amberwolf wrote:Not exactly: If the charger is removed, the BMS would then drain down the high cell(s), but it cannot then top-off the still-low cells that were below the HVC.
If the BMS is designed so that it does not use an HVC, but instead depends on waiting for all cells to be shunting before it cuts off, then it would indeed balance all the cells after the charger is removed.
But I doubt many BMSs work that way, instead cutting off as soon as the first cell reaches HVC after it's begun shunting. Then after the shunt drains the excess off of that cell, it turns on again, pulling power from the charger until a cell reaches HVC, with one or more cells in the process of shunting. That process would then repeat until any cell that is still significantly lower than the others is brought up to the same state of charge as the rest of them. On badly imbalanced packs, it can take days or more for this to happen, at the very low shunting rates most ebike BMSs have.
On our Zephyr and Zephyr Lite BMS units, we actually have two HVC trip points. The higher of the two is used as a "failsafe" that keeps a cell voltage from going too high.When this happens, the charge current will be cut for about a second, or so, and then if the voltage is still too high, the cycle will repeat. This won't ever happen under normal conditions because the this "failsafe" HVC set point is higher enough higher than the charge to voltage that the shunt circuits alone are usually enough to keep the cell voltages in check, even for moderately imbalanced packs.
The second set of HVC signals are set to trip once the shunt circuits are in full operation. What the charge controller does is it waits for all of these to be set, and then it signals the end-of-charge condition and shuts off the charge current. This way the charge current is still there to help the low cells catch up, while it keeps the higher cells at the charge to point.
You are right about one thing. Most of the "commercial" BMS units that are out there (Signallab, etc...) have extremely low shunt currents, on the order of 50-75mA. That's way it can literally take days to balance a pack. The full Zephyr BMS boards have about 600mA of shunt current and even the "Lite" versions have about 150mA.
-- Gary
