ThinkPad t42 has power problems.
Hey there! Without having your T42 in front of me for diagnostics, I can only give you my best educated guesses, but here's exactly how I'd approach this puzzling battery situation if it landed on my repair bench.
I'd start with the simplest things - checking your AC adapter with a multimeter to confirm it's delivering a steady 16V at the proper amperage. Sometimes adapters work enough to power the system but not enough to charge. While I've got the easy stuff out, I'd perform a quick power management reset by removing both battery and AC adapter, then holding the power button for 30 seconds to drain any residual power - this often resets confused power management circuits.
Even though the T42 is ancient by tech standards, I'd check for any available BIOS updates from Lenovo's support site. Old ThinkPads sometimes had firmware updates specifically targeting power management issues, and this takes just a few minutes to try. Similarly, I'd reinstall the ACPI and power management drivers in Windows, as corrupted drivers can cause exactly these kinds of phantom charging indicators.
Next, I'd take a good look at the DC-in jack and the small circuit board it connects to. These jacks get loose over time, and if there's intermittent connection, the laptop might incorrectly detect AC power. Wiggle the connector gently while watching power indicators - if they flicker, you've found a likely culprit. Replacing the DC-in board is relatively straightforward and inexpensive (usually $10-20).
If that's not it, I'd try calibrating the battery if possible, or testing with another battery to isolate whether the issue is battery-specific. Since you mentioned the battery works when charged externally, I'd check the pins and contacts for corrosion or damage. I'd also run any battery diagnostics available through Lenovo's Power Management tools to check battery health and status.
Getting a bit deeper, I'd open up the laptop to inspect for obvious signs of damage - looking for corroded traces, burnt components, or bulging capacitors around the power circuit. Using a multimeter, I'd check the fuses (particularly F2) near the DC input for continuity, as blown fuses are common failure points. I'd also test for proper voltage at the battery terminals when AC is connected - if power isn't reaching there, we've narrowed down the problem.
For the phantom AC detection issue, I'd examine the resistors and diodes around the DC-in area that handle the ACIN signal. This signal tells the system when AC is connected, and if it's stuck "on" due to a faulty component, that would explain why the system always thinks it's plugged in.
As a last resort, I'd check the charging IC (often a TB62501 in these models) and the power management chip for signs of failure. These can sometimes be reflowed if there are solder joint cracks from thermal cycling, though replacement is more reliable but requires advanced soldering skills. I'd also look for any signs of liquid damage or corrosion on the motherboard, especially around the power circuits.
Given the machine's age, it might eventually come down to component-level repair versus finding a donor board, but don't give up - these old ThinkPads are built like tanks and often worth saving!
Good luck troubleshooting! Take it step by step, and don't hesitate to reach out if you hit a roadblock or need help interpreting test results. These classic machines have plenty of life left in them when treated right.
-Robert Cedar ' Owner, Geeks On Command Computer Repair