How can i fix it?

Hello, The voltage regulator needs to be replaced. Normally, they are built-in to the alternator, so they must be purchased as a unit. The voltage regulator will be limited somewhere around 14.1-14.9 volts, and when it fails, your situation is replicated. If you know how to work on cars, this could be a nice DIY to save some cash, but yes, repair it. If you were to listen to your battery while the alternator is charging, most likely it would be hissing. If you replace it now, you keep your battery, if not, you replace the battery too, and most likely many fuses and wires. Also, alternators put out fewer amps at lower speeds. Keep the engine in a 2000 or lower RPM cruising patter, and you can decrease the chance of harm to the electrical systems on the vehicle. Good catch, by the way.