[Caleb Sykes]

Originally Posted by timytamy

Interesting question, I'm hoping someone more qualified will step in but I'll try anyway.

The easiest answer is that 100% of charge is when 204Wh (12V*17Ah) of energy can be drawn from the battery, and 0% is when 204Wh has been drawn. This of course requires a few assumptions, such as we ignore any aging affects and the battery is able to push reasonably large currents at near 12V (say 10.5V and above).

It seems that you are saying that state of charge has no formal definition, but rather that it is defined uniquely for each system to provide a useful value representing the state of a system. Would you agree with that?

Originally Posted by timytamy

Maybe a more useful answer is that 100% corresponds to what a useful charger will say is charged, and 0% is the point at which most robots will no longer completely function (ie have difficulty driving/turning). Perhaps you could go one further and say 0% is when non-motor electronics start failing (such as the RoboRIO or the VRM) and have another point, say 10% which is when robots stop turning. ie 0% is when the robot can no longer "idle". This one would require characterising some batteries in that you would need to find an amount of energy that you could draw before reaching this point.

If we are defining S for FRC batteries ourselves, then I would suggest the relationship S = Eavailable/Emax where Emax is the difference in energy of the battery between the state that some standard charger says "fully charged" and some standard 0 energy value E0, such as the energy at which non-motor electronics on the robot start to fail. If we just define these two points, the relationship between S and E is linear, but if we also try to define a third point (such as the point at which motors start failing), we will not generally be able to use a linear relationship to describe S in terms of E.

Originally Posted by timytamy

An even simpler answer is that state of charge is just what a battery analyser such as the Battery Beak or the CBA will say. Does anyone know how the Battery Beak works out what it's state of charge is?

Unless we can get an answer on how the CBA/Battery Beak defines state of charge, I'd suggest we go with the 0% - idle, 10% - motors, 100% - off chargers model for this discussion. Of course I'd be happy to be corrected by someone with a better understanding of lead acid batteries.

I too would be interested to learn how the battery beak defines state of charge. If the beak says 78% charged, does that tell me anything quantitative about the battery or does it just mean I should leave it on the charger longer before using it in a match?