I suspect you have a bad alt or connection. If you have a charger available, give your battery a good charge first disconnected from the tractor. Like Bulldog says measure the battery voltage difference with a multi meter between running and not. I don’t know about the tractor world but in automotive you should have about 13.2 volts at the battery with the tractor not running and about 14.5-16 volts running. This will tell you if the alt is working but won’t tell you the amperage output. You could be putting out 14.7 volts and only 1 amp, that won’t keep the battery charged. If you have a friend at an auto repair facility he most likely will have an inductive amperage gauge. You simply lay this device on any battery cable and it will display the rate of charge/discharge in amps of the battery while running, it can also be used to check the amperage load from the starter while cranking. If you don’t have access to any meters or gauges you can remove the battery and alt from the tractor and take to most any large auto parts store and have them tested at no charge. If it turns out to be the alternator try to find a starter/alt shop in your area and have it repaired, much cheaper than have to replace through Kubota.