. . . are like check-valves, diodes, and every street you need to take in Madison. One-way devices, if you will.
The rectifier just keeps electrons, that are being used to charge your battery, from going in the opposite direction. Without it functioning, you can get:
1. Fried batteries.
2. Batteries that seem like they are fried because of the lack of charging from the electrons bouncing back and forth (forward and backward) with the AC input.
3. A motorcycle you want to kick everytime it strands you wondering why your battery isn't charging.