Since seeing that, I've been pondering the question of how to tell what a cell is really doing, while it's running in a system. The question that's been rattling around in my head is whether there's a way to tell the difference between a cell that's at 2.5V because it's under heavy load, and a cell that's at 2.5V ocv because it's fallen off the cliff and is in mid-plummet? I mean, obviously, there's the difference in the current going through the system under those two circumstances, but I don't think that's indicative. Let's ask the question a different way:
Is there a way to tell when one cell becomes 'reverse biased' in a running system? Let's say you've got 1/3rd of your pack charge left, and you start driving across Nevada on a flat level road at 50 mph. You're pulling a nice steady 100A off the pack, and you're just cruising along. 25 miles later, your lowest cell hits the proverbial cliff. How can you tell? You still see 100A going through the system, but now the rest of the cells are starting to shove current through one cell that's misbehaving. Is there any sign of this that can be detected before it's too late? Does the voltage across that cell and the current through it in any way indicate that the cell has become a resistor instead of a battery?
In the AC world, you could imagine looking at the phase of an AC device, and measuring whether its phase is leading or lagging the line voltage, to determine whether it's sourcing or sinking power. Is there anything like that on a DC battery? Can you tell by looking at "edges" in the current/voltage domain, to see where voltage edges lead current edges, and where they lag?
Is there a way to tell when one cell becomes 'reverse biased' in a running system? Let's say you've got 1/3rd of your pack charge left, and you start driving across Nevada on a flat level road at 50 mph. You're pulling a nice steady 100A off the pack, and you're just cruising along. 25 miles later, your lowest cell hits the proverbial cliff. How can you tell? You still see 100A going through the system, but now the rest of the cells are starting to shove current through one cell that's misbehaving. Is there any sign of this that can be detected before it's too late? Does the voltage across that cell and the current through it in any way indicate that the cell has become a resistor instead of a battery?
In the AC world, you could imagine looking at the phase of an AC device, and measuring whether its phase is leading or lagging the line voltage, to determine whether it's sourcing or sinking power. Is there anything like that on a DC battery? Can you tell by looking at "edges" in the current/voltage domain, to see where voltage edges lead current edges, and where they lag?
Would you learn anything by intentionally pushing a voltage ripple down the wire, and watching what it does as it flows through the system? Or would the capacitance and inductance of the cells just filter such a signal out? What if every BMS board had a PLL oscillator on it, that pushed a small ripple signal down the big pipe, and the next BMS board in the chain was watching that ripple to see how its own cell changed the shape of it? Could that tell you somehow what each cell is doing, and whether it's sourcing or sinking power?
I don't know the answer, or even enough about the field of batteries to predict what the answer might be, but it's an interesting question.
