How to Charge a 12V Battery With an Exercise Bicycle

When hunting down a good way to recharge a 12v battery or battery pack, it doesn’t often occur to people to use the power in their gym bicycle – but it works, and it can keep those batteries going for much longer than they usually do. Using a stationary bike in this way sounds like a great idea: encouraging you to exercise for energy recharging. However, there are certain details that everyone needs to know about how to charge their batteries using an exercise bicycle before they begin

Batteries can be Overcharged

When using a stationary bike to recharge 12v batteries, there is a slight risk that you might pedal too quickly, and push too high a voltage directly into the power pack. A 12v battery will not usually require more than 14.8 volts during charging, but using a bike means that often the voltage will go much higher. This high voltage will actually prevent the battery from charging properly, and may permanently prevent the cells in the battery from recharging-exactly the opposite of your intention.

Use a Charge Controller

The best way to prevent too much current from going into the battery is to use something called a ‘charge controller’. This does precisely what it says: it controls the amount of energy being charged to the battery. The most common type of charge controller is called an “NC25A”; and works by detecting how much voltage is travelling to the battery, and if the necessary amount (14.8v for a 12v battery) is exceeded, then it can divert the power elsewhere.

The charge controller uses a diversion load output-which acts as a limiting function for the cyclist-and then pushes the extra energy into another location, away from the battery. Inside the charge controller is a typical on/off relay switch, which turns the current to the battery on and off, depending upon the strength of the voltage.

For really enthusiastic cyclists, they may need an extra-strong charge controller, as normal ones can be damaged if the voltage exceeds 35 volts. There are more powerful controllers on the market, but they do not have the diversion load ability. Instead, it might be more practical to encourage the excess voltage to flow to another path. The charge controller and diversion load output cannot work as resistance training or regulators, as they have slow responses, and would be unable to maintain a constant level of pressure.

Powering Two Batteries at Once

The charge controller acts to prevent extra voltage from passing directly into the battery, and diverts this excess voltage into a ‘diversion load output’. This means that there is excess energy, created by the cyclist, not being used. For the economically minded, this extra voltage can be used: simply fix a second charge controller onto the diversion load output, and add a second 12v battery or battery powerpack. Other things to do with the energy include attaching a fan, floodlights, or maybe a small kettle-work out then have a cup of coffee!