Negative value for Morale?

In my current campaign I have defeated most of my foes except the British. Since I was fighting in the east with my generals and Horatio Nelson was still around I sent a small 5 unit cavalry force to British Isles. Pillaged and looted the country and kept distance from the British armies trying to capture me. So, when I finally killed Nelson and destroyed their merchant fleet I forgot to move one of my cavalry units away from the British army.

They attacked my lone light cavalry with their entire army. Since there was no way I could win I was ready to autoresolve it. But then I checked enemy army and found their Morale really low. Some of their units had -1 Morale? I know I play with Easy but still.. negative Morale? Anyway, I tried fighting the battle. Managed easily rout 2 enemy cavalry units until their cannons routed my men.

I would like to know what determines the Morale value?

That is one I don't know. I have not gotten that deep into the files.

Maybe someone else knows something.