shinyballs wrote:thanks, both (LVC & BMS board) have the same dimension?
Close, but the BMS board is a little bit bigger. Its dimensions are 3.45" x 7.0". Here's what it looks like now:
Over the weekend I got my 8-channel test board working with the latest parts, and it works quite well. I've tried it with several 8s4p a123 packs I have here, and it even found that in one of those packs, I had a weak block of cells. Normally, while the charger/supply is in the CC phase, pumping out the max current, the orange LEDs are off and the main LED is red. When the cells get to the cutoff point (3.65-3.70V...), their individual orange LEDs start to glow. That means the shunt for that channel is starting to bypass current, holding the voltage at 3.65V. The amount of current it will bypass is controlled basically by the size and value of the shunt resistor. My test board has 20 ohm/2W resistors, which limit the bypass current to about 150mA. This was mainly to get a baseline of the heat dissipation. We've also tried 15 ohm/2W versions, which boost the current up to about 200-250mA. I think this is going to be the limit, for boards that have to be sealed in with the cells, with no real airflow. For the bigger setups, like the 40Ah ThunderSky-based packs many are using, we have some big, fat 6.8 ohm/5W resistors that will boost the shunt current up to about 500mA, but with these, you need to have room to allow the board to "breathe".
I'm not sure how hot these are going to get yet, but I'm guessing hot enough that you wouldn't want the board duct-taped to the cells, stuffed away in a bag.
The higher the shunt current, the less time it takes for the low cells to "catch up". Most of the Chinese BMS boards only use 50-100mA, which is why it can take 10 hours, or more, to balance a Ping pack. What I'm seeing is that with my a123-based packs, the LEDs are all coming on within a couple minutes of each other. I purposely discharged one cell down about 10%, and it took about an hour for it to catch back up to the rest. In any case, when a cell's shunt circuit gets close to getting swamped, an opto for that channel starts to conduct, which cause the FET logic to start cutting off the charge current completely, which in turn causes the cell voltage to drop, turning off its opto. This oscillation runs at about 50kHz, and when this mode is active, the main LED will be a yellowish color, as both the red and green portions will be lit. Actually, the LED transitions from red to green. When the first channel turns on its opto, both the red and green portions are lit about equally. As more channels come on, the green portion ecomes more prminent. When all the channels are in full bypass, the LED is just about solid green. Basically, all you do is wait until all the orange LEDs are on and the main LED is green. Then you are done.
With one of my 8-cell packs, one channel's orange LED came on way too early. I wasn't sure what was going on, so I just let the charge go on. It took awhile, but eventually all the rest of the cells caught up. I then tested this 8s4p pack with another one, on one of my bikes. Sure enough, the LVC circuits tripped, and it cut the throttle as soon as the controller got above about 30A. I also tried unplugging the LVC opto output from the controller, and tried it again. I could only get to about 35A and the voltage wsa being pulled down from about 55V to 41V. Later, I checked the pack, and one block was quite a bit lower in voltage than the rest of the cells (2.8-2.9V vs 3.2-3.3V...), and it was the channel whose orange LED came on too soon. It turns out that in that 4-cell block, one cell was dying (it was one from one of my early RC packs, and has been tortured to no end...
), and it was pulling down the other three. Since these three were also from RC packs, I decided to replace the whole block with four "healthy" cells. The pack is now as strong as the rest, and I was able to hit 55A peaks again on my new Cyclone 1000W setup on my Mariner folding bike. Anyway, the point is that by watching when the orange LEDs come on, in relation to the others, you can get a lot of info about the health of the cells in the pack.
I'm getting the boards shown above today. I think we are done, except for one remaining issue, which is that the voltage regulator transistor we are using (KSC1009...) wsa getting pretty warm. I'm going to do some tests today to see just how much current this part of the circuit needs, and then we will pick a new part that can dissipate more heat. Worst case is that we have to go to TO-220-sized part, but I'm really trying to avoid that, as it would force another layout change. We're going to try some other parts that wil still fit in the TO-92 footprint. I think Randomly suggested a one, and Richard's found several others. Other than that, I think we are done.