I'm surprised no one has mentioned the simplest diagnostic test: looking at the battery screen icon to verify that the charger is indeed charging.
The icon does 2 things. It displays the amount of power remaining and indicates that the laptop is in charge mode both for the main power source & the PRAM battery.
The green-to-amber light on the charging plug can be deceptive. It will go to steady amber, indicating the battery needs charging. But if you have a loose or broken connection, the battery charger may be doing nothing at all.
I found this out when trying to recharge my Powerbook after a long period of disuse. I relied solely on the amber light, and 24 hrs. later, concluded the battery was dead/no longer accepting a charge.
Bought an aftermarket battery on eBay, and made the same error. Then I noticed the battery icon was not showing the charger was plugged in.
A couple twists later to get a better connection, both the original & PRAM battery came back to life and my Powerbook is now working fine.
It's also not wise to plug the charger into an desktop APC. W/O a blocking diode, the APC's lead acid battery is in continuous trickle charge mode, and can literally suck the Powerbook's battery dry.