Why does my tablet charge so slow?

so ive had my tablet for only half a year. Its bought second hand because my family isnt in the best financial situation at the moment. It wasnt always like this, up until recently it charged quickly. Now though, it takes a long time to get even 5%. I had to replace the charger because the original one broke, and plugging it into different sockets doesnt help. I have to admit, ive been using it while its charging sometimes, but usually only to text some friends for a quick minute or so. If its a drained battery, how do i get to it? I cant find any way to take off the back off the tablet like you do with a phone.