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The batteries are in series. It's just a bit unusual because the tractor frame is smack-dab in the middle of the 24 Volt circuit, so 1/2 of the lights and accessories operate off of (-) 12 Volts and the other half operate of of (+) 12 Volts, while the generator and starter work with 24 Volts. The system is not hard to work with, if you have a manual with a wiring diagram, and are open-minded to the unusual system when you study the diagram. The battery cable on the starter solenoid should be from the (-) battery post, and the cable on the side of the starter belly next to the engine should be from the (+) battery post. The opposite connection from each battery is connected to the tractor frame. There are two heavy-duty circuit breakers on a metal plate mounted next to the starter solenoid. The source for the (+) 12 Volts and (-) 12 Volts for the instrument panel, key switch, and lighting circuits are through these breakers. You can use the output side of these circuit breakers as a source for powering accessories, such as lights. For the 24 Volt charging system to function properly, you must keep the load on each 1/2 of the system as nearly equal as possible. The circuit breakers mentioned above are VERY IMPORTANT for safety in case of a wiring short. I'm not sure if all the 4020's had them. Deere had a kit to update the 4010's to include the circuit breakers, and that may apply to early 4020's as well. If you do not have them, I would check with Deere to see if the kit to install them is still available. I'm sure a replacement wiring harness is still available from Deere, and, while costly, if your wiring is torn up, it might be better to start with a new harness than to risk an electrical fire if the poor wires short out. Often, the two smaller breakers and the light and key switches, all mounted beneath the steering wheel, have seen their better days, too.
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