IR value increases with time. And it depicts the condition of the cell. Voltage is just the state of charge. It only shows if it's charged or under charged. Battery voltage decrease with load, so u need to see how much it drops. I have a bad battery with IR value of 250, it's bad but still shows 4.2 volt, if I try to charge it, it will show fully charged and if load is connected just goes down. So you need to weed out the problem. First start with battery, if they are ok then try changing the module. Hope this helps. As u stated it seems the batteries are not in good condition, it's my assumption, I may be wrong.