Can I read smartphone batteries gas gauge info with a PIC?
Here is an excerpt from an article I found:

&quot;To determine the available charge in a battery, simple monitoring methods are preferred. They should consume little energy and should (ideally) allow one to deduce the charge level from battery voltage. Such a voltage-only method can produce unreliable outcomes, however, because no clear correlation exists between voltage and the available charge (Figure 5). Battery voltage also depends on temperature, and dynamic relaxation effects can cause a slow increase in the terminal voltage after a reduction in load current. Thus, purely voltage-based monitoring is unlikely to provide charge-level accuracies better than 25%.
The relative charge level, often called the state of charge (SOC), is defined as the ratio of residual charge to the battery's charge capacity. Hence charge flow must be measured and monitored through a procedure called &quot;coulomb counting.&quot; In practice, coulomb counting is accomplished by integrating the currents flowing into and out of the cell. To measure these currents with a high-resolution ADC, one typically connects a small resistor in series with the anode.&quot;

Here is a datasheet I found for an Atmel AVR352 that you could use for coulomb counting:
Even better would be to refer here.
And there are devices.
Though as far as I can see there is plenty of info on the web; i don't see how you could have missed it.