For charging a battery, when it reaches the desired voltage, charging is complete. You'd then remove it from the supply. Lowering the voltage will cause a reverse current into the supply, effectively forcing the supply to become an electronic load, which is not desirable and could damage the supply.
When charging batteries with a bench supply, it's a good safeguard to put a power diode in series so that current only flows in the correct direction (i.e., out of the supply's positive and into the battery's positive).
Regarding schematics, you're unlikely to find them for modern gear unless someone reverse engineers them. It's even rare these days for A-brand companies to release schematics, although they used to back in the day. As you peruse the forums, you'll find that many of us intentionally buy older gear because of the available service information.