@roomba870owner I don't think we have enough information here. You need to let us know the exact specs of the batteries as well as the charger. In order to determine what is going on you need to attach an ammeter and see what amperage your iRobot’s draw and see what the difference is within each. For now, I suspect that you are using the same battery in different Roomba's that may have different power requirements, so you should not expect the same "runtime". Let us know what you find.