Dale on Twitter: "Flipping the board over reveals the battery. It's a CR2032 button cell, but annoyingly it has tabs spot-welded to it, and must be desoldered. You can buy these batteries
Battery for Yamaha DX7, 7II & 7IIFD Synths - Internal Memory Replacement Battery | eBay
CR2032 Battery PCB adapter for Yamaha DX7 synth – OSH Park
Yamaha DX7 Cartridge Battery | Analog Classics
DX7 Battery Replacement and Battery Holder Install - YouTube
Orig Yamaha DX7 FM synthesizer + kaces bag & New battery | eBay