how does windows measure % battery capacity?

[joeld]

My old laptop battery was giving up on me, so I purchased an aftermarket one. It's the same specs but 4400mAh instead of 4300mAh. The battery life is great, but check out this graph...

Graph GIF link

As you can see, the laptop drains at a constant rate down to 7%, where it stays for over an hour until finally completely draining. I used the BatteryStatus\RemainingCapacity and the System\SystemUptime fields in a counter log to get the graph, and the percentages are just the value reported divided by the maximum value reported times 100. 7% corresponded to 4144 in whatever units is used by the log for the capacity.

Anyway, got any ideas as to why this is happening? Thanks.

 

[Nothinman]

Probably either a problem with the firmware or the battery's reporting, AFAIK Windows just gets it's information about the current discharge rate from ACPI.

 

[bersl2]

Seconded. It's probably hardcoded at 4300.

If you wanted to, you could look at the mess that is ACPI up close and personal by disassembling the firmware and looking at the code.

 

[RandomFool]

I dunno if this helps but my old Compaq laptop had a tool that (i think) calibrated the battery remaining time. Basically it drained the battery then charged it fully and drained it again. Now the time remaining works a lot better than it used to.

 

[frostedflakes]

Yeah my laptop has a battery calibration option in the BIOS, so check there or online if there is software that will do this.

 

[corkyg]

Battery calibration in effect, calibrates the "gas gauge" in your laptop. It is common to may brands, and is usually found in the laptop's BIOS. The procedure is, fully charge the battery - then run that program. It totally drains the battery and then recharges completely. In the process, it sets or calibrates the % readout.

Some OEMs suggest doing this about twice a year. It helps keep the battery in good condition as well.