what exactly does "system battery voltage is low" cause on bootup of a computer

Bad batteries have caused boot-loop.

"when pc boots into OS, low battery does not matter any more."

I have to differ on that one.  When the battery was low in my PC, it would freeze and give me BSOD's.  When I thought I needed to reload OS, it wouldn't do it.  This was on my SSD.  Yet my other boot drive, an HDD, worked fine.  Also, no problems with a low battery in my previous 755 other than having to press F1 to continue.

So in other words, symptoms can vary by PC and configuration.

A "system battery voltage is low" message on bootup typically indicates an issue with the CMOS battery on your motherboard. This small, coin-shaped battery provides power to the CMOS (Complementary Metal-Oxide Semiconductor) chip, which stores essential system settings like date, time, and hardware configuration even when the computer is turned off.

Here's what a low voltage in the CMOS battery can cause:

• Losing system settings: The most common consequence is losing BIOS settings. This means you might need to reconfigure the date, time, and boot order every time you turn on your computer.
• Booting issues: In some cases, a very low voltage can prevent the system from booting properly or cause unexpected behavior during startup.
• Data corruption: While rare, severe voltage issues could lead to data corruption in the CMOS memory itself.