ZapPat said:
The BMS looks nice, but I don't see anywhere on the kit page where it mentions the maximum charge current this BMS can handle while still doing adequate balancing? If I'm charging at something like 12 amps, is the limited 500mA shunt capability enough? It might be good to add this info to the kit page...
Thanks,
Patrick
Actually, one doesn't really have anything to do with the other. There is a practical max charge rate, which is really limited by the size of the wire used, and the FET rating. With 12-gauge wires, and a 4110 FET, 20-30A is probably a good number for a limit.
The amount of shunt current has more to do with how long it takes for the cells to each reach their 100% full level. Another major factor in this is the capacity of the cells. The main reason it is nice to have large shunt currents is for large capacity packs. If the cells are well matched in capacity, internal resistance and thermal characteristics, it really wouldn't matter how much shunt current could be bypassed, because the cells would reach the "full" level at the same time. If there are slight differences, which there almost always are, especially over time, it is going to take longer fo some cells than others, in order to get full. With SLAs, the cells have the built-in ability to absorb some "extra" current when it is full, so if left on the charger long enough, the low cells can catch up. Lithium-based chemistries don't have that "absorbtion" capability, which is why we need the shunts.
Looking at a single cell, the way it works is that charger starts out at the maximum current it can supply, and the voltage is controlled by the cell. As current is pumped into the cell, it becomes harder to accept it, so the voltage rises. Think of a large, raised water tank, with a hose going down to a pump. When the tank is empty, it doesn't take much water pressure from the pump to fill the tank. As the tank gets fuller, though, there is a back pressure from the water trying to go back down the hose that the pump has to overcome, so it takes more pressure to keep pumping water into the tank at the same rate. In a cell being charged, the voltage is analogous to the water pressure and the amount of water being pumped into the tank is the same as the current.
Anyway, when a cell gets to about the 85% full level. It starts to become much harder to let more current in, and the voltage starts to rise at a much quicker rate. In a LiFePO4 cell, this point is around 3.65-3.70V. In order to get the last 15%, or so, into the "tank", the charger holds the voltage at that point, and the cell starts dropping the current it can take in. When the current is down to a trickle, the cell is about as full as it can get. With individual cell chargers, each cell is allowed to go through this process independently. With bulk charging, with many cells in series, this becomes a problem because the cell that reaches this point first, and then starts to reduce the current down to a trickle, is also reducing the amount of current the rest of the cells see as well. All the current has to go through all the cells. What the shunts do is first to hold the cell at the cutoff point, but then as the cell starts to reduce the current it lets in, the shunt srtats to bypass current, up to its limit, so that the next cell in series can at least have that much in order to "catch up" with the higher level cells.
When the cells get to this cutoff point, and are in the "constant voltage" (CV) mode the max charge current the charger can provide doesn't really matter, as the cells are going to start reducing the current anyway, all the way down to nothing. The max current will, however, determine how long it takes during this "constant current" (CC) part of the charge process, for the cell(s) to get to the 80-85% level when the the CV mode kicks in. With larger capacities, it will take longer, which is why there are high current chargers. The same thing applies to the shunts. A 10Ah pack with, say, a 2-3% difference in cell levels, won't need near as much shunt current as a 40/60/100Ah pack with the same 2-3% difference in levels, in order to allow all the cells to all get full in a "reasonable" amount of time.
In any case, it really all depends on how far apart the cells are after typical use. I have some a123-based 4p/9.2Ah packs, with healthy cells that were well matched in the beginning, and have stayed that way. These rarely have differences greater than about 1%. With differences this low, it doesn't matter what size shunt resistors I use, because the orange LEDs all come on within seconds of each other. I have some other a123 4p packs, however, that have lots of "stressed" cells in them and the differences are usually 3-4%, or more. For these, using the big shunt resistors do make a difference in how long it takes for all the LEDs to come on.
One thing to keep in mind is that even the 15 ohm/2W resistors will allow about 250mA of shunt currents, whisch is about 5 times what almost all of the Chinese BMS boards will allow. Even the VMS boards used in the LiFeBatt HPS automotive packs only allow 100mA.
I'm not sure I've answered your question, but basically you should use the capacity of your pack as a guide for how much shunt current you need, not the maximum current your charger can put out.
-- Gary
I'm ordering more parts today, which I should get before the end of the week. I need a few days to get these first orders out, and then I'll re-enable them on the site. I'll also add an option for partial kits (8, 10 and 12 channels...).
