I had an el-cheapo Chinese Android 7" tablet once. (I fact I still have it but haven't used it for about a year. Any buyers? ) Android has a nice feature to show what uses power on the device. I was surprised to find that the Phone app uses about 75%, most of the rest used by the display. Phone app? On a tablet without a phone functionality? Well, yes, as it turned out. The SW package was quickly hacked together from a phone, including the Phone app.

Anyway, I found dedicated forums and marinheiro's conclusion was listed as a reason. And yes, rooting the tablet and removing the phone app cured it.