Corrosion from batteries that leaked on Wii Balance Board (more explanation in comment) : r/consolerepair
Wii Balance Board Battery Case Replacement - iFixit Repair Guide
Wii Fit : Rechargeable Battery Pack for Wii Fit™ Balance Board - SHOP01MEDIA - console accessories and mods, retro, shop - One Stop Shop!
Nintendo Wii Fit Plus + DDR Exercise Combo #1 (Balance Board + Wii Fit Plus Game + Battery + Bag + Mat + Sleeve + DDR Hottest Party Game + 2 x Original Konami Dance Pad)
Wii Fit Battery Kit for Balance Board - Walmart.com
Wii Balance Board Battery Case Replacement - iFixit Repair Guide
Fixing corroded battery contacts - EdVoncken.NET
Nintendo Wii Fit Balance Board Rechargeable Battery
Amazon.com: Nintendo Wii Fit - Rechargeable Battery Pack : Video Games
Accidentally left the batteries in my wii balance board for 9 years and it corroded the converters away, is it possible for me to fix? : r/consolerepair
CTA Digital Battery Pack for Wii Balance Board - Newegg.com