Diesel power is now almost 4X the cost of the grid. Battery cost is
2X - 3X grid cost and the combination is just now about equal to or in some areas already below the cost of diesel.Unlike electric road vehicles and plug in hybrids, battery energy density is not much an issue with farm tractors which can recharge every few minutes. 90 watt/kg is more than enough energy density for almost all farm operations.
For example, for a 400 HP articulated diesel tractor engine equivalent, two 350 LB batteries -- about the same energy as 2 gallons of diesel -- are cantilevered off both sides of the electric tractor, each with a vertical conductor mounted on top to contact wires at the ends of the field.
When the tractor reaches the right hand U turn end of the field the left outrigger picks up a recharged battery. After the U turn the outrigger drops off the discharged battery for charging where it can be picked up on the next lap.
The right side battery is swapped out at the other end of the field when the left hand U turn is made.
Depending on use the batteries last a month or so, changed and recycled much less frequently than motor oil.
If such a system was available now, it would be more cost effective than replacing with diesel.
There are all kinds of farm situations and there will be all kinds of solutions. In the long run for some applications, it might be cheaper to eliminate the battery cost and run straight from the grid, either by trolley wiring the entire field or with something like a pivot structure to deliver the power to a tractor.
The original single battery single wire idea where the driver waits at the end of the field for a recharge was the absolute cheapest easiest electric tractor to prototype and demonstrate. It was just a way to get started.
Bret Cahill