The battery on my Dell 1720 is good, recognised by the computer, charges but will not power the computer. Removing the power lead causes immediate shutdown

SOURCE: not charging

Dell PA-10 & PA-12 have internal serial ROM chips that communicate with the PC via the tiny center pin in the powerr plug.

If that pin gets bent or is not making proper contact, the laptop USUALLY complains and runs from AC adapter but refuses to recharge the battery ..

SOURCE: Dell inspiron 6400-runs slow on AC power but not on battery

uninstall any energy management software provided by dell and then check.if the issue is not resolved then the issue is with the converter board as it is heating up.

SOURCE: laptop will not turn on, mains adaptor box green led light flashs

Sounds like the power jack is damaged or faulty.

SOURCE: Inspiron laptop Battery won't charge

Try taking out battery and plug in power. If it works, battery is faulty. If not,
Try taking out your hard disk plug in power, if it works, hard disk is faulty. If not,
Try taking out memory card, if there's a sound, memory card faulty. If no sound, it's your motherboard that is causing the error.

Note: If machine is under warranty no need to pay for the repairs.

SOURCE: Dell Inspiron 9400 will not charge - AC adaptor not recognised

Does the laptop power up just no charge the battery? If the laptop does not power on then you may have a bad power supply in the unit. If it does power up than you most likely have a bad battery connector of blown fuse on the motherboard. The flashing of the BIOS is done with a bootable cd or floppy but this will not keep the new battery from charging. The reason Dell states that you need at least 10% battery to flash a BIOS is because if your AC power goes out while you are doing a BIOS upgrade and you don't have battery power the BIOS will be incomplete and your laptop will not boot again without replacing the motherboard.