How can i fix my Vauxhall Astra 1.7 TD?

2 Wires Coming off my aleternator got burned. I made replacements. Charged up my battery overnite. Started the car following morning, it started. Drove my car to work 5mins away from house..... Turned engine off then within mins i tried restarting and my battery seemed flat. I left it for 5 hours, went back to the car and it started no problem. Run it 20 miles. Stopped it left it 10 mins got back to it restarted it and it would not start , battery seemed flat.