The grey top of a large bank of lead-acid cells forming a battery with thick black interconnectors. There are 12 cells for the 24v battery. It's hard to tell from the pics, but the cells are tall, at knee height. Each cell weighs 55kg.
https://media.mastodon.scot/mastodon-scot-public/media_attachments/files/109/698/506/112/102/509/original/83db9904f73be975.jpg