Why does the battery charge in sleep mode with direct power supply enabled?

So, I like to use direct power supply mode whenever possible to extend the life of the battery, and I'd prefer when I put it in sleep mode for a sec that it wouldn't start charging, but it does. It seems the setting doesn't apply at all when in sleep mode, is this a bug or is intended this way? The only semi solution I've found is I have to turn off direct supply and unplug it before putting it in sleep, this way it doesn't keep charging, then when I use it I have to plug it in again and turn on the setting again. Pretty annoying to have to do this every time, so I'd like to know if there is any way around this.