Smartphone battery dropped very quickly from around 50% to close to 0%

 My smartphone battery level seems to have dropped very quickly from around 50% to close to 0%  quite a few times so I went back to the sales vendor. He told me to charge the phone for 8 hours. I am not really sure if this applies to Li batteries but I  remembered that for the older handphone batteries, I needed to charge them for 8 hours. Anyway I charged mine for around 5 hours. I also installed Avira antivirus. It detected and removed a malware. After that it seems like the issue has not  occurred - at least not as  dramatically as before.