how can i fix my nissan micra 1986 colette (998cc) electrical problem?

When engine is switched off and key removed the red batterie warning light on the dash stays on. That does not look normal to me. A few hours later the battery is totally discharged. What could be the problem? I hope it can be fixed easily and it will not be too expensive. I hope someone can help me out here.
My car is a 3 door hatch Nissan Micra Colette 1986, 998cc, manual, runs a sweet as a nut otherwise and I try to get some fun with it.
cheers