I understand that the system can read the current voltage and determine battery remaining percentage from that. Say, if a Li-ion battery in a mobile phone is at 4.3V, it's likely the battery is full. This method is quite reliable even if the battery is worn-down, i.e. when it cannot generate as much power (Watts) as it could when it was brand new, but its voltage will remain mostly the same.

My question is, while voltage can be used to determine remaining percentage, how does the system give an accurate "estimated usage time" for a worn battery? Assume there's a device running at a precisely fixed power (e.g. 1.0W), and it's able to run for 6 hours on a half-charged (50%) new battery. When the same battery is half-worn out and also at 50% charge, the system can correctly estimate that the device can run for 3 hours. Consequently, the system is able to determine by how much the battery is worn out. How is that achieved?

  • \$\begingroup\$ Does it actually ever give an accurate estimate? \$\endgroup\$ – pipe Apr 16 '18 at 13:42
  • \$\begingroup\$ @pipe Not sure what you mean. The system obviously give estimation based on current battery output current/power and remaining capacity. I'm surprised it respects battery wear. \$\endgroup\$ – iBug Apr 16 '18 at 13:51
  • \$\begingroup\$ I mean that I can also estimate the battery time - that doesn't mean it's accurate. \$\endgroup\$ – pipe Apr 16 '18 at 13:57

The way that the estimations for remaining time work using either historical data and / or current being drawn from the battery of a known capacity, or most usually a combination of the two.

Most laptops and systems will be able to ascertain charge current and discharge currents at a given moment, these values may come from a current sensor OR often by reading the % charge status over time, and a known 'full' capacity, and estimating the current draw (or charge current, etc). These readings are usually time-moving-averaged over several points to provide the estimate of remaining time based upon the usage conditions at that point in time.

With respect to the precision, they are rarely THAT precise TBH, though with the accumulation of past data they can become usable in the way you describe.

If they have battery voltage and an estimate of current, then power usage of the entire system become trivial to calculate. (Again this is just an estimate)

With respect to battery 'wear', this is generally more calculated by the amount of charge that has been successfully put back into the battery in, versus a known 'full' capacity. A lithium battery will be charged initially at constant current, and then switch to constant voltage later on. I am not sure where on the the charge 'curve' they would usually take the reading for wear, but my suggestion would be that the simplest approach would be to record the point at which CC switches to CV, however systems that can record the current and voltage, would be able to integrate the charge curve to provide the actual charge inserted into the battery, and this would be far more accurate.


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