If you have the 3 amp charging system, you won"t see that large of a voltage increase.
the 3 amp system is actually 3 parts. one part charges the battery, the second runs the lights, and the third is used to put a load on the engine when its shut off, so it will stop faster.
the lights run off the AC coil under the flywheel, not the battery, UNLESS you have the engine speed below 2600 rpm, and they will then draw from the battery, and you"re charging output is greatly reduced as well.
If you have an electric PTO clutch, then you have the 15 amp system, and you should see 13.8 volts or so. If you don"t have electric PTO, then you most likely have the 3 amp system. Deere did have a service bulletin concerning this, and the fix was to upgrade the system to the 15 amp system.
Here is the theory of operation from the service manual.
Function: To maintain battery voltage at 12.4 volts or higher, to provide electrical power to light the headlights, and to reduce the time it takes for the engine to shut down. Operating Conditions: The engine must be running for both the charging system to operate, and the headlights to be on. The shunt circuit is functional when the engine is turned off. System Operation: The stator provides DC current for charging the battery, an independent AC circuit for headlights, and an additional independent AC circuit that is used to place a load on the alternator when the engine is shut off. The battery is not used for lights, so lights are available even if the battery is disconnected or removed. However, the lights will draw current from the battery when the engine is running and below 2600 rpm. Current for lights is available as long as the engine is running. The output depends upon engine speed, so brightness of the lights changes with engine speed. Magnets mounted to the flywheel rotate around two coils of wire, (the stator), mounted underneath the flywheel. As the magnets rotate, current is generated in the coiled wire and converted, with a diode, to DC current to charge the battery. The current from the DC side of the stator is unregulated and is rated at 3 amps. The output rises from 2 amps at idle to 3 amps at full throttle. The shunt circuit is used to cause the engine to stop faster when the engine is shut off. When the key switch is in the RUN position and the PTO is off and the brake is on, or the operator is on the seat, the shunt relay is energized and opens the contacts that connect one phase of the alternator to ground. This reduces the load on the alternator to 2-3amps for recharging the battery and 3-5 amps for illuminating the headlights. When the ignition is grounded to shut the engine off, (See “IGNITION & SHUTOFF CIRCUIT OPERATION– LT133/LT155”.) the shunt relay is deenergized. This allows contact across the normally closed set of contacts, grounding the alternator. This places a load on the alternator, which in turn places a load on the engine. Since the engine ignition system has been turned off, the engine has no power to turn the extra load. The result is that the engine stops spinning faster.
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