Random Question Regarding Correlation Between Memory and Dead Macbook

Ok so here's the thing, I have a Macbook and a few months ago my battery died and I'm currently using outlet power to run my laptop because I lack the sufficient funds to purchase a new battery at the moment.

Occasionally I want to move my laptop to another room or I accidentally unplug the cable and my macbook shuts off. The technique that I've been using is to wait for about 30 minutes before plugging the charger back in, this way here all of the stuff I had going on my desktop is still there when it turns back on and I don't have to reboot it to the screen. The weird thing is when I immediately plug the charger back in and press the power button it reboots and I lose all the stuff I was doing.

So the question is why does my computer save what I was doing only if I wait between 30 minutes to a couple hours later to turn it back on, opposed to immediately plugging the charger back in and turning it on?
It seems like the little bit of energy it uses to save what I was doing would be more prevalent immediately after disconnection that half an hour later right? I don't know...

So if some smart techy guy could answer my question it would be cool, but if not it's alright it's just a stupid thing that I've been curious about for a while.