Why does my tablet lose charge too quickly

My 'Samsung Galaxy Tab 4' is loses charge too quickly and when it's left charging for hours, it only comes out with 20 percent. Even after 8 hours of continuous charging it will only have 40%.

The problem started around 6 months after it was bought and it's still fairly new at 1.5 years. I suspect a battery issue or a faulty charger. What do you think is wrong and is it worth fixing.

I appreciate if you could reply.

S. RM