Corrosion from batteries that leaked on Wii Balance Board (more explanation in comment) : r/consolerepair
ZZZZS 6S Battery Balance Board Battery Equalization Board Inductive Active Equalizer Balancer with Cable Light Indicator 1.2A 2.0V to 4.5V : Amazon.fr: Commerce, Industrie et Science
Nintendo Wii Balance Board Battery Compartment Fix Repair. - YouTube
Wii Fit Balance Board Rechargeable Battery 3800 MAH (Not For Wii Fit 2) | eBay
Okuyonic Batteries Balance Board, 0-5.5A Balance Courant Large Application Haute Précision Résistance Interne MOS Égaliseur de Batterie 1.8V-4.5V pour LTO LFP LPO NCM(4-5S) : Amazon.fr: Instruments de musique et Sono
2.3a Balance Board 1s 3.2v Lifepo4 Lithium Iron Phosphate Battery Balance Board Balanced Current 2. | Fruugo FR
Amazon.com: Yunnyp 3800mAh Balance Board Battery,High Capacity Rechargeable Battery for Wii Fit : Video Games