Buy these two apps to monitor your battery:
Then install the battery percentage icon at a large enough size for comfort in the upper right hand corner.
Use the Fire down to 10% to 15%. Open to the GSam battery screen and fully charge to 100%. The GSam icon will show that percentage when it is reached. A 100% charge can take up to SIX HOURS.
Preferably, do the charging overnight.
At least one full cycle like that is necessary to train the battery measurements and to mix the Lithium chemicals thoroughly to perform at full capacity.