As you may know, I’ve been a bit at odds with the cognescetti of the EV community on the topic of Battery Managements Systems, and particularly of the form of shunt balancing circuits. They’re pretty much unified in the position that you have to have them or you will kill your expensive LiFePo4 batteries.
My position is that they are dangerous, a fire hazard at most and an annoyance at least.
One of the problems I have in life is that I’m often surrounded by people that are extremely sure of themselves and their positions. I’m never quite sure. Almost everything COULD be a couple of different ways, and most probably is, and I’ve probably got at least part of it wrong.
The oddity is that the ones most certain, are the those most certain to be in error. And if I run into a man with a theory, who isn’t quite sure, I can often find valuable, and sometimes extremely valuable information there.
Those who most loudly voice their absolute certainty, almost inevitably lead me into something totally erroneous, and they then bleat piteously about “unintended consequences” and the simple unavailability of such information back when they were so sure.
Last week we aired a kind of a tutorial on using the Manzanita Micro PFC-75 charger. Despite some kind of bizarre design choices, I like this device and it is undoubtedly the most powerful single phase charger on the planet at this point. Along the way, I had a conversation with Rich Rudman, about the device of course.
But we also discussed his “Rudman Regulator” and the new MkIII device he’s working on for LiFePo4 cells. Mr. Rudman was EXTREMELY emphatic that without some form of battery management system, I would kill numerous cells. His solution would have been $7000 for the Mini Cooper. He told me that they had spent THOUSANDS OF HOURS testing these cells and that balancing was imperative.
This made me feel quite badly. In truth I have NOT spent THOUSANDS of hours testing batteries. I’ve spent a lot of time, but it is not very rewarding work. It takes HOURS to charge a cell. You have to pretty much observe it closely the entire time to log any meaningful data. It then takes HOURS to discharge it, and again, you can’t even really turn away from it. So it’s long, boring, and tedious.
Some of it IS fascinating. Mostly once you’ve collected all this and are going over it. But the actual testing is pretty gruesome.
There are 2040 work hours in a year at 40 hours per week. I may have HUNDREDS of hours testing batteries, but certainly not THOUSANDS. And he was so certain of his results, it rather sent me back to the lab.
The “lab” isn’t precisely so. I can test single cells on the back workbench where I have a lot of test equipment. And I often test 4 cell or 8 cell banks. But a string of series cells to be tested poses some problems in dumping that much power during discharge. So we use GEM’s and in fact have from the beginning.
GEM’s are Global Electric Motorcars. I had three of them, now down to two. Ours are like little pickup trucks. They require no licensing. They are Neighborhood Electric Vehicles, typically limited to 25 mph. They have a small 7.5 HP motor and a simple GE controller.
So this week, I took some time, and brought my really pretty nice Agilent 5 1/2 digit multimeter from the back bench, along with the test device I built for the Mini.
It took 2 full days, but I very precisely hand balanced all 24 cells to precisely 4.000 volts.
The tester lets me add 3 amps of charge to a cell. And it also lets me bleed 3 or 5 amps from a cell through some 50 watt resistors. Although it has a built in voltmeter, I used the Agilent for precision.
Of course the problem is that if you add a bit of energy to a cell, or for that matter delete a little energy from a cell, it kind of “bounces”. The voltage indication will change, for example going higher when charging. But when you quit, it will sag back down a little bit to it’s stable level. Similarly when discharging, it will decrease the voltage alright, but when you remove the load, it bounces back up a bit. So this “manual” balancing is a bit tedious. But I can make sure this way that they ARE in fact balanced.
This is normally the function of these shunt regulator active cell balancing devices. This is typically a small circuit with a voltage regulator chip controlling a larger MOSFET or transistor that “shunts” current across the cell terminal when the cell reaches a set maximum voltage. This usually uses a current limiting resistor which dissipates some of the heat.
The theory is that you hook up your serial string of cells to a charger. When one cell gets to the voltage set point, 3.8 vdc or 4.0 vdc, or whatever, the shunt goes into conduction. The rest of the cells continue to charge, but the cell in conduction is held at this maximum voltage.
Once ALL the shunts are in conduction, the cells are thought to be “balanced” in that they are all held at the same maximum voltage.
I’ve examined the cases of several fires wherein electric cars burned to the ground. Two culprits emerge as likely causes. Battery modules made of large numbers of small cells, and shunt balancing circuits.
So I’ve avoided them.
But after my discussion with Mr. Rudman, who has been doing electric cars for many years, has one of the most respected charger products in the community, and who personally assured me that after THOUSANDS of hours of testing, he’s utterly convinced you MUST have a battery management system, I simulated it in this fashion.
Then I went and drove the car. Actually I went through this process THREE times this week. And with the same result all three times – a totally destroyed battery cell. Irrecoverably discharged to 0.0000 vdc.
So I AM in fact destroying cells. And if I’m destroying them on this little 72 vdc 24 cell GEM system, IMAGINE how much difficulty I was going to have with 112 cells in the Mini and 72 cells in the Beck Speedster.
I was pretty depressed about it. Not only was I murdering cells in groups, but I was apparently pathologically unable to balance them sufficiently well to prevent it. And in fact the more precisely I balanced them, the worse the carnage became….
Uh..oh. Is this the sound of a clue?
Finally Friday morning it all came together. It is so obvious I’m embarassed by my own intellectual limitations and overall backwardness. But worse, I have to go public with it because there are a LOT of people spending a LOT of money on these shunt chargers to do precisely the same thing.
The problem is, the batteries vary in capacity. While capacity diminishes very gradually with time, there’s really nothing you can do to change the capacity. It is what it is and it is that for each cell.
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.