Battery charge Indicator. Directly connect this circuit board to you battery , positive to battery + and negative to battery - . It will show battery charge left. If 4 led glows then its 100% charge, if 3 led then 75%, if 2 led then 50%, if 1 led then 25%. And if no led then battery drain out and need to charge again.This module is for only 3.7v 1S lithium Ion or lithium Polymer Battery. not for lead acid B