HDMI only works when plugged into the charger?

Basically what the title says, (through a dongle) the HDMI port seems to work when the macbook is plugged into the charger, but not on battery power. But it'll continue to work on battery if I plug it into the charger, plug in HDMI, and then remove the charger. Is this supposed to happen? I think it used to work fine on battery power but I'm unsure.