Notebook suddenly shuts off when battery at ~20%

[bar_foo]

I'm having an issue with my notebook's battery: when it gets down to 20-21%, the system suddenly shuts down without warning and won't restart unless plugged in/recharged. I've run the HP battery check diagnostic and it reports that the battery is good. I've also eliminated a Windows issue because the same thing happens if I drain the battery to about 25% then reboot to BIOS and, for example, run the hardware tests: after a while the system shuts down without warning.

My best guess is that there's a bad cell in the battery. Any other ideas?

[banhien]

@bar_foo 

Please try

(a) Right click Battery icon,

(b) Open Power Options,

(c) Select your current (active) plan,

(d) Select Change advanced power settings,

(e) Change 2 places as shown

       

(f) Click Apply to save.

Note: Please do not use my numbers and actions because I use all may machines on Main 99% of its time.

Regards.

[bar_foo]

Thanks for the suggestion but changing the power plan won't fix the underlying problem. I  can set the critical battery level to, say, 25%, and have it shut down at 22%, and I will avoid the problem, but it means losing 1/4 of my battery life.