My laptop power supply is rated for output to 19.5v, 7.7A.

However, it's kicking out 41.5v. Strange thing is that before I measured this voltage, I have been using it to power on my laptop and it works fine. There's a problem, however; I've been trying to understand why my battery doesn't hold a charge. I have it plugged in and it charges (according to my battery manager) up to 84% and never higher than that. The status also reads to be "plugged in, charging." When I disconnect the power cable, the machine shuts down and cannot be restarted with only the battery. I have to plug it back in to get it to restart.

Why would it do that and can I fix it? Is it still safe to use to power up the laptop, which requires a 19.5 VAC power supply even if it's kicking out so much more voltage?

  • \$\begingroup\$ did you measure the output voltage when connected to the laptop? \$\endgroup\$
    – jsotola
    Apr 5, 2021 at 23:33
  • \$\begingroup\$ no, I didn't but Tony's last sentence made me check my multimeter settings and everything checks out. Thanks, jsotola! \$\endgroup\$
    – Abe Alaniz
    Apr 6, 2021 at 0:10

1 Answer 1


Your battery has dead cell.

Your charger is missing the 1.25V voltage feedback input.

If it is 3wire type, it senses voltage the plug and ratios it down to the Vadj reference voltage. That wire could be broken otherwise internal. The 41V is typical maximum for universal boost regulators with selectable taps when open loop or the feedback wire is open or shorted to 0V. I hope/trust that the output caps are rated higher than 41V.

I have used this fact to make one 12 to 30V for a 50W LEDS array with a dimmer pot and a couple discrete passives, just like any adjustable SMPS regulator.

It just happens that since laptop OEMs stopped competing with each other with unique voltages and plugs that 19.5V 3.5mm became a standard or perhaps by peer agreement or some industry std. The tolerance on the 19.5 is likely >10% and the laptop will have its own regulators for running without a battery and a separate one for battery CC,CV & cutoff.

If your charger isn’t DC then disregard.

  • \$\begingroup\$ I am so sorry to have wasted your time, everybody. I made a HUGE rookie mistake when reading the voltage coming out. Tony, your last sentence prompted me to check my multimeter... and I had it set to read A/C; makes total sense why it would read double. Thanks so much for that easy fix. \$\endgroup\$
    – Abe Alaniz
    Apr 6, 2021 at 0:08

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