The best BMS would be something where we could determine the "knee" point and then maybe count how many AH of current is stored from the knee point. Then when you use it back, the computer or meter would count back how many AH you have used ....and how far away you are from the knee point.
EDIT: Actually Jack is correct for most of the things he says however, bottom balancing can also be bad because when we charge, we have a potential of overcharging if we charge in bulk. Sorry, I'm just learning this stuff. Maybe you guys already know this. Maybe this can be useful for the average lifepo4 user. The best way to ensure that you don't overcharge is by top balancing, but the best way to ensure that you dont' destroy cells when discharging is bottom balancing. Hope that makes a little sense. The best of both worlds would be top balancing with LVC for each individual cell.
Here is some quote from his blog if you want to read it in his own words
By very carefully charging each cell to precisely the same 4.000 level, I did indeed "balance" the cells - at least at the top of the charge.
But as I discharged the cells, they reached any arbitrary point on their discharge curve at DIFFERENT times. So at the end of the charge, where the knee of the discharge curve turns sharply down, they became more UNBALANCED at the bottom.
The graph below shows the number of seconds a cell has at a 100 amp discharge rate to 3.00 vdc from a full charge with all of the cells balanced at the top of charge.
The problem here of course is that some cells go over the knee first and start down the steep discharge wall at the end before the others. This has a very bad result. The cells still up on the plateau, making current, drive current through this smaller capacity cell and drive it down to zero volts and ultimately to destruction..
So I was repeatedly destroying cells by carefully top balancing the cells, precisely as a current shunt balancing circuit would, and then discharging past the knee of the discharge curve. The other cells turn on the weaker one and eat it like a pack of wolves.
Worse, your overall pack voltage masks all this - remaining up in the supposedly safe area.
The solution appears to be BOTTOM balancing. With all the cells discharged, I replaced the dead one, and balanced all the cells at 2.90 vdc. Then recharged the pack to 87 vdc (3.625 vdc per cell).
Now the cells are very unbalanced at the top - some slightly over 4 volts and some quite under the 3.625 average.
But I don't care about the top. I don't lose cells at the top, and we're charging at 20 amps. During discharge, even the GEM can go over 200 amps of current. That is a 10x more violent event in the life of a battery. And a weak cell can drop from 2.8 to 2.0 to 1.0 to 0.0 in a matter of a dozen seconds or so at 200 amps.
This pretty much explains why I was able to lose cells on the GEM while balancing to the nth degree, but the Speedster, whose cells have never been balanced at all, wheels merrily along without problems.
In fact, we recently completed a 107 mile test drive with the new tires and really did push the little car to the limit. At the end, all of the cells measured between 2.8 and 2.9vdc in quite balanced fashion - at the BOTTOM of the discharge curve. Things were good BECAUSE we had never top balanced.
What I conclude from this is that these simple current shunt balance circuits are not only a needless expense and a fire hazard, they are doing exactly the opposite of what they purport to do. They are UNbalancing the pack at the bottom where it matters, and potentially leading to the untimely death of cells.
So we're still in search of the perfect Battery Management/Monitoring system. But the current shunt balancers are certainly not it. Save your money, and your batteries.