how Windows XP measure laptop's battery level

[mdanh2002]

I recently replaced my laptop battery. The old battery lasts only 20 mins even with minimum usage. Even if fully charged, the battery level reported by Windows XP is only 91% and can never get to 100% (why?). If the charger is removed, e.g. during discharging, the battery level slowly decreases at a rate of 1%/1min until around 60-70% where the battery level suddenly drops to 3%, resulting in a 'Critical Battery' warning. At this time, the laptop attempts (unsucessfully) to go to standby mode but before it can ever get to standby mode, the battery drains and the notebook turns off.

I am amazed by the sudden drop of the battery level. I wonder how the battery level is measured, is it by Windows or is it by the battery circuit? What are the factors used to measure the battery level, e.g. voltage/current? I am not sure what formula was used that can cause such a sudden drop in the battery level. Strangely the estimated battery life reported by Windows is accurate - it reports around 20-30 mins right from the beginning. Any ideas?

Btw, my new battery can last for around 3hrs with average usage.

 

[arunb]

Batteries have something called 'memory effect'.

They show full charge even if the battery is not fully charged.

thanks
a

 

[3v0]

When I replaced my battery the instructions asked me to charge and run the battery down several times. Each time it lasted longer.

 

[mdanh2002]

Batteries have something called 'memory effect'.

They show full charge even if the battery is not fully charged.

thanks
a

I thought only Ni-Cd batteries have memory effects. The newer Li-ion battery don't have. My laptop has a Li-ion battery.

 

[Hero999]

The battery level is measured by a state of charge sensing circuit either on the motherboard of in the battery module. The reading is connected to the CPU bus and a device driver written for the operating system tells the CPU how to read the battery level.

It's possible that the state of charge sensing unit needs to be recalubrated as it's used to the charging and discharging the old battery which was dead. It'll either sort itself out or there'll be a procedure that you have to follow to remedy it.

 

[3or24fan]

Check your laptop's BIOS for a battery calibration program. Some laptops have them, so don't.

 

[davidbball13]

try to flash the BIOs it helped for my IBMT41

 

[mdanh2002]

The battery level is measured by a state of charge sensing circuit either on the motherboard of in the battery module. The reading is connected to the CPU bus and a device driver written for the operating system tells the CPU how to read the battery level.

It's possible that the state of charge sensing unit needs to be recalubrated as it's used to the charging and discharging the old battery which was dead. It'll either sort itself out or there'll be a procedure that you have to follow to remedy it.

Thanks. You are right. The measuring of the battery level was a bit strange after the battery was replaced. However it became stable once I went to BIOS and calibrate the battery.

 

[Salgat]

I opened up a laptop battery once. You'll find a circuit that goes across the positive and negative ends of the battery along a thin PCB.

 

[Hero999]

That's a battery protection circuit that prevents the battery from blowing up if the charge malfunctions.