Cannot install old BIOS

[Mori-Sabii]

Hi.!
My laptop won't charge batteries, which according to some search, seems to be caused by the BIOS update I've done (F.66 Rev.A).

I've tried downgrading the BIOS, In windows, the BIOS utility says my BIOS is newer and it cannot proceed.

In DOS interface, it tells me: "The computer must be running on AC power to perform this operation" and won't proceed. (I'm already on AC power)

In the BIOS upgrade utility, it will either give a generic error message or a message similar to the one above
In emergency BIOS recovery (pressing the power button while holding Windows + b), Caps Lock LED will keep blinking and nothing happens.

I'm out of Ideas, when I charge the batteries some place else, they work on the laptop, but it won't charge them. the new BIOS also seems to have caused other issues as well (system automatically turns on after being plugged in and displays a message related to Intel ME, and turns off, and there are "BIOS Checksum error" messages each time I turn on the laptop)

How can I downgrade the BIOS?

Thank you!

Edit: I've managed to downgrade to F.63 Rev.A, but it won't downgrade further. This seems to have solved the Issue with laptop turning on after being plugged in,showing Intel ME related messages, and invalid checksum errors. But still it doesn't charge the battery.

One thing I've noticed, When turning the laptop on with AC attached and batteries in place, after taking out the battery, laptop turns off even though it's plugged into AC.

Edit: previous problems are happening again, Voltage reading on battery pins are around 2.5 volts without battery attached.