Is it my Roomba or my battery? You can go to the "How Do I?" page to help you identify whether your Roomba is suffering from a defective charging system or just a poor performing battery. Sad to say but if the battery will accept a "DIRECT CHARGE" from your power supply but won't charge inside your Roomba then you can suspect either the U2 or U4 MOSFET or both on the circuit board have failed. The two pictures presented here show the physical location of these two transistors.