This is what I found when I googled it from DSI site
If liquid of any kind has been spilled in the Nintendo DSi or Nintendo DSi XL system, remove the battery cover and battery and allow the system to dry. If a significant amount of liquid touched the battery, it must be replaced. For information on ordering a replacement battery directly from Nintendo, please click here. If allowing the system to dry and replacing the battery do not solve the problem, your system will need to be repaired. Please click here for repair options.