Lights

I put it in the fridge, it doesn't work. I take it out, it works. Therefore I assumed it was temperature related.

I also waterproofed the battery with a fair amount of sealant.

Does it have a set of built-in rechargeable batteries?

If so, your batteries are dead... no more recharge cycles left in them.

The fridge trick is used in prisons to prolong the life of batteries. It helps restore some of the charge capacity after it's been run down.

Biting batteries achieves the same... but shortens the life dramatically in a different way.

Basically... the super-cheap bit resulted in them shipping super-cheap batteries.