Alternator not charging?

Okay, so, my car does not hold a charge for some reason. I just recently replaced both the alternator and the battery. Both were bad. The battery light and the brake light turned on when the car first quit running. We replaced the alternator and the lights when off for a little bit and after less than a day of driving they came back on again. Then the car shut off again, I got the battery tested and it was bad so replaced the battery and the lights were still on. It shut off again today and won’t start. Also, when it is started the car won’t give any throttle, but the engine is still on. Then all of a sudden, the lights go dim on the car and it shuts off. Can anyone give me an idea as to what’s going on, please?