Why is my battery draining so fast (100% to 3%) overnight?

My battery problems coincided with starting a study abroad program in Europe. I had bought an adapter for a US to European plug, and it works fine to charge. However, if I leave my laptop unplugged overnight, a full battery just completely drains itself. I reset the SMC, but it didn't work. It's also weird because during the day, if I leave my laptop unplugged, the battery is normal and does not drain as severely.