The BMS thing is a problem. I don't agree that the solution is easy. I do recognize that my views are a little controversial. But they seem to be working. Yes, I do advocate a more advanced charger than is quite available at an affordable price.
First the RC thing. Actually I have BOXES of eBay purchased RV balancers and chargers. And actually, they are the closest thing to a workable and affordable device even for large cell work. They typically only "balance" at the rate of 250 ma or so, although a few do a half amp. In this way, you could very SLOWLY balance your pack over time. And they SEEM to pose little fire danger.
The problem is they mostly work as least common denominator - they bleed connected cells to equal the lowest cell by voltage. This causes them to step down your pack over time if you leave one sitting.
They are useless while charging because you are charging at much higher currents than RC guys. They may even be dangerous and so you are faced with some way to connect/disconnect while charging.
But I've played with a lot of them and actually like them. The cute little aluminum cases are part of it probably.
The cell bleeder shunt concept I just have a problem with. Yes, I played with Blochers as well. I hooked four of them up to 4 cells and left it for 3 days. One of them bled a cell to zero volts and it never recovered.
They started conducting at 3.6 volts, which I didn't like, and it was not adjustable.
I devised a number of circuits that were actually quite similar, typically LM431 designs with a FET or darlington and a 50 watt resistor. Actually, they worked pretty well at the design voltage. If I turned up the voltage to say 1.5 or 1.6 of the 4.0 preset, they went into thermal runaway and terminated when the copper heat sink melted and broke the connection. Remember, you are talking about connecting these across the cells of a battery than CAN put out 1000 amps briefly.
I've tracked down three or four cases of cars burning to the ground. In every case, there was at least a probability, and to my way of thinking, more than a probability that the current shunts actually caused the fire. I think the batteries are "basically" safe. They don't start fires. But they are great fuel if you melt one and short it. And you can do that with a current shunt.
I'm working on the same concept right now. But the shunts aren't mounted on the batteries. They're mounted on a foot square heat sink. And the thing will roll around on a cart. And I'll hook it up to the car, use a special charging algorithm to bring them up to a 4.00 volts and do this as a maintenance procedure. The shunts, if they fail can't burn the car to the ground because:
1. They aren't in the car at all.
2. I'm standing there watching them.
And after it is done, I'll disconnect them.
But in practice, my cells have been REMARKABLY well "balanced" most of the time. The comparator works quite well at detecting anomalies. It's basically built into the EVision. Lee Hart orginally described it. And it's not only simple, but simple enough to use while driving.
It certainly can detect imbalance, all vendors views to the contrary. It can't detect them or do much about them if you are charging your cells to max voltage.
And that brings me to another little thing I'm coming to feel strongly about. We just don't charge them to max voltage. On the thundersky's we're charging to 3.6-3.75 volts. We'll do the Blue Skys' sprobably to 3.45.
I have been testing the Blue Sky batteries quite a bit the last couple of days. We just received our shipment for EVcomponents last Friday.
And I love them. A couple of things. The dimensions we got from the specs are NOT accurate. They are EXACTLY the same size and shape as the THundersky 90Ah cells. The ribbing is the same. The color is blue instead of yellow.
They come with threaded rod and brackets for the ends. Not good. They take up much more room than the flat straps and plates of the Thunderskies. We are just going to have devise our own clamp system using the battery boxes themselves in the Mini Cooper.
Their voltage is lower. They describe 3.6v down to 2.0. Which APPEARS different from the THundersky. I'm not sure how different they are. I'm starting to think they are just reading my mail.
In any event, if anything it has a FLATTER charge discharge curve. There really isn't anything above about 3.45 volts on the high side. On the low side, I'm a little unclear about the 2.0 volts. They take the dive right about at 3.0 volts just like the Thundersky's. Perhaps they are more tolerant of the lower voltage, but it doesn't matter. From 3.0v down to 2.0volts, there isn't anything in the way of stored energy anyway.
In fact, the ends seem a little sharper on the Blue Sky's and the 3.3 to 3.0 volts a little wider and flatter as a result.
I got 111.83 Ah out of one of these 100 Ah cells picked at random.
So again, my belief is the strategy should be to avoid the ends and stay in the middle. Balance really only comes into play when you fall off the lower end. The problem is that one cell always reaches the 3.0 volts first. And it starts to plunge. The others are still at 3.05 and just digging it - hoping for another massive acceleration so they can show their stuff. By the time they reach 2.5 volts. The one that hit 3.0 volts first is shorted - completely dead and irrecoverably so.
Actually JRP, I HAVE cratered some batteries. But I'm in a little bit different situation from you guys. You're trying to preserve your batteries. I'm trying to learn how to kill them. This is a good deal for you. I buy the battery, and you get the info.
Overcharging doesn't precisely kill the batteries, within reason. It causes lithium plating on the cathode and will reduce the life of the cell. But over discharging is death. Some you can bring back. Some you can't. The ones you can't, feel free to bandsaw and take a look inside.
So I view the 3.0 volts, basically the knee of the discharge curve where it turns downward, as the end of range. The little you have from 3.0 to 2.0 on the Blue Skys is your "pad" and really it is precious little.
I'm coming to admire Victors EVISION more all the time. It lets me see how many AMP HOURS I've taken out of the battery. Voltage is actually a pretty good indication, but if know your batteries after awhile, you pretty much know how many amp hours you can take out and live. It does have the balance indication, and actually a quite nice battery charge fuel gage that probably works fine, I've just never trusted it. It monitors amp hours going out, and indeed it will monitor amp hours going back in when you are charging.
So when I say I don't like BMS, I'm talking about most of the systems out there and all of the current shunt balancers. I guess the EVISION is a kind of "monitoring system."
Having a smart charger is an advantage. But many of the chargers now can do basic constant current/constant voltage stuff and let you set the voltage. Really MOST of the problems go away if you just charge to a lower voltage and the range you sacrifice is typically between 1 and two miles.
The other end is just making sure you don't run them too far out - a key component to battery cycle life. Remember that 3000 cycles is at 70% discharge. At 80% you're looking at 2000 cycles. And I would guess at about 3.0 volts per cell, you are pretty much at 92-93% discharge on these Blue Sky cells
And I am actually amazed at how well they stay in balance. But when marrying a new battery into a pack, I do have to do some manual balancing. Basically a 12 v charger and a big 5000watt 1/4 ohm load I made makes even that pretty easy. But for drill, I'm kind of playing with this cart level shunt thing that will do about 5 amps per cell and allow me to "top em all off" as a once every month or so maintenance procedure. If I charge them normally to 3.6 volts, I can hook this up, run a charge cycle up to 4.00 volts which should hardly take 20 minutes. There is NOTHING in the land from 4.00 to 4.25 volts. It' sjust wasted effort.
I do have a video showing the insides of a Thundersky if anyone is curious.
http://evtv.me/videos.html.
If you want a full Linux Ubuntu server in your car monitoring your batteries and current shunts wired over the top of your cells everywhere it's all good with me. I'm not trying to kill the BMS industry, and indeed if I could find a good one, I'd buy it obviously. I've bought most of the bad ones so I'm not opposed to investing in a good one. I just havent' found one.
But ff you do have a fire, please send photos.
Jack Rickard
http://evtv.me
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