New Battery, apple logo blinks on and off, but doesn't boot.

So, I followed the incomplete guide to solder in a new battery, and it worked!

But now when I try to turn it on (before I fully close it up and get everything going again) it starts with the apple logo, stays there for maybe half a second, goes black, and repeats the process until I press the power button to turn it off again.

What do I do?

Additional info: I used flux.

(as a note, but not relevant to the question, I did poke my screen a bit, so it’s “damaged”, but functional. It’s like a wrinkle under the glass, on the side.)