You need to test the charging system on the tractor. You might have a bad regulator or alternator.
A good, fully charged battery will put out 12.6 volts dc without a load.
Read the voltage across the positive and negative terminals with the engine off and the ignition switch off. You should read 12.6 volts or more.
Start the engine and set it at full throttle. You should read 13-14 volts dc. If the voltage doesn't go up with the engine running, you have a problem with the alternator or the regulator.
To test the alternator and regulator locate the regulator on the engine. The alternator is underneath the flywheel of the engine. The regulator is a small plastic box with 1 red and 2 yellow or white wires attached to it. Disconnect the 2 white or yellow wires and run the engine at full throttle. You should read 28 volts ac or more across the white or yellow wires. If the voltage is less than 28 volts ac, the alternator is bad. If you read 28 volts or more the regulator is bad.