Market is flooded with batteries. It is hart to select and trust the ratings. If you want to be sure your project will run predictably from selected power source it is best to measure characteristics by yourself. Elm-chan has built a battery discharge analyzer some time ago that proved itself.
It is based on Atmega48 microcontroller which reads data parameters while it’s being discharged. Battery analyzer supports three load modes that imitate different applications: constant current (imitates linear regulator), constant power (imitates switch mode power circuit) and constant resistance (imitates lamp, heater). Circuit logs data via RS232 interface that can be recorded to file for graphing and analyze. Analyzer additionally can measure internal series resistance of battery (ESR).