Nevilleh wrote: ... So you can either start at empty and fill them up by the same amount or fill them all to capacity and stop discharging when the first one reaches empty. One way you are putting in only the amount of energy that the weakest one can hold, the other you are taking out only the amount that the weakest one can hold.
Right, nicely summed up. But the symmetry breaks down with the convenience of top or bottom balancing. For bottom balancing, you have to get to a low state of charge to do it; if you happen to be at about 40% SOC, say, then you either have to wait till it gets closer to 20%, and risk running out of energy, or waste a lot of energy, or postpone balancing until you happen to end up closer to 20% SOC (or wherever you choose to do the bottom balancing). By contrast, with top balancing, no matter what the SOC, you can charge to the top and balance every charge, and always have a full pack, or a 95% or 90% full pack if you choose to charge to lower than maximum voltage. You can even choose between a "short charge" to 90%, if you don't expect to need maximum range, or 100% if you expect to need maximum range, without planning in any way other than setting the switch on your charger or BMS master.
I guess the open-circuit voltage is pretty much the only measure we have of the state of charge, unless you were to exactly measure the capacity of each cell and then monitor the energy flows precisely. That sounds a bit hard, and measuring the voltage is much easier.
True, and I also worry about drift of the measured cell capacities, and the effects of temperature, how "hard" you drive, and other factors. Voltage seems more "foolproof" to me: regardless of what the actual capacity of the cell is right now, when it gets towards the low voltage knee, you can detect it with the cell voltage. If you assume a certain capacity, you might get caught out if you just count amp-hours in and out.
And probably a fairly accurate measure of the state of charge at the full and empty ends of the spectrum.
Right - so it's good for protecting the cells. The thing that voltage is not good for is predicting the range to empty; for that we have little choice other than to count amp-hours and hope that the capacity is about what you measured last time, or what its nominal capacity is.
Has anyone destroyed any Li cells yet? - running in a car, that is. And if so, what conclusions were reached?
Don Saxby (mcudogs) has had to replace 24 of his 45 (?) Sky Energy cells. I think his conclusion was "don't use the dodgy Chinese BMS that I used". It was a bizarre design whereby only eight of the cells could be in bypass at once. Apparently, this works OK as long as the cells are reasonably balanced to start with, but somehow his weren't.
But don't trust my poor memory, send Don a PIM to get the information first hand.
Ah, here it is: 5 defective CALB cells
I'm sure there are more if you look hard enough. Lithiums are now common enough, and the idea of not needing to use a BMS is widespread enough, that there must be plenty of ruined packs out there to learn lessons from. There are also a few fires documented; one of them at a school in the US promised details after an investigation was completed, but I never saw the results published (I could easily have missed it).
weber and coulomb will be interested to know that the shunt resistors rise approx 70 deg C with the cell voltage at 3.28.
Yes, I thought it might be of that magnitude. And of course hotter again at 3.6 - 3.65 V, which is where we want to use our bypass resistors. At an ambient temperature greater than 20 degrees Celsius, that's hotter than the 90 degrees that typical wire can handle. I would not want bypass resistors melting the insulation of pack wiring.
The other issue is that the heat is right there on the PCB (even more so if you use vias and copper on the other side), which is close to the cells. Cells hate heat (apart from performing better in extreme cold once warmed up, but that happens automatically as they are used). Hence we prefer our ceramic resistors, even though they are very much larger and a little more expensive. We space them about 2mm above the PCB, so that air can circulate under them, cooling them, and keeping the heat away from the PCB and the cells. Of course, the heat may largely not escape the battery box, so it remains to be seen whether our approach is really better than SMD bypass resistors.
[ Edit: formatting; responded to 70C resistor temperature rise ]
Nissan Leaf 2012 with new battery May 2019.
5650 W solar, 2xPIP-4048MS inverters, 16 kWh battery.
1.4 kW solar with 1.2 kW Latronics inverter and FIT.
160 W solar, 2.5 kWh 24 V battery for lights.
Patching PIP-4048/5048 inverter-chargers.