MINETY, England — On a clearing at the edge of a farm field, 40 big rectangular boxes the size of shipping containers sit behind a tall fence. Packed inside are stacks of lithium-ion cells and other electrical equipment.Wired together, these units will form Europe’s largest battery, the operators say, able to pump out powerful bursts of electricity to offset fluctuations in the power grid when ebbing winds or cloudy skies slow the generation of renewable electricity. As more power comes from wind and solar, the need for giant batteries will grow.One of the companies behind this 40-million-pound (about $56 million) …