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I am wanting to connect a small 300W 12/24V DC generator motor to an exercise bicycle to put charging current into a 24V battery system. But when I hooked up the generator, the power from the batteries turned the "generator" bike into a "motorized" bike. A friend told me that I need something like a "diode" so that the power can only flow from the motor to the batteries but not back to the motor. I don't have much electronic know-how - What kind of diode do I need to search for? Thank you.

A diode is the simplest semiconductor component, just two terminals. There is plenty of information on wiki. I would recommend using four diodes in full-wave rectifier configuration, which you can buy as a single package with four diodes in it. The package is conveniently marked with two ~ terminals (input) and + - terminals for the output. I expect you would like to use just this. However, a real professional might add circuits to control the battery charging and to protect the generator. Or you might like to include an ammeter to see just how much charge you are putting into the battery and a voltmeter to tell you when it is full. As you have a 24V 300W generator you need a beefy rectifier, say 20A or more. Amazon do them, as do Mouser (USA) or RS (UK). Connect it as shown here http://www.electronics-tutorials.ws/diode/diode_6.html and add your volt meter across the battery and your ammeter in series with it. Any multimeter will do, or use 20A or more ammeter and 30V or more voltmeter. Your battery will be fully charged at 25.2 volts.

PS - Bridge rectifiers seem to be paradoxically cheaper than single diodes. Here is one from UK supplier BitsBox which offers 35A 600V at £1.66. They are very good on postage and delivery for small quantities. https://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=140_141_146&products_id=2750 Please note you may need to mount it on a heatsink. Any large piece of metal should do.

Thank You:)