Just use a 2-part plastic epoxy which will have a beige to white color and cost about $4 US dollars.
If you have a black battery just use one drop of black paint when mixing the 2-part epoxy. It uses very very little paint.....just use one drop to start with.
Clean the crack and surrounding areas of the plastic with brake-cleaner that is sprayed on a rag really good. You want a cleaner that has no oil, fat, soap or wax in it.
Then mix your plastic epoxy (equal parts) and if wanted one drop of paint to match the battery color.
Fill your crack and let cure according to your epoxy instructions on the package.
This fix should last for years to come.
I am a maintenance man and the last battery I fixed that way lasted for years until it died on natural causes.
I never had much luck with silicon, it never lasted long enough for me.
Uhu? I remember Uhu as a kid and I used it back then, but at the time they had only one kind of glue. I guess they are coming up in the world.
If you still have warranty, I would see if they would exchange it.