normal ebike batteries - just plug and play.
You can buy your own buck converter for a couple of quid, but ones that can work with a 36v battery take a bit of hunting out. you need one with HV after the LM2596 designation, then check the voltages in the listing.
https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=LM2596HV+Buck+Converter&_sacat=0
As its is a LED, with a far lower current usage than a conventional bulb, it is 5 times a very small amount of current, theoretically!
Even a very efficient DC to DC converter will only be a very small amount better (though I have not checked up yet, I will come back with further details if needed!) if at all, depending upon the chip used!
But a DC to DC converter will add complexity, with a far larger possibility of a fault, like a voltage spike for example, killing the chip used. Chinese designers are well known to avoid safe guards they do not feel are needed, and thereby make stuff even cheaper to produce, but more likely to fail!
The 2 watt resistors in my rear light, will handle voltage spikes all day and every day, without ANY problems, and are cheaper to use and/or replace if it finally one gives up the ghost!
I will open the lamp again sometime and read the colour bands on the resistors, when I have time, just to see what current is actually flowing, but from memory, it was only 10 ma., so they have apparently used high efficiency LEDs to achieve their aim. 10ma is fairly normal and average. Good!
Also, remember, the rear light may be on for an hour or so while riding, but the brake light will probably not exceed even one minute of that time....
So guessing a little per hour of rear light to brake light usage, if we say the ride was actually 59 minutes, and brake usage a total of 1 minute, which equals Amps x Hours = 0.01amps x 1 hour = 10ma/hours.
You are not going to ride far on that sort of power wastage now are you?
But each to his own....
I personally feel that the cheap resistor used in my rear light, is a clever alternative to any converter, and will far outlive any electronics.
Plus the DC to DC converter, will have to be designed brilliantly to save any power against the resistor in this particular case. That will not come cheap!
It is true that resistors are wasteful when the current pulled is high, but in this case not, because the current is so very low!
I hope that I have brought your thinking more into the reals of the real world, and not just the theoretical one!
I will leave the details of efficiency for you to calculate accurately, but the difference, if any, will be very, very tiny! And your factor of 5 you quoted, must be wrong, probably not taking into account and power used in the converter, as if the same rear LEDs only got a fifth of the current, that would be around 2ma, the rear light would be possibly be quite dark.
Also, remember LED have a low internal resistance, that can be safely ignored when making such calculations.
Andy
