Start by checking the AC output from the alternator.
There should be 2 wires coming out from under the flywheel that are the same size & colour, these are the AC feed from the alternator and they should go to a box of some sort with a single heavier wire coming out of it,
That is the DC feed to your battery
You should see 26V AC or better between the 2 wires that are the same colour when the engine is running.
Next check the DC volts on the other wire, you really need to disconnect it if it is low to double check but if it reads 13V or better between it and the engine case ( ground) it is fine and your problem is a break or short somewhere in that wire between the rectifier ( the box ) and the battery.
Get that done & get back to us