2015 Beta Testing - The Components are Here.

[Jared]

Originally Posted by inkling16

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?



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.

I still don't see this working well for what you're trying to accomplish. Your value of "E available" is not really quantifiable for the battery. If you read the datasheet, you'll see that the battery's capacity (which affects E_available) varies greatly with current draw.

Basically, this means that your effective energy consumed is proportional not only to the energy you're actually using, but the rate at which you're using this energy (or some weird function of the rate).

Assume that we can estimate state of charge in a battery by timing how long it takes a 10 amp load to cause the battery voltage to drop below 10 volts.

If we play one match where we draw 43,200 joules (12 amp hour volts, or 1 amp hour for our battery), and we run the test on the battery, we may see that the battery can power the 10 amp load for 15 minutes before dropping below 10 volts.

We may then completely charge the battery, and play another match where we also draw 43,200 joules (again, 1 amp hour for our battery), but the load test will cause the voltage to drop below 10 volts after only 2 minutes.

Our integration of current over time gives the same result both times, but in the second match, we ended up depleting a larger portion of the batteries capacity because we used our energy in short, high power, fast spikes, rather than a slow steady draw in the first match.

It would be interesting to see if a function for effective energy used (out of the rated 18 amp hours) could be used if we took into account both the current and the integral of the current with respect to time.

Also, does anybody have specifications for the new PDB's current sensing (latency, resolution, sampling rate, maximum current)?