On my Android smart-phone, I have the third-party “GSam” battery monitoring app installed.
This app can be a useful tool, to determine which other apps are causing the greatest battery drain. It gives very detailed information about the battery and its charging behavior.
Further, this app will state the battery voltage – in addition to the percentage charged – any time it is clicked on. This is where I obtained the numbers I used in this earlier posting.
At lower current-levels of battery-drain (-13mA), this same app showed the battery as 96% charged, but with a voltage of 4.24V . The app continues to run in the background, when the phone is asleep, and when the phone may be drawing much less current than it does with the display on. Then, after we wake up the phone, this app initially displays with its remembered values, until a few seconds later, the app-data updates.
(Edit 12/14/2016 : In the case of a soldered-in battery, it would make perfect sense if the O/S of the device computed the State Of Charge as a linear function with two fixed voltage end-points, as well as to compensate for the amount of current drawn, as if the battery simply had an assumed series-resistance. This is because a soldered-in battery is not assumed to be changed. However, multi-pronged battery-packs also exist, which possess internal chips. Those could be exchanged easily by the user.)
(Edit 12/12/2016 : Actually, this app does not tell the phone, what the State Of Charge of the battery is – the Percentage Charged.
And so there will be a scattering of relationships, between voltages as measured by the device, and percentages. However, one concept which intrigues me, is that if each battery-pack has 4 prongs, there is no way for me to rule out, that 1 prong could be for discharging, while 1 prong could be for charging.
If that were the case, then the charging circuit would detect that the battery suddenly seems to stop drawing current from its charging terminal, and could then immediately measure the voltage on the discharging terminal.)
The advantage this would offer, instead of setting up an arbitrary communications-protocol between a battery and its device, is a simpler internal chip as well.
But If somebody did that, it would still assume a fixed low-endpoint voltage, corresponding to a Sate Of Charge of 0%. This might as well be the voltage, at which Li-Ion Batteries generally start to produce Li2O , which I think is at 2.5V .
(Edit 12/13/2016 : Actually, the battery of the Samsung Galaxy S6 Phone is soldered in. Therefore, it does not need to be an info-battery, and only has 2 terminals.)
But I do not want to over-rely on this app to give me technical information about my phone battery, because just like with any app, the algorithms have the potential to fudge whatever values they display. My actual voltmeter will be better-able to give me true voltages, once the ‘18650-Type’ batteries arrive with their charger.
The ‘18650-Type’ batteries roughly look like AA-format batteries, except that I think they are slightly bigger, and those only have 2 (external) Terminals, a (+) and a (-) . Thus, knowing at what voltage they are fully-charged either needs to be a strict standard as applied to the design of chargers, or needs to be applied by a circuit inside the battery, which is in-series with the supply current, and which disconnects the flow, when the battery is fully-charged.
I should add, that while my Samsung Galaxy S6 Phone, and my Samsung Galaxy Tab S Tablet, possess hardware-sensors that report both voltage and current, ‘GSam’ also works on devices which only have the sensors for voltage. My older Samsung Galaxy S4 Phone was an example of that. In that case, no current level was displayed, but only a voltage, and a percentage charged.