Cannot charge due to low battery temperature on Lenovo P780
So what you have sounds like one of two things. 

One, there is a software issue giving false readings off the temperature sensor. In order to diagnose this I would recommend installing a third party battery monitor which can monitor temperature. If this gives you a normal temperature you know its software and a factory reset should fix your problems. If it still is -20 than it could be hardware or just firmware level software.

Two, the temperature sensor could be broken or malfunctioning giving a false or default reading.

Either way if doesn't cause any problems a factory reset would either fix it or tell you what is wrong.

Let me know how it goes, I may have some other options for you if need be.