iPhone wont power up after it had no battery left

Hi everyone,

A costumer came to the store for a new charging dock, but i didn't disconnect the battery for this repair after i replaced the charging port the phone wont power on,

so i disconnect the battery and replug but still same result,

i tried a new battery in it and it works fine.

phone works perfect untill battery drains to 0% and the costumer cant charge it again then.. it gives the charging logo but it wont go to 1% or even start up.

but as long the battery don't drain lower then 1% everything works fine you can charge you can call you can do everything..

anny1 knows this issue?

thanks alot for reading this

greetz