Diode just keeps the batery being charged, not overload. let me explain, I tested both situation, so practically speaking, a schottky diode is better
A normal silicon diode needs 0.6 to 0.7V to to start conducting, it works as a battery. On a schottky diode, you only need 0.3V to do it. So when you you turn off your computer , RTC uses a 3V battery , 1n4148 takes 0.6V, wich makes the rtc working with only 2.4V, it will take less time to discharge with that diode. With a schottky it only takes 0.3V, making the rtc work with 2.7V, wich will live more time. You have that explanation from the same site we took the 2mb chip mod.