/diy/ - Do It Yourself

I bought a new 12V motorcycle battery, filledit to thz upper line with acid, and then began charging it. The manual said that the battery is already fully charged but it's good to charge the battery for like 5 hours before using it.
When i started charging, i measured 12.54V. but suddenly after 3.5 hours i measured 18V. I immediately took the charger off the battery. I measured 15V on the battery and 18V at the charger. I was like wtf and let it sit for a couple of days. The battery stabilised at 12.54V, and i measure 12V on the charger when it's not connected to the battery. But as soon as i connect the charger to the battery i measure 15.5V and keeps increasing slowly.
I'm afraid that it will or already damage the battery or charger. Can someone explain what's happening? Pic related my battery and charger