Accessory Battery Not Charging

Hello, I'm hoping that this might be something simple. In short, the accessory battery on my boat stopped charging suddenly. I noticed that the small black wire from the BEP VSR had come unattached, but was not sure where it had originally been. I don't see a common bus bar for the grounds, so I hooked it to the ground of the starter battery. That caused the red light to come on for the VSR, and I thought that I had fixed it. Nevertheless, the accessory battery still will not charge. Should I try connecting it to the negative terminal on the accessory battery? Thoughts? There is a large 40 amp fuse in the positive lead to the second battery, and I was also wondering if maybe that blew. I tried removing it, but could not pull it without using extreme force. and was concerned that I might damage it (or something else) if I kept trying. The starter battery is fine. I can charge the accessory battery but it just dies if I use it, even if I have the VSR set to "both". Thanks for your input

Jeff

FWIW, I fixed it. Essentially I ran another wire between the negative pole of the accessory battery and the starter battery. It seems that all of the grounds need to be tied together.