How to solve Inaccurate battery status after removing battery

My phone battery is just about 2 months old. It can last for many days without browsing and over 12hours streaming HD videos on YouTube. But something is strange. Any time I remove the battery ( high temperature or to switch SIM card or SD card ), phone doesn't care if battery was at 99%, on reboot, I realized battery has drastically reduce down to even 5%. At times I would remove the battery again and wait for some minutes (about 30) and reboot again getting up to 40%. What's the problem and how to solve it Device : MTN Smart L 860 on Marshmallow 6.0