Need some help before I drive my ebike into the river.....

[dnmun]

"Does this mean I have a bad cell? Could this be the cause? Perhaps the BMS is in fact working - If this cell is bad and tripping the battery BMS LVC during discharge, it would all make sense, no?"

you should verify that the cell is not just balancing to the 3.33V level, and not allowing more than the 3.33V to build up before the shunt turns on. because of a miscalibration of the resistor divider bridge to the comparator on that channel.

no reason not to open it to work on it. i don't think paul really warrants things once you use them and if the BMS channel is miscalibrated, then i suspect he would wanna know that.

so you really have to actually charge that cell up to at least 3.65V while the others are fully charged and run it through several discharge cycles to see where the cell balances again. that's why it helps to measure the individual cell voltages while charging. to see where they balance. you do not know if it is a runt or if it is just not getting fully charged.

 

[Lenk42602]

Thanks Dnum. I'm on it.

Len

 

[Ypedal]

a cell that has lost capacity ( damaged or otherwise ) will charge to 3.65v and discharge to 2.0v, delivering less than it's brothers...

a cell that is self-discharging faster than it's brothers has a higher internal resistance ( usually ) or is being drained by a faulty BMS.. that's what you determine by topping off with the cell ph charger.

as dnum pointed out, fully charge the pack first, so that all the other cells are fully charged.. then top off the suspect cell by itself, if it takes hours to reach 3.7v it's probably good to go ... if you put the cell ph charger on it and it jumps to 3.7v in minutes.. it's a dud cell.

 

[Lenk42602]

.... because of a miscalibration of the resistor divider bridge to the comparator on that channel.

no reason not to open it to work on it. i don't think paul really warrants things once you use them and if the BMS channel is miscalibrated, then i suspect he would wanna know that.

so you really have to actually charge that cell up to at least 3.65V while the others are fully charged and run it through several discharge cycles to see where the cell balances again. that's why it helps to measure the individual cell voltages while charging. to see where they balance. you do not know if it is a runt or if it is just not getting fully charged.

Thanks Dnum,

BTW if I do begin opening up the pack, I will need a bit of guidence identifying corresponding resistor/comparator components that manage that partiular cell..

Thank you,


len

 

[dnmun]

hahahaha! like i know anything about the BMS.

but like ypedal says, if the cell jumps right up to 3.7V then it is not capable of holding charge as easily as the other cells. it will drop faster than the others too.

but if you charge it up to over 3.65V and the voltage holds initially but slowly declines while sitting then the shunt transistor is diverting the extra charge through the shunt resistor and it will stabilize back at the 3.33V again. but if you get it up there where they are all at 3.65V or better then you can run the bike again to see how many Ahs you can pull out of it before it shuts off. we know your BMS is working that way.

 

[liveforphysics]

You need to charge and watch the low cell's voltage relative to the others while charging. If it's first to fall off the cliff and first to reach peak voltage, it's a weak cell group, likely just had a paralleling tab-weld disconnect (and no, this won't change it's voltage, just capacity of that cell group).

 

[Ypedal]

You need to charge and watch the low cell's voltage relative to the others while charging. If it's first to fall off the cliff and first to reach peak voltage, it's a weak cell group, likely just had a paralleling tab-weld disconnect (and no, this won't change it's voltage, just capacity of that cell group).

it's a single 20ah pouch cell.. not 26650's..

 

[John in CR]

This pack is new enough that Cellman should cover it. Was it the same cell that was at 3.48V the cycle before, and then wouldn't charge above 3.33V . If the cell will take near 20ah charge from it's current status, then there's something wrong with that channel of the BMS. If not then that cell is a problem, and needs replacement. If the cell is stable, just lower capacity, and if you're on your own with it (hopefully not the case), then the fix might be as easy as paralleling just one or more 26650 A123 cells with that weak cell to make up the difference...not pretty or elegant, but a fix. I've got a number of those cells at various capacity, so we could get you up and running in a jiffy.

John

 

[liveforphysics]

You need to charge and watch the low cell's voltage relative to the others while charging. If it's first to fall off the cliff and first to reach peak voltage, it's a weak cell group, likely just had a paralleling tab-weld disconnect (and no, this won't change it's voltage, just capacity of that cell group).

it's a single 20ah pouch cell.. not 26650's..

I am betting he is mistaken. I thought all the cell man packs with a BMS were tab welded 26650's. Unless very very early on you got one of his first couple packs using pouches.

 

[Lenk42602]

This pack is new enough that Cellman should cover it. Was it the same cell that was at 3.48V the cycle before, and then wouldn't charge above 3.33V .John

luke,
Paul and I spent a long time discussing my pack prior to his assembly. if my "?" after the pack implied I do not know the difference between a cylindrical A123cell and a prismatic A123 pouch cell, I apologize. Just wasn't certain if A123 manufactured different pouch cells w/different model # designations.
Pack is constructed with A123 20ah prismatic cells.
John, I would like to think he will offer some assistance considering the pack has 30 cycles at 8.8 ah discharge max. I sent him a second email today.

If the cell will take near 20ah charge from it's current status, then there's something wrong with that channel of the BMS. If not then that cell is a problem, and needs replacement. If the cell is stable, just lower capacity, and if you're on your own with it (hopefully not the case), then the fix might be as easy as paralleling just one or more 26650 A123 cells with that weak cell to make up the difference...not pretty or elegant, but a fix. I've got a number of those cells at various capacity, so we could get you up and running in a jiffy.
John

John, thanks for your input and your offer. Seems like the next phase is to try and bring that cell up to 3.65 ~ 3.7 so it can play with the others.

You need to charge and watch the low cell's voltage relative to the others while charging. If it's first to fall off the cliff and first to reach peak voltage, it's a weak cell group, likely just had a paralleling tab-weld disconnect (and no, this won't change it's voltage, just capacity of that cell group).

So Luke, I need a little help understanding "cell group" - my understanding of the pack's operation is that the BMS controls and monitors charge on each cell via the balance wires. Based on what you have indicated, it sounds like the BMS monitors cells individually and in "groups"?

BTW, based on space limitations in the frame, the 24s pack is constructed of two sub packs - 9s & 15s connected to a single 24s bms.

Thanks agian for all the input guys. As much as I have tried to avoid it, sounds like my battery pack construction and function education is formally beginning.....

Len

 

[cell_man]

Hi Len,

sorry there was a trade show here last week and I've had some visitors so my emails have slipped, I am now trying to catch up as best I can but do have some backlog to get through.

Sorry to hear you are having some problems, (it is a pack from 20Ah ouch cells BTW, not 26650s) I have a few comments:

The voltage measurements you last took do suggest 1 cell is at a lower state of charge than the others, that could be a bad cell, it could just be slightly higher self discharge on 1 cell that is not being addressed when charging, due to the charge voltage not being high enough for the BMS to do anything. All other cells would need to be at 3.65V plus before a low cell is brought back into balance. The charger you are using only brings the high cells up to approx 3.55V, that is not enough. A charge voltage a little higher than the standard 87.6V will help help to bring any low cells up to 3.65V. If you can charge the low cell using a single cell charger or power supply that would be good. Regarding Headway BMS, Headway do not and never did make their own BMS, they have bought from different companies over the years and their latest supplier are much better than some of the earlier BMS they sourced so my contact in Headway tells me. They had a lot of BMS issues in the past with units bought from other suppliers, so no I do not buy anything from Headway and the BMS I supply are 1 of the most reliable products I do supply, very few issues have been reported, so there is nothing so far that suggests a BMS issue, so there is no need to do any mods to it or the wiring of the pack.

Also, you really need to have some way to measure the current draw accurately, if Lyen didn't measure the CA shunt value and as far as I know he doesn't, that number is no more than a guess. You need to either accurately measure the R Shunt value and also ensure the CA wires have been fitted to the controller PCB correctly, or get an independent way to measure the current, such as a watt meter and use that to discharge the pack. Watt meters are generally 60V max, so you can only measure the discharge current, you cannot measure the total pack voltage or you will fry it. Bottom line, the Ah figures mean nothing right now, you need a reasonably accurate way to measure the current and Ah used, preferably at home with a test rig. There are just too many unknowns and variables in your present setup.

If you can charge the low cell up to 3.65V and then discharge the pack into a reasonably accurate test system, preferably at home in a controlled way then we can be sure if there is a bad cell or not. Once the BMS trips, all cells should be checked to determine the cell voltages of course. The CA should also be ok, but it would be good if at least the current measurement has been verified to be reasonably accurate.

I have replied to your email, can we please continue there if that is ok.

Thanks
Paul

 

[liveforphysics]

So Luke, I need a little help understanding "cell group" - my understanding of the pack's operation is that the BMS controls and monitors charge on each cell via the balance wires. Based on what you have indicated, it sounds like the BMS monitors cells individually and in "groups"?

Ahh, ok, it is indeed made from the 20Ah pouches, so you have no parallel cells and no cell groups. I had been thinking it was made from the 26650 M1 cells, and had something like 8p24s, in which case each 8p cluster to get ~20Ah would be a cell group. My mistake, you've got no cell groups.

Charge that low cell up to 3.65v with the others, and then run a discharge and see if it drops first. If it does, you've got a bad cell. Sometimes the foil connection internal where the tab gets welded to the foil layers gets torn a bit if its tugged on or jostled a bunch.

 

[Lenk42602]

btw,

waiting on one of these:

http://www.batteryspace.com/smartcharger30afor32vli-fepo4batterypack1cellstandardfemaletamiyaplug38vcut-off.aspx

should see it this upcoming week.

Len

 

[nicobie]

Do you have an extra cell phone or camera charger lying around? The ones with a mini UCB connector on them usually put out a voltage that is close enough for lifepo4 if you keep an eye on it toward the end.

 

[dnmun]

you can adjust the output voltage up on the charger too.

 

[John in CR]

One easy way to verify that the CA is calibrated correctly is to use a multimeter in line with the controller and compare no-load current on the CA to what the multimeter shows.

 

[GCinDC]

so what's up with the Skill? batt issues still, time for some lipo? hate to see you throw the baby out with the bath water..

or are you enjoying pedal power once again?