HP laptop powers on with adapter but won't charge battery - charging circuit?

Question

HP EliteBook 840 G9 powers on fine with the AC adapter connected but the battery won't charge. Battery shows 0% and 'plugged in, not charging' in Windows. Tried a known good battery and same issue. Also tried resetting the BIOS. The AC adapter outputs correct voltage. Starting to think this is a board-level charging circuit issue. What should I look at?

Danny Tran · Accepted Answer

On HP EliteBooks, the charging circuit involves a few key components. Start by checking the charging MOSFET — on the 840 G9 it's usually near the DC jack area. Use a multimeter to check for voltage on both sides of the MOSFET. If you're getting input voltage but no output, the MOSFET or its gate driver is likely blown. Also check the EC (Embedded Controller) firmware — sometimes a BIOS update gone wrong or a power surge can corrupt the EC and it stops sending the charge enable signal. Try doing an EC reset by disconnecting the battery and AC adapter, holding the power button for 30 seconds, then reconnecting. If you need to go deeper into the board, this is where microsoldering skills come in handy. You'd want to check the schematic (available on various repair forums) to trace the charge path. The issue is similar to what MacBook techs deal with — there's a great thread about MacBook charging issues that covers similar diagnostic approaches.