I am having this same issue. There are some Firmware updates that you can download from the Apple site. Just look up SMCUpdate and find the one that matches with your model number on your Mac. Just download the right version and install that. These address some issues that may be present in older Macs that are using more modern charging units. Unfortunately for me, these updates won't work on my Mac. My system keeps rejecting it, saying that the computer doesn't need it. That's the source of my issue. The update is supposed to fix problems with power management and allocation, which may cause your older computer to not recognize the newer charger as a valid power source. Give it a try. These updates are supposed to automatically done, but older Macs are lacking in support. I don't believe that they always auto-update if they are old.
Hope it helps you, at least! :D