What is the correct way of measuring remaining battery capacity?

[.Scott]

A major issue here is the definition of a discharged or uncharged battery. ... And I am really surprised at the way that battery discharge is being described now compared to how it was described 30 years ago.

Traditionally, batteries were considered "discharged" when they could not longer power the device they were powering. So whether a battery is "discharged" or "fully discharged" isn't exclusively depended on the internal battery state - the load it is expected to support is also part of the "discharge" condition.

Playing with Google for a few minutes hasn't turned up that definition for me - but it use to be topic one in any discussion of battery life or battery charge condition. Certainly part of this change reflects that modern rechargeable batteries are often used in devices with very wide load ranges. If you're running GPS apps on you cell phone, the battery drain and the discharge point will both be higher than if you only are playing sudoku.

Still, if you are expecting your device to continue functioning until your battery meter goes to zero, don't expect any off-the-shelf solution to work. You will need to match your meter to your whole device - not just the battery.

 

[HansH]

There are also methods based on measuring the AC impedance of the batery. The AC impedance relates to the state of charge while the frequency where the impedance is measured is still a degree of freedom.