[spdas]

Aloha y'all. A while back I started a thread about my 5 new 180 calbs discharging down to 0.7v while the rest of the pack was over 3.2v or so after I came back from 70 miles. Nothing was figured out and Calb position was tough luck, since you do not have a BMS, there is no warranty. I was reluctantly going to accept this because 4 cells took a charge and came right back up and have been cycled 7-8 times and are acting fine. Yesterday I went 100 miles and this morning I checked every cell and ALL were 3.26 or 3.27v. An hour later the previously defective cells ranged in voltage from 1.2v down to 0.7v. They self-discharged as there was no load on them. I feel they are defective and emailed Keegan and the factory in China, so we will see how I am treated.

Francis

 

[EVfun]

If I understand you correctly, the cells seem normal if kept nearly fully charged when not in use but if allowed to sit at around 20% SOC they will quite rapidly self-discharge. Is that correct? If so, that is an interesting failure mode. I've seen abused cells that just self discharge, but I haven't hear of or seen any that self discharge or not, based on the SOC they are stored at.

Will everything else work if you just pull the 5 cells? (mostly referring to the DC to DC converter and charger) Reading your previous thread it seems they are together, hopefully making them easy to pull. You could put some TS cells back in there if you need the voltage, just remember that you will execute them if you discharge the pack past the 160 amp hours.

 

[spdas]

Correct, there were a couple of cells at 3.25 and a couple at 3.28v in the rest of the pack, so I figured I would bottom balance. At least see how much I had to add to the 3.25v to get it to 3.26. Maybe not even one hour after I checked all the cells and found them to be mostly equal I rechecked the bad cells and they were 1.2, 0.7 etc. So my position is that this is what happened the first time, when 5 cells self-discharged when at about 20% SOC. Had nothing to do with a BMS or not. And since have been "normal" at around 35% SOC which is where my last 5 cycles were discharged to.

Francis

 

[DavidDymaxion]

I have seen something similar in that cells that vented electrolyte would then have a high self discharge. That's one reason I am weighing all my new cells, and if I have a problem cell I can reweigh it and see if it has lost any mass.

If I understand you correctly, the cells seem normal if kept nearly fully charged when not in use but if allowed to sit at around 20% SOC they will quite rapidly self-discharge. Is that correct? If so, that is an interesting failure mode. I've seen abused cells that just self discharge, but I haven't hear of or seen any that self discharge or not, based on the SOC they are stored at.

Will everything else work if you just pull the 5 cells? (mostly referring to the DC to DC converter and charger) Reading your previous thread it seems they are together, hopefully making them easy to pull. You could put some TS cells back in there if you need the voltage, just remember that you will execute them if you discharge the pack past the 160 amp hours.

 

[frodus]

From what it sounds like, you probably damaged them to begin with (before the first time you noticed), now they won't hold a charge.

Running without a BMS is just fine if you keep an eye on things, but running without a cell-level monitoring system is risky because there's no way to catch a cell or two that went too low.

You probably overdischarged a few of them.

 

[cruisin]

From what it sounds like, you probably damaged them to begin with (before the first time you noticed), now they won't hold a charge.

Running without a BMS is just fine if you keep an eye on things, but running without a cell-level monitoring system is risky because there's no way to catch a cell or two that went too low.

You probably overdischarged a few of them.

Travis, this would be a good time to recommend how he should monitor each cell during use to avoid what has already happened. What are his cell level monitoring options that are available? Is there a thread that has already addresed this subject to avoid duplication?

 

[JRP3]

That may have been covered in this thread, which he started:
http://www.diyelectriccar.com/forums/showthread.php/bms-not-bms-53768.html

 

[JRP3]

To review, from that thread:

Cells were brand new. First time use. All were top balanced, with putting in parallel for 24 hours. They all settled to a resting voltage about 3.40 or so. There was nothing hooked up to drain the cells. The 5 cells depleted to 0.5v or so and the rest were at 3.19v or so. I charged the remaining cells up and counted the KWH and by specs they were 35% SOC, not dead like the 5. I have now bottom balanced all the cells and the 5 cells volted up to 3.9v and the rest were still at 3.37 (about 85% charged). All cells came in at 195 to 196v and close resistance when I got them.
I would believe the scenerio if you were to tell me the 5 were 130ah and the rest were 180ah, as that is what it seems like to me.

http://www.diyelectriccar.com/forums/showpost.php?p=251294&postcount=944

 

[JRP3]

It certainly sounds as if those 5 cells were defective, and while a BMS could have stopped them from being deeply discharged they would still be defective cells.

 

[spdas]

It certainly sounds as if those 5 cells were defective, and while a BMS could have stopped them from being deeply discharged they would still be defective cells.

That is my position too. The original time it happened, ALL cells were top balanced and on my return journey at the 50 mile mark (I have 100 range), the 5 cells dropped to "0" immediately, when the rest of the cells were still over 3.25v or so. (counting amps in, the pack was less than 2/3 discharge or still had over 1/3 left .... probably almost 1/2 left). Why did the cells drop off. How would a BMS avoid this? A BMS would only tell you that you have a cell getting low or that you have a defective cell.

Francis

 

[dimitri]

All were top balanced, with putting in parallel for 24 hours. They all settled to a resting voltage about 3.40 or so.

This is not top balance at all. This is very rough approximation of being at the same SOC, assuming all cells are the same capacity, which they are not in real life. Most likely his 5 cells were few AH less than the rest, perhaps being from a different batch. By just paralleling cells and not charging them to the upper knee together he did not accomplish much at all. Then, when cells of slightly different capacity, but same SOC were put in series and put to use, smaller ones drained before others and took some damage. From that point you are on the down slope as damaged cells keep accumulating damage since you can't keep them in line with the rest since you have no tools attached to them.

Tracing back this story one can assume that 5 cells were simply few AH below others, but as long as they were not less than rated capacity, you can't claim they were defective.

Simple cell level monitoring tool would have clamped usage of smaller cells at their LVC level and prevented them from taking a dive. Alternatively, if he bottom balanced the pack to begin with, they would all drained together and not gotten damaged, but this means 5 smaller cells would shoot up at the end of charge and would require charger adjustments to make sure it stops when those 5 are full. All of this also assumes he knows how to bottom balance correctly, and seeing if one can't top balance correctly one may not be any better at bottom balancing.

The issue here is that pack management terminology is discussed until everyone is blue in the face, but when it comes to practice many tend to do it wrong and then blame the cells.

Spdas, please do not take it personally. I made bunch of assumptions here based on what you said, which may not be the actual story. I do not discount the possibility of defective cells, its certainly possible, but when you refuse to use simple tools the you can't prove anything one way or the other.

 

[spdas]

Sometimes it is hard to describe a persons methods and assume everyone understands. So let me explain more clearly how I prepared the cells.

1: I charged all cells to the upper knee and when they started to climb in voltage fast, I stopped.
2: I then put all into parallel for over 24 hours. (all cells were very very close in voltage)
3: I then installed into the car and charged the whole pack to the top knee. (edit: verifying that I was at the top knee already. Maybe only 0.2kwh went into the cells charging to 3.478vpc)
4: the next day I originally drove the for the first time and the 5 cells went bad.

I estimate the good cells had 60-70 amps left in them and the 5 defective cells had 0-5amps remaining. I have seen no explanation other than a defect to have caused this. And I feel it is a moot point to say this or that system would have warned you that your cells are going low.
I don't take any offense of good sound arguments.
francis.

 

[dimitri]

In this case, 5 cells were most likely defective in some way by the time you got them. Rare case, but certainly not impossible and not the first time I encountered such case myself.

 

[JRP3]

CALB cells simply do not rest at 3.4V unless they have been charged well into the knee of the curve, which means even without paralleling they will all be very nearly completely top balanced, within an amp hour or so at worst, certainly no where near 30 amp hours apart at the bottom, unless they have significantly different capacities, which they supposedly did not if they were all between 195-196 ah.

 

[dtbaker]

I don't think his pack was actually initially top-balanced in the very beginning if all he did was wire and parallel after chargnig individually and wait for a while. That whole middle flat of the curve would balance voltage while having potentially close but different SOC from the factory. I see no mention that the initial top-balance was brought to any 'top knee' voltage while cells were IN PARALLEL to insure all cells were actually balanced at the top.... and thus the pack never really was balanced either top or bottom after that in use, resulting in damage. Also I seem to recall that this pack has been direct charged in a split pack from solar panels.... or am I mixing up threads now?

 

[dtbaker]

Sometimes it is hard to describe a persons methods and assume everyone understands. So let me explain more clearly how I prepared the cells.

1: I charged all cells to the upper knee and when they started to climb in voltage fast, I stopped.

was this done while the entire pack was wired in parallel?
stopped at what voltage?

 

[JRP3]

As I said, CALB cells simply do not show a resting voltage unless they are charged well into the knee of the charge curve, meaning fully charged. He charged all cells into the knee of the curve, then paralleled them, then charged them again in the car in series into the knee of the curve. Even if they were not perfectly balanced there is no way they were more than the 60 amp hours difference he ended up with.

 

[dtbaker]

He charged all cells into the knee of the curve, then paralleled them, then charged them again in the car in series into the knee of the curve.

the initial charge of individual cells 'into the knee' is what I see as the key issue here. if there were 5 cells maybe 1ah or 2ah higher than all the rest, and then the pack was wired in series and charged.... those 5 could well have cooked on the very first charge.

 

[JRP3]

I see what you're saying. I guess we need to know exactly how closely he monitored the max charge on each cell and if any could have spent any serious time in an overcharged state. A minute or two at 4.0V won't hurt them.

 

[dtbaker]

a BMS is not going to do you much good if the initial balance is not *close*, and really not going to help much in a case like this when the OP is charging a half-pack of thunderskys, and the other half CALB directly from PV panels.