iPhone 5 or earlier Help! iPhone irregular battery percentage after replacement

[3:16]

I replaced a battery on old iPhone 5 the percentage skips from 100% to 98 % in just 2 minutes and decreases fast until below 40 % and slow after that

Time since last full charge
Usage 5 hr 29 min
Standby 8 hr 11 m
Currently at 29%
What could be the problem?

 

[ab22]

Most likely a faulty battery.

 

[Trader05]

Full charge and completely drain a few times. See if it normalizes, if not its the battery.