IRL a Power Bank has around 80% of a charge even when it says 100%. Unless you use the battery immediately. The charge lowers 15-25% for such batteries. Sucks but true.
Also, it is sadly common that unless you investigate a quality battery. Some are literally filled with sand to screw people over. Look up cheap cells on youtube. Plus, some batteries just suck @$$ by poor design…or rather the board they are attached to. They maybe good cells, but the people attaching badly design boards to them will end up discharging the power irregularly. Which means the battery will seemingly not be as powerful.
In game terms most of this matters nigh, but good info when buying batteries. Look up what you need and see what is considered better and why.