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[MrDude_1]

So quick technical question.  When and how are the zero cells balanced by the BMS?

I assume it bleeds power off of the higher voltage cells to match the lower ones, like most other BMS systems.
It may even shunt a little current across the cell so that the cell doesnt charge any longer.

But at what point does this happen? Only at full charge? does it start at a specific voltage? will it give the "charger off" signal when it begins?

Lets say you had an extremely powerful aftermarket charger... you dont want the BMS to start bleeding that much power as heat, it would burn up... does the BMS account for this?
If it was built to be fail-safe, I assume it would first give an "charger off" signal.. and if the power goes too high after that, it would open the contactor... am I correct?


long story short... lets say you had a way to dump a J1772 (or two) into your bike within the 1C range... could you set the voltage limit to 116.0v so it kept the higher current all the way up until .2v short of full (assuming 4.15v per cell is full)  Or would the BMS restriction kick-in and require a cutoff at a lower voltage limit?

[BrianTRice@gmail.com]

I'm starting to learn some of this but I don't know it well enough yet to explain responsibly. Stay tuned? Or hopefully some of the more knowledgeable types can speak up about this.

You seem to have the right idea, but my experience so far is that the BMS seems to handle ~1C fairly well.

[MostlyBonkers]

From what I've learnt, the balancing happens at the very end of the charging process. On the dash, the SoC will be 100% but the green light will keep flashing until balancing is complete.  After three days the BMS checks the SoC and cell balance. That continues as long as the bike is plugged in.

Regardless of how much power is available for charging, the BMS controls how much power goes to the cells. It will pull up to 1C, if it's available, until the cells start to reach their maximum voltage and the the power is tailed off.

Therefore, you don't need to worry about it. Unless you're thinking of hacking the BMS for some reason?

[Kocho]

I don't think this is correct. The BMS does not control the power of the charger to taper down towards the end of the charge. The charger does that on its own based on the voltage it measures across the entire pack. The charger must reduce power near full SoC to a low enough level so the BMS can handle shunting individual cells if needed. The charger must be programmed to taper down near full charge and shut completely off at full charge even if there is no BMS. I think the charger also must have input from the battery temperature sensor(s?) to reduce/not charge in extreme temps. Again, without BMS intervention (although the BMS might be reading the same sensors and use the info for its own needs independently of the charger).

Regardless of how much power is available for charging, the BMS controls how much power goes to the cells. It will pull up to 1C, if it's available, until the cells start to reach their maximum voltage and the the power is tailed off.

[BrianTRice@gmail.com]

The BMS intervention for temperature and voltage is to open the contactors.

[ultrarnr]

Cell balancing starts as son as you plug in the charger. On the last long trip I took on my 2014 SR I had power cut at 17% SOC. The cell balance was over 150 mv. As soon as I plugged it in the cell balance began to drop. The cell balance dropped a lot faster than the SOC increased. Once the cell balance got down  to around 40 mv I took off. After a few miles power got cut and the cell balance was over 150 mv. Same thing, plug it in, get the cell balance down and take off. Made it home after the second time charging.

[Kocho]

I do not think this is from the BMS. I do not "know" for the Zero for sure though. My theory is that because at very low SoC the cells are in the range of the discharge curve that is very steep, even just a few Ah of use can result in large differences between cells. Just a few minutes of charging at that point that puts those Ah back in the battery brings the cells back to the flatter range of the charging curve where they are much closer in terms of V, even though their SoC has changed very little. Probably some other factors too.

Cell balancing starts as son as you plug in the charger. On the last long trip I took on my 2014 SR I had power cut at 17% SOC. The cell balance was over 150 mv. As soon as I plugged it in the cell balance began to drop. The cell balance dropped a lot faster than the SOC increased. Once the cell balance got down  to around 40 mv I took off. After a few miles power got cut and the cell balance was over 150 mv. Same thing, plug it in, get the cell balance down and take off. Made it home after the second time charging.

[MostlyBonkers]

Thanks for setting my erroneous spoutings right. Apologies for being less than helpful on this occasion...

[MrDude_1]

I do not think this is from the BMS. I do not "know" for the Zero for sure though. My theory is that because at very low SoC the cells are in the range of the discharge curve that is very steep, even just a few Ah of use can result in large differences between cells. Just a few minutes of charging at that point that puts those Ah back in the battery brings the cells back to the flatter range of the charging curve where they are much closer in terms of V, even though their SoC has changed very little. Probably some other factors too.

I've observed this behavior too with hobby lipo...  they stray alot as the voltage dips toward the bottom below 3.6, but as the cell voltage comes up above 3.9v or so, they're about equal.
The whole time, Im using a bulk charger and some cell monitors, so theres no balancing being done...


I guess the answer to my question is... no one publicly knows exactly how it works.
Mostly I just want to know at what point I have to limit my charger too. I know it cant be too complex of a deal, since they use very basic CC/CV power supplies as the chargers, however the wattage of them is much lower than what I intend to use. I hate the idea of cutting off power at an arbitrary early number, and then relying on the undersized charger prematurely.
My gut tells me the BMS doesnt even balance until the wattage going in is pretty low (low amps because of voltage cut off and cells are almost full).
If thats the case, I can set my charger to 116.2 and not worry about it.
If thats not the case, what is the limit?

BTW if someone PMs me information, I can keep my mouth shut. I'll sign a non-disclosure if needed.
I know who reads this. lol.

[BrianTRice@gmail.com]

The Super Charger will have a more than adequate operations manual.

[Doug S]

I can't swear exactly how the Zero BMS's behave, but I did build an electric bicycle a while back and I read up pretty thoroughly on the operation of the BMS I used for that battery pack.

For bulk charging, a high-power switch (a MOSFET in this case) connects the charger directly to the battery pack, and all the cells are charged in series (that pack had only a series string, no parallel hookup). This gives all the cells exactly the same charging current, but since no two cells are ever truly identical, one of them will hit 100% capacity first, and when that happens, the high-current charging path is opened up. Battery charger voltage is then applied to a series resistor string, one for each cell, but the current through each resistor can be diverted by a switch (smaller MOSFETs, two per cell) through each individual cell instead of through the resistor. Each cell "decides" whether it needs to continue charging or if it's done. The current through the resistor string is far smaller than the main charging current (to keep heat generation down), and will charge each cell only very slowly. Eventually all cells will achieve the same voltage, the maximum safe voltage for that cell, and the whole pack is considered fully charged.

If this is similar to Zero's BMS operation, which I suspect it is, there is no equalization done until the bulk charging phase is turned off. I think Kocho's right, during the bulk charging phase, the cell voltages may equalize some but it's just due to the cells all approaching "fully charged" voltage together. Cell voltage is pretty stable until the end of the discharge cycle, where the difference between a tired cell and a not-so-tired cell increases much faster.

[MrDude_1]

The Super Charger will have a more than adequate operations manual.

Thats awesome news! I am looking forward to reading it.

But the supercharger isnt the only CC/CV charger supply. Mine is more of a DIY setup.