Also I like to share my experience by charging the device, so it could be taken as an advice.
Never left the GPD XD charging more than the necesary, until it reaches the 100%. If you leave the device connected for a long period of time after it gets full battery, it will still warming up. Usually with the phones, the system automatically cut off the energy when the charge is complete, and that's for avoid overcharging, hence overheating too. When I forget to disconnect my phone after few hours it gets full, when I disconnect it, it's cold, no trace of heat.
But it seems that that feature isn't that good on the GPD, so if you leave the gpd connected 3 hours after the battery gets full, those are 3 hours that the battery will be getting warm, not as hot when its charging, but the enough to note that it's not normal.