Are you using the same charger and cable it came with? If so either the charger or the cable may be bad. See what happens if you plug it into a running computer for 5-6 hours. See if it improves.
20 is too much for sure there's a problem either on Tab software or hardware. Try to restart the Tab ans used other charger and also try to uninstall apps that related to battery saver something like that cause i read some articles regarding those apps might be a source of those problem.