Dell Inspiron 6400 indicates the wrong power adapter is plugged in and will not charge the battery

Having just paid over £100 to have the the power board inside my daughter's Inspiron replaced and had the explanation confirmed by the Dell engineer, here is what happens:

The power connector consists of 3 pins, 2 are power and earth, the 3rd is a signal line to identify the PSU as a compatible (Dell) unit. If the Laptop does not get the signal, it does not allow the PSU to charge the battery, it will, however, allow it to power the laptop. The fault is a common one and happens because the plug sits so proud from the side of the laptop that it constantly gets knocked and causes the connector on the power board to fail, most commonly open circuiting the signal line.

To clarify, the power board on some Inspiron models is a separate unit to the motherboard and has the power socket mounted directly on it.

As the message is happening outside Windows, it something going wrong at the BIOS level. I.e. a hardware failure of some variety. A loose connection, cable or a faulty chip on the motherboard. You can go into your BIOS (F2 during POST) and check the status of your AC adapter there too if you are curious. It will most likely be under the Battery Info subheading, under System.

If your laptop is still under warranty with Dell, give them a call. If you do not have a spare AC adapter that you can borrow from a friend or work colleague, I see no reason why they would not send a replacement out to you. Batteries are not covered under warranty so great, but AC adapters definitely are.

If the replacement AC adapter from your friend or from Dell also fails to work, it would indicate to me that there is a fault on the motherboard. Again, Dell should be able to replace this under warranty. If out of warranty, they should still at least be able to offer the part at a cost to you. Given the cost of the replacement part from memory, it will probably be cheaper to buy a new laptop or just live with it.