I have the same problem with my 1520 since having a new battery installed. It only charges up to 80% or so, at least that's what it reports. It could be that it's fully charged but it is just reporting wrong. I have tried draining it to the point of shutting down but that didn't help, nor did a soft reset. I read somewhere that a hardware reset didn't help either so I'm not going to bother trying, it's too much hassle.
I am on 8.1 and have never put 10 on it. I don't know if it's a problem, although I suppose there might be a potential safety issue if it is trying to charge a fully charged battery for a long period of time. That's just a guess, it might not be a big deal.