y720 laptop keyboard error. number 5 trigger.

I'll start from the beginning, a few months ago there had been some fluctuations in electricity while my laptop, by mistake, was charging. I'm not saying that this was the problem but I suspect that it was. Shortly after I took my laptop and left for a place. When I arrived to the place, 30min later, my laptop was at 0% battery, which was strange because it was connected (I have it in conservation mode). I connected it to charge and it was there for one hour a follow at 0%, as soon as I disconnected it from the power supply, it would turn off. I deduced that it could be the battery as such. But then pattern errors started in the loop, for example "'= 560" were marked alone or even pages were closed, the magnifier went crazy, web pages were restarted unless you used an external mouse or keyboard. So after many checks in the software, I bought a new battery for the bios. And also buy the main battery. After installing the BIOS battery everything worked great (the main battery hasn't been deliver to me, so I depend on power supply) the problem is that everything goes well except for the 5 key (not the num pad) when I press this key does not work, it does not reflect a 5. But it is not until the moment I press the 5 that the aforementioned events about the loops, magnifier ... etc. begin to occur again... It's like the number 5 trigger something in the system. 

I would like to know if anyone has an idea of ​​what it might be, I thank you in advance.