Looking for feedback on my bike-powered circuit using supercapacitors - Page 1
Forum section: electronics / projects

author: lou_rnt
author_rank: Newbie
timestamp: June 30, 2025, 10:03:35 am
content: Hi everyone,I'm currently a Master's student in electronics engineering, and I’m doing an internship within an association promoting sustainable mobility. My project is to design a system that generates and converts electricity from a bicycle, for educational and awareness purposes.I don't have any technical supervision during this internship, so I’d rather ask here, rather than make a poor design decision, especially since I’m working with supercapacitors, which I know can be dangerous.I am quite concerned about the way I've managed the supercapacitors, and I do not know if the protection I've added are enough or not.For the energy storage I've planned to use 12 supercapacitors of 2.7 V / 500 F, which I often see in diy projects. I don’t fully understand why theses values are choosen most of the time, so I'm open to suggestions if better values exist for my system.My idea was to use an esp32 to send data via Bluetooth, and then display them in a mobile app and allow user to control the outputsAny feedback would be a huge help, so if anything in the schematic seems off or incorrect, please don't hesitate to let me know !Thanks a lot in advance

author: jwet
author_rank: Frequent Contributor
timestamp: July 01, 2025, 01:42:32 pm
content: Saw you got no responses- I'll try to help.1. The reason that you see those values of SC's is because they are the most commonly available.2. I understand why you're stacking the caps to give you HV so you can take advantage of V^2 term in the energy equation but remember that the capacitance will be 500/12 or about 41,7F3. When you stack capacitors, you generally want to have a resistor string or active circuitry to keep them balanced. They won't charge equally by them selves due to tolerances.4. The total energy of that cap bank is 20K Joules which sounds impressive but is only about 5 watt-hours. Light a night light for an hour?5. I didn't see a fuse, this is extremely important. Big charged supercaps can produce a LOT of current.6. Silicon diodes conducting 20 amps will dissipate real power- likely 15 to 20W, this will likely require heat sinking. This stuff is often done with big FET's.7. You shouldn't rely on an embedded uP for all your control. Include some simple comparator circuits as a fail safe. uP's get busy, crash, miss interrupts, etc.8. Super caps while amazing are still several orders of magnitude less energy dense than batteries. (about 1/30 th today)

author: Kleinstein
author_rank: Super Contributor
timestamp: July 01, 2025, 06:12:37 pm
content: The rectifiers are shown for a negative output voltage, while the zener diode is shown for a positive polarity. So the two parts don't work together as shown.Ideally one would want some switched mode regulator / charging current, as with most generators the voltage is proportional to the RPMs. As shown one would get a heavy load when the capacitor is near empty and need a relatively high speed to get charging at all ehen the capacitor is near full.Also a simple zener diode has a rather soft corner. So quite some leakage already at some 30 V and possibly more than 33 V under higher current.The divider with 100K for the voltage reading is relatively low resistance and thus taking some load for keeping the charge over a longer time.

author: SteveThackery
author_rank: Super Contributor
timestamp: July 01, 2025, 06:30:37 pm
content: Quote from: lou_rnt on June 30, 2025, 10:03:35 amI'm a novice and have a question. May I ask why you decided to use supercaps rather than rechargeable batteries?

author: Benta
author_rank: Super Contributor
timestamp: July 01, 2025, 09:53:04 pm
content: Quote from: SteveThackery on July 01, 2025, 06:30:37 pmThe challenge is to use the lowest energy-density technology available in a limited space.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: July 01, 2025, 10:06:08 pm
content: I believe this video may be relevant to the discussion;https://youtu.be/i8ok7S3G29s

author: Jesse_G_EE
author_rank: Newbie
timestamp: July 01, 2025, 10:38:05 pm
content: Quote from: UnijunctionTransistor on July 01, 2025, 10:06:08 pmWell played.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 03, 2025, 04:45:30 am
content: You'll need a switching converter between the rectifier and supercaps to allow implementation of MPPT. Otherwise, it will be really hard to pedal at first and then require pedaling very fast as it gets near full.You can use LiFePO4 batteries with BMS, those are much safer than the usual lithium batteries and wouldn't involve dealing with the very wide voltage range a supercap would have.BTW, back when I worked at Intel, some 13 years ago, the company gym had stationary bicycles with USB ports for charging phones and tablets. Those presumably work similar to what you're trying to do, although I don't know the internal details. For your version, I say skip the AC output, implement USB-PD using off the shelf modules.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 03, 2025, 06:36:25 pm
content: Quote from: Benta on July 01, 2025, 09:53:04 pmLeyden Jar?

author: bson
author_rank: Unknown
timestamp: July 03, 2025, 07:55:52 pm
content: The bridge rectifier diodes look backwards to me. You'll get + on ground and - on the supply rail.Also, instead of bypassing the caps with a zener, use a power MOSFET to shut off the supply, controlled by the zener.If it were me I'd be more inclined to just grab the alternator and regulator from a small MC or scooter, hook up the caps or a battery to it, and then use a DC/DC boost converter to get whatever I need out.And fuse the caps/battery or you might have a bad day.Non-cyclists (I mean people who don't train and race, as a sport) tend to vastly overestimate the amount of power they can produce for more than even ten minutes.

author: Someone
author_rank: Super Contributor
timestamp: July 03, 2025, 10:16:57 pm
content: Quote from: bson on July 03, 2025, 07:55:52 pmCyclist or not! This forum had several of its members claim they are undiscovered world class endurance athletes:https://www.eevblog.com/forum/chat/a-possible-way-to-keep-your-fridge-cold-in-long-term-blackouts/?all

author: lou_rnt
author_rank: Newbie
timestamp: July 04, 2025, 08:23:36 am
content: Hello everyone!I’ve already made changes to the system thanks to your suggestions, and I’ll keep you updated with the results (wether it works or not) !I really appreciate the time you took to give me a relevant feedback, your advices were really helpfull !Thanks again!