Why is my macbook pro battery not working?

Question

macOS

Hello! I installed SSD, 16 RAM, optibay with native HDD instead of CD on Macbook Pro 13 2012. Made by video on youtube. As a result, I turned on the laptop, and it began to work only from the network. And on the icon of the battery is a cross. During the upgrade, I disconnected the battery cable from the motherboard. Tell me, please, what can it be? Otherwise, you will have to take the laptop to the service, which I would not want to do :(

Answer 1

People are smart enough to delve into laptops out of ignorance, watching videos.
It is better to take it to the service if there is no desire to lose the poppy

Answer 2

Check contact

Answer 3

There are 2 loops, one for battery power, the second battery level. They are nearby, you need to reconnect them again.