Inspiron 8600 will not power up with battery attached...

I have a Dell Inspiron 8600 which did not power up this morning: the light on the power cord is lit and glowing. There are no lights on the panel lit.

However, I have shown the following:

1. the computer will not start if the power cable is plugged in with the battery.

2. the computer will not start with the battery without the power cable.

3. the computer will start normally with the power cable without the battery.

4. the computer will start normally and run off the battery without the power cable attached if I remove and replace the battery. The battery is 99 % charged and runs the computer well (the battery is 4 years old).

I reseated the memory chips. Any thoughts? Thanks!