# Super capacitor vs. battery

I've made a turn signal system for my bicycle that uses power generated from the wheel. The wheel puts out 0-9v and is irregular depending on the speed. My lights take 3v, originally from two 1.5v batteries. I used a step-down board from ebay and the system works well, but when I'm stopped at a red-light it is unpowered. I would like to store a few seconds of power, a buffer, and I can think of two ways to do it.

I could add a super capacitor after the step down (or would before be better?) the step down. Or I could add a rechargeable battery.

Which solution is easier to set up? Which solution is better suited to my application?

• It's probably easier to put the cap after the step down regulator as most all supercapacitors are 5V or less (the big ones are 2.4-2.7V). To run a light for any decent amount of time you'll probably need some decent fraction of a farad (3V @ 1F gives you a total energy of 4.5Watt-seconds if you totally drain it, or only ~1.4Ws if you drain it from 3V down to 2.5V as E=0.5C*[V1^2-V2^2]). Most suitable supercapacitors have really low ESR (like melted screwdriver low), but it's still pretty hard to hurt yourself with one – Sam Jun 27 '16 at 0:06
• Wheel -> buck (or buck-boost) -> supercap -> boost -> light – Jacob Jun 27 '16 at 5:53

Just adding a cap seems easier to me, (rechargeable battery will need recharge circuit, and pass-through diode [kind of like a voltage source-selector] at the very least, no?) but the question remains whether that will be enough to hold you over.

You've gotta do the math though to see how much juice you need. Current will have to be measured from the leds + any leakage / quiescent through the step-down system. Voltage max might be 9 V but more than likely is 9VRMS if you're turning the crank steadily. Voltage min is min input to the step-down circuit.

http://www.circuits.dk/calculator_capacitor_discharge.htm

Here's a good time to make you aware that capacitors store a lot of energy. If you short them, especially large ones, you're in for a bit of a surprise. I think anything over 500uF starts to garner my attention, and you're going to need a lot more than that. You'll need to consider a discharge circuit to make sure you don't hurt yourself or blow stuff up with the super cap (no joke), but I believe if the light circuit is constantly connected / on you should be ok (anyone else care to pitch in on that?).

edit1: and on second thought (for the very same reasons described above) just do the battery. If you don't know what you're doing things can get problematic quickly. Search for battery power source. There's plenty of info around on this sight. Main points. Separate with diodes. Make sure your step-down output is HIGHER than the battery output (but that your led's can still run off battery output). If the step-down is putting out 3.3 (probably?) then use 2 nimh (2.4v). Done.

Battery in parallel with main power source

edit2: Okay so maybe a 9V cap isn't going to kill you..., but it's going to to put on a show if it shorts.
Guidelines for determining shock hazard of capacitors

• Can you explain how one could be killed by a capacitor charged to 9 V given that it's safe to put your hands across a 12 V car battery? – Transistor Jun 26 '16 at 20:42
• Caps have low internal resistance so they'll get rid of all their energy quickly. The risk is more via a short or if your hands are wet or something else going on. Given that this thing will be outside, I'd want it water-proofed etc. I added a link on the dangers of capacitors. They say anything under 48V is ok for small caps. Super caps are very very large, though, and should not be taken lightly. – bathMarm0t Jun 26 '16 at 21:03
• @bathMarm0t: a typical car battery can put out enough current to melt a wrench if you place it across the terminals. – whatsisname Jun 26 '16 at 21:33

Maybe you could try one of these cheap lipo charging boards available on online stores. You need the "step-down" board output voltage to match their input voltage (usually 5V). Maybe use an old flat lipo cell phone battery, or if a little more power is needed, an 18650 salvaged from an old laptop battery.

For a few dollars each, you could use one on each left and right side if the load math requires it.