There is one thing which is obvious once stated, but not until then.
Your phone tells you it has "37% Charge Remaining". How do you know that's accurate? It's probably not.
The software may be doing some estimating based on average current draw since it was fully charged, average time between charges, and of course the discharge characteristics for the specific battery. Then it presents you with its best guess.
Over time, it can build up a reasonably accurate profile for the battery and use that to improve the estimates. But it is usually an estimate.
In my experience of developing battery based systems (with smart batteries, dumb NiCad, and everything in between) the only times you are confident of the charge level are 100% and 0%.
Usually, a smart battery will let you know when it's fully charged, and with a dumb one you are probably doing some calculations with current and temperature. That takes care of the 100% case.
The 0% case is where the sneakiness comes in. Whatever the battery chemistry, there is often a distinctive pattern in the discharge curve as you approach voltage collapse. But allowing a battery to go into deep discharge is generally a "Bad Thing" (TM).
So firmware looks for that pattern and decides when the battery is at a virtual "0%". Then it shuts the system down so that there's enough residual charge in the battery to avoid deep discharge and, more importantly, a sudden loss of power. This allows a graceful shutdown.
If this seems a little unlikely, let your phone "run down" and shut itself off. Then turn it back on again. If the battery were truly at 0%, it could not boot and power up the screen to tell you it needed charging.
The 5% (or perhaps 10% depending on the precision of the measurements and the batteries tolerances) warning is also often somewhat artificial, again representing a point on the discharge curve when the firmware starts thinking "Going to shutdown soon".
Ironically, this is the level at which someone in marketing insists that you turn on that bright LED to tell the user they are about to run out of battery power.