Use Your Bike as a Backup to Your Backup Power Supply

Illustration: James Provost

As this article goes to press, Hurricane Delta is making landfall not far from where Hurricanes Sally and Laura came ashore earlier in the season. I live on the East Coast of the United States, and fairly far inland, so such storms are not as frequent or intense as they are in states bordering on the Gulf of Mexico. But they are still a concern, if only because they can topple trees and cause widespread power outages. And ice storms during the winter here are also apt to bring down power lines.

I don't mind the resulting darkness so much. What I really don't relish, though, is losing Internet access-especially now that it is my main connection to the world due to the pandemic. And the pandemic is reducing the number of field crews available to fix power lines, making outages last that much longer. So this year, I figured I'd get prepared for a blackout in the most self-sufficient way possible.

I could, of course, just purchase a conventional small gasoline-powered generator. But I didn't want to do that for a few reasons. In particular, I recalled stories about what went on after Hurricane Sandy in 2012, when many people using such backup generators had trouble finding fuel, including the IEEE Operations Center, in New Jersey.

My first thought was to use photovoltaics, so I purchased two 100-watt panels on Amazon for less than US $1/W. I had a 35-ampere DC-to-DC converter from an earlier project to use as a charge controller, so my next step was to spec out a deep-cycle lead-acid battery to keep things going at night.

Some experiments with a watt meter led me to conclude that a battery of at least 300 watt-hours capacity could keep four laptops, a cable modem, and a wireless router running for about 4 hours, while also charging the family's phones and flashlights. That should get us through dark evenings. It would also suffice at a reduced load if the sun were hidden behind clouds all day. So I purchased a 12-volt, 35-ampere-hour battery (which nominally can store 420 Wh).

But what if skies remained gray for many days in a row? Rather than trying to purchase enough battery storage to cover all reasonable eventualities, I decided that my backup source of electricity needed a backup itself, one that I could use to charge that battery during times when my photovoltaic panels won't function. When necessary, I'd simply detach the battery from the panels and attach it to my backup source to be recharged. I briefly considered whether a wind turbine might serve that role, but then opted for something I figured would be more dependable: my two legs.

Illustration: James Provost Parallel Power: My system primarily relies on photovoltaic panels to charge a deep-cycle lead-acid battery via a DC-DC converter. When there's not enough sunlight to use the panels, I switch over to a generator driven by the back wheel of a conventional bike mounted in a stand. This requires a rectifier and a meter mounted on the handlebars to monitor the power produced and fed into the battery.

I cycle regularly for about an hour a day, during which I probably put out an average of 80 W, based on some rough calculations (I'm small and typically cycle around 22 kilometers per hour). That 80 Wh is only a fraction of what my solar panels can provide in a day, but it would be enough to keep my laptop connected to the Internet for a few hours, charge phones, and so forth. And pedaling my own power seemed like a healthy, stress reducing, activity to pass the time during a power outage.

I discovered from one blogger that it wouldn't be hard to modify a stationary bike stand to generate electrical power. Although I had a bike stand already, mine provides frictional drag using a fluid-filled chamber, which I was reluctant to crack open. Instead, I purchased one similar to the one that the blogger used, which employs magnets and eddy currents to create drag forces on a shaft that presses against the back wheel to increase exercise intensity.

I ripped out all that drag-inducing stuff and attached a brushless motor to the shaft using a flexible coupler and a wooden spacer. Then I connected the three leads of the motor-originally intended to motorize a skateboard and now acting as a generator-to a three-phase bridge rectifier. The output of the rectifier in turn is connected to my battery through a Drok meter. This meter allows me to monitor the voltage, current, wattage, and total energy produced.

Testing my power-producing bicycle stand quickly revealed a flaw in my logic. Pedaling at a comfortable pace, meaning one that I could keep up for a long time, produced only about 60 W, not 80. In retrospect, I decided that I had failed to consider the inefficiencies of power conversion, which surely are significant because my motor/generator gets pretty hot after a while. But even 60 Wh would do in a pinch. And there's no rule that says I couldn't cycle for longer than an hour. Even better, I can get my kids to contribute a little sweat to support their phone and computer use during the gray days of a winter power outage.

Actually, generating power with their muscles provides a valuable lesson for kids, whether or not the power goes out. Every time they switch on a lightbulb or a television, they will think more about what this energy consumption means, given the considerable effort it takes to produce those watts yourself.

This article appears in the November 2020 print issue as Pedaling Out of the Dark."