Computer is not charging battery, although it says it does

Hello all, and thank you in advance for your help. Here's the situation:


I have a HP G71 laptop computer. A few weeks ago, the power supply died (confirmed by a multimeter) and I bought a new one on eBay from China (big mistake, I know). The computer worked fine for a few hours, but then a new problem occurred: Windows tells me that the computer is plugged in and charging an the charging light is on, but the battery is depleted. If you take the battery out, the computer doesn’t work at all. I have Linux on that computer too, and it's the same on Linux.

The light next to the AC jack is on so long as the computer is plugged in, whether the battery is in or not. I had that eBay adapter checked, and it's only producing three volts (instead of 18.5). However, I checked the computer with two other adapters that are working (tested with a multimeter) and the problem is exactly the same: it won’t work without the battery, and with the battery it says it’s plugged in and charging but it’s not charging.

Questions:
(1) Is this a motherboard issue? If so, can it be fixed or must the motherboard be replaced?
(2) Could the adapter have done that? Did it fry my computer?

Thank you for your help and advice. If you need more info, please let me know.
MJ