Is the battery replaceable on this product ?

Given the comparative cost of an entire new light versus the rather high price of lithium batteries, I'm betting that even if you can find the right one, it would be cheaper and simpler to just buy a new spotlight and throw the old one away when it's no longer functional. Murphy's law says right after you replace the battery the trigger will break or the charger will fail and the whole unit will once again be useless anyway. Probably not worth the effort.

As I understand it there are no user-replaceable parts on this light.

Technically no... but knowing the life expectency of a lithium ion battery, I'm not going to worry for about 5 years. (the case is a typical clam shell design held shut by screws so if you tinker in electronics, I'm confident one could order replacement cells and trade them out)