Chevy Bolt EV Forum banner

81 - 100 of 175 Posts

·
Registered
Joined
·
593 Posts
I understand why it is good to have a tight pack tolerance for range, as every cell is doing its job. I don't think .02 volts is causing fires. This is twenty millivolts, about what a thermocouple in gas fireplace puts out. It is an incredibly small amount of voltage. It is only .5% of a 4 volt cell's final voltage. I do not believe a battery maker would produce a product so unstable, so hard to control or balance that this value would cause a cell to catch fire. As batteries age we all know internal resistance goes up and batteries drift even more from new specs. Inferior cells pull down the voltage of good ones and range decreases. The BMS needs to be designed to handle new cells and cells that have 5000 cycles on them. If the manufacturer could not accomplish this task, then God help us, we are all sitting on bombs. This voltage anomaly we are seeing is very strange as it tends to be in the same cell groups so far. We all know LG didn't pick inferior cells and decide to put them exactly in position 65-72 just for fun. It could the location in the battery pack or a fault in the BMS that is causing this but I don't believe it is causing the fires. Maybe decreased capacity, but not fires. There would also be more fires as someone else mentioned. No, pouches were made by LG with some defect in them. I read in one story posted of microscopic holes in the separators. This is the kind of problem we are up against.
 

·
Registered
Joined
·
890 Posts
I agree. I don't think the weird variations would cause a fire either. At that level, it may not be a problem at all, much less be a fire risk. The only question is whether those weird variations might add up over time and eventually lead to a bigger variation which could cause a fire (not likely since the 2017's have been around a while)... or maybe whatever causes that slight variation may deteriorate over time or be worse in some vehicles. For example, what if whatever is causing that weird variation is exagerated in some Bolts and causes more than a .02V variation? Just something to ponder.

Mike
 

·
Registered
Joined
·
4,176 Posts
I disagree that "tight" voltage tolerances couldn't cause fires. There's a reason the first thing GM engineers pointed to was 100% charging. While below 100%, a 0.02 V variance isn't anything more than a nuisance that might slightly affect capacity. At 100%, however, it could cause a serious issue, especially because the Bolt EV's battery provides almost no buffer on the upper end, and the buffer does have can be overridden by "burping" the battery.

The Bolt EV's cells should settle on a post-charging voltage of no more than 4.17 V. A 0.02 V variance at that point could cause a lot of damage to cells, and if the Bolt EVs BMS is attempting to balance cells at that point, a 0.02 V variance could create a lot of heat both in the electronics and the pack.
 

·
Registered
Joined
·
418 Posts
Discussion Starter #87
Here are my Torque Pro numbers... look familiar? :unsure:

2017 Chevrolet Bolt EV
Vista 1.0 Battery
130,000 miles
61.9% SOC
56 kWh Capacity

View attachment 31711
Looks a LOT like my 2017 pack.

Do we know if Chevy changed the BMS software for the cars with the new chemistry packs? I can't think of any other reason why the low / high cell groups would be the same for all these 2017 - 2019 packs. And I agree we need more data from 2020s.
 

·
Registered
Joined
·
4,176 Posts
Looks a LOT like my 2017 pack.

Do we know if Chevy changed the BMS software for the cars with the new chemistry packs? I can't think of any other reason why the low / high cell groups would be the same for all these 2017 - 2019 packs. And I agree we need more data from 2020s.
Yes, the BMS software for the 2020 is definitely different. In fact, the BMS itself might be different.
 

·
Registered
Joined
·
890 Posts
I disagree that "tight" voltage tolerances couldn't cause fires. There's a reason the first thing GM engineers pointed to was 100% charging. While below 100%, a 0.02 V variance isn't anything more than a nuisance that might slightly affect capacity. At 100%, however, it could cause a serious issue, especially because the Bolt EV's battery provides almost no buffer on the upper end, and the buffer does have can be overridden by "burping" the battery.

The Bolt EV's cells should settle on a post-charging voltage of no more than 4.17 V. A 0.02 V variance at that point could cause a lot of damage to cells, and if the Bolt EVs BMS is attempting to balance cells at that point, a 0.02 V variance could create a lot of heat both in the electronics and the pack.
I thought it was more conservative like 4.12 is the max? I'll go with your 4.17 though because I know you use Torque Pro a lot and I only use it occasionally so you would know better than me. But here's my question. Will the BMS stop charging when the highest cell is at 4.17? Or does it go by pack average? If the former, there shouldn't be much to worry about. Even if it went by pack average, the highest cell should only be about .01V higher than the average because the higher cells are usually higher by .01 and the lower cells are usually lower by .01.

Now, if it doesn't stop charging until the LOWEST cells reach 4.17, then that's a BIG problem! Not sure what you've observed on this front.

Mike
 

·
Registered
Joined
·
4,176 Posts
I thought it was more conservative like 4.12 is the max? I'll go with your 4.17 though because I know you use Torque Pro a lot and I only use it occasionally so you would know better than me. But here's my question. Will the BMS stop charging when the highest cell is at 4.17? Or does it go by pack average? If the former, there shouldn't be much to worry about. Even if it went by pack average, the highest cell should only be about .01V higher than the average because the higher cells are usually higher by .01 and the lower cells are usually lower by .01.

Now, if it doesn't stop charging until the LOWEST cells reach 4.17, then that's a BIG problem! Not sure what you've observed on this front.

Mike
I don't know specifically for the Bolt EV, but I do know that with lithium batteries in general, you cannot apply float voltage. The concern here is, if the BMS is attempting to constantly balance cells after they are "full," it's the equivalent of applying float voltage that could damage lithium cells over time.
 

·
Registered
Joined
·
6,259 Posts
I don't know specifically for the Bolt EV, but I do know that with lithium batteries in general, you cannot apply float voltage. The concern here is, if the BMS is attempting to constantly balance cells after they are "full," it's the equivalent of applying float voltage that could damage lithium cells over time.

Float voltage only applies to lead acid cells. For balancing, you basically overcharge the highest cells until all the cells reach a minimum voltage. This doesn't hurt lead acid cells, they just boil away some water. Lithium ion is not like this at all. If you overcharge them the electrolyte outgases, and they vent, or puff up, in the case of pouch cells. This ruins the cell. The purpose of the BMS is to prevent cells from exceeding the design voltage. For most lithium cells this is 4.2 volts. They can certainly be charged higher but, much higher and they fail outright. The lower voltage you charge them the longer they last. So the top voltage is always a tradeoff on life versus utility.

Cells that are not defective or failing can reach 4.165 volts easily, and having the BMS bleed off anything beyond this is no harder on the cells than any other time you pass current through them. As long as the bleeder resistor is maintaining them at your design voltage they are fine. The trouble comes if they can't reach, or maintain that voltage, or the BMS fails, and allows them to exceed the design voltage.

If all cells are good, the BMS should shut off charging when all cells have reached the design voltage. At that point they will all sit at that voltage , or a few hundredths of a volt lower, for months to years, if there are no parasitic loads.

Lead acid cells don't do this. They suffer from self-discharge, and need to be topped up frequently, hence float charging.
 

·
Registered
Joined
·
4,176 Posts
Float voltage only applies to lead acid cells. You basically overcharge the highest cells until all the cells reach a minimum voltage. This doesn't hurt lead acid cells, they just boil away some water. Litium ion is not like this at all. If you overcharge them the electrolyte outgases, and the vent, or puff up, in the case of pouch cells. This ruins the cell. The purpose of the BMS is to prevent cells from exceeding the design voltage. For most lithium cells this is 4.2 volts. They can certainly be charged higher but, much higher and they fail outright. The lower voltage you charge them the longer they last. So the top voltage is always a tradeoff on life versus utility.

Cells that are not defective or failing can reach 4.165 volts easily, and have the BMS bleed of anything beyond this id no harder on the cells than any other time you pass current through them. As long as the bleeder resistor is maintaining them at your design voltage they are fine. The trouble comes if they can't reach, or maintain that voltage, or the BMS fails, and allows them to exceed the design voltage.

If alll cells are good, the BMS should shut off charging when all cells have reached the desighn voltage. At that point they will all sit at taht voltage , or a few hundredths of a volt lower for months to years, if there are no parasitic loads.
Yes, because you don't use float voltage on lithium, I was trying to use a common term to explain applying a continued current to lithium batteries after they've reached their peak voltage. The concern is, if the BMS is trying to balance cells, is it allowing certain cells to continue receiving power even after they've reached their peak voltage.
 

·
Registered
Joined
·
593 Posts
Yes, because you don't use float voltage on lithium, I was trying to use a common term to explain applying a continued current to lithium batteries after they've reached their peak voltage. The concern is, if the BMS is trying to balance cells, is it allowing certain cells to continue receiving power even after they've reached their peak voltage.
The single most important function of a BMS is to protect the battery. That means disconnecting the battery or discontinuing the charge if the correct voltage can not be achieved. A well designed BMS will not just sit there and allow electrons to be poured into a cells if in a certain time frame and compared to the other cells it does not come up to snuff. This is exactly the opposite of what is suppose to happen. If this is how the BMS was designed, then I say again, god help us because we are driving time bombs.
If GJetson is correct, and this is a passive system then all the cells are charged a little on the high side and then a resistor bleeds the voltage of the highest cells down to the lowest.
Each cell is 55 amp hours and since we know we can get a voltage reading from all 96 of them, each cell voltage can be individually bleed down. Watts = Volts x amps. Trying to bleed 55 amps down by .02 volts should release 1.1 watts of heat. That is 1/60 of a 60 watt light bulb. Let's say half of the cells need to be bleed down. That is still only 53 watts of heat energy.
Most likely the resistors are in the BMS as I saw on indication of them around the pack in the Weber video. 53 watts is not a lot of heat. I may be wrong with some of my math but you get the general idea, dissipating this much heat should not be a problem and if the BMS is pumping juice into the whole pack in a futile effort to bring all the cell into alignment then it is doing the absolute opposite of what a BMS should be doing and is a total engineering disaster.
 

·
Registered
Joined
·
4,176 Posts
The single most important function of a BMS is to protect the battery. That means disconnecting the battery or discontinuing the charge if the correct voltage can not be achieved. A well designed BMS will not just sit there and allow electrons to be poured into a cells if in a certain time frame and compared to the other cells it does not come up to snuff. This is exactly the opposite of what is suppose to happen. If this is how the BMS was designed, then I say again, god help us because we are driving time bombs.
If GJetson is correct, and this is a passive system then all the cells are charged a little on the high side and then a resistor bleeds the voltage of the highest cells down to the lowest.
Each cell is 55 amp hours and since we know we can get a voltage reading from all 96 of them, each cell voltage can be individually bleed down. Watts = Volts x amps. Trying to bleed 55 amps down by .02 volts should release 1.1 watts of heat. That is 1/60 of a 60 watt light bulb. Let's say half of the cells need to be bleed down. That is still only 53 watts of heat energy.
Most likely the resistors are in the BMS as I saw on indication of them around the pack in the Weber video. 53 watts is not a lot of heat. I may be wrong with some of my math but you get the general idea, dissipating this much heat should not be a problem and if the BMS is pumping juice into the whole pack in a futile effort to bring all the cell into alignment then it is doing the absolute opposite of what a BMS should be doing and is a total engineering disaster.
No, that's not how the BMS was designed, but as others have noted, according to the Weber State video, the BMS must be connected in the proper sequence. So again, it could be an issue with the assembly process rather than the components themselves.

As far as bleed down resisters, yes, I would assume those are in the BECM itself, but also, yes, your math is off. Each cell is 57 Ah, but each cell group is 171 Ah, so over three times as much energy as you were calculating. The bigger issue is, if something is wrong with how the BMS was set up, it might allow cells to continue charging past their high voltage cutoff. And if bleed down resistors are attempting to pull excess voltage off of cells that are still being actively charged, it can create a lot more than just incidental residual heat.
 

·
Registered
Joined
·
593 Posts
No, that's not how the BMS was designed, but as others have noted, according to the Weber State video, the BMS must be connected in the proper sequence. So again, it could be an issue with the assembly process rather than the components themselves.

As far as bleed down resisters, yes, I would assume those are in the BECM itself, but also, yes, your math is off. Each cell is 57 Ah, but each cell group is 171 Ah, so over three times as much energy as you were calculating. The bigger issue is, if something is wrong with how the BMS was set up, it might allow cells to continue charging past their high voltage cutoff. And if bleed down resistors are attempting to pull excess voltage off of cells that are still being actively charged, it can create a lot more than just incidental residual heat.
So are you saying the BMS has no battery protection function? This cannot be possible. If the BMS doesn't detect even the most basic malfunction then it is not a BMS but a crap shoot! No wonder cars are catching on fire. Not sure what you mean by each group? Are we balancing each cell of 3 pouches or each group of 3 cells? If what you say is true the graphs would not have such per/cell individuality as every group would have the same voltage. that would mean every three lines in a row would be the same, clearly each cell has it own height in all the graphs.
 

·
Registered
Joined
·
6,259 Posts
if the BMS is pumping juice into the whole pack in a futile effort to bring all the cell into alignment then it is doing the absolute opposite of what a BMS should be doing and is a total engineering disaster.
The charger is pumping juice into every cell during charging, because they are in series. There are no relays between cells, allowing them to be bypassed, to prevent this. As long as the charger is on, current is flowing through every cell. The voltage of each "cell" will be capped at the BMS designed limit, however, by bleeding off the excess current from each full cell to its bleeder resistor.

At the end of a 100% charge, using the onboard charger, the charger is putting out less than 2000 watts, at 400 volts, so under 5 amps flowing through every cell. Bleeding off 5 amps x 4.165 volts = 20.8 watts per cell, During balancing, for some length of time, every one of the 96 cells is bleeding off that much power...the whole 2000 watts.

This is easy to see here where module #6, which the BMS sits on, is 9 F warmer than the others, and module #5 which is permanently attached to it is almost as warm.

12-29-18-1.jpg 1605918279247.png

However, I agree that they need the BMS to be more sophisticated that that. If there is a failing cell that can't reach or maintain the target voltage, that failure is exhibited as heat in the cell. Sooner or later it will go into thermal runaway.
 

·
Registered
Joined
·
4,176 Posts
However, I agree that they need the BMS to be more sophisticated that that. If there is a failing cell that can't reach or maintain the target voltage, that failure is exhibited as heat in the cell. Sooner or later it will go into thermal runaway.
Again, though, I think that the question might not be whether the BMS is sophisticated enough; the question might actually be, "Does it still function with adequate sophistication if improperly installed?"
 

·
Registered
Joined
·
593 Posts
The charger is pumping juice into every cell during charging, because they are in series. There are no relays between cells, allowing them to be bypassed, to prevent this. As long as the charger is on, current is flowing through every cell. The voltage of each "cell" will be capped at the BMS designed limit, however, by bleeding off the excess current from each full cell to its bleeder resistor.

At the end of a 100% charge, using the onboard charger, the charger is putting out less than 2000 watts, at 400 volts, so under 5 amps flowing through every cell. Bleeding off 5 amps x 4.165 volts = 20.8 watts per cell, During balancing, for some length of time, every one of the 96 cells is bleeding off that much power...the whole 2000 watts.

This is easy to see here where module #6, which the BMS sits on, is 9 F warmer than the others, and module #5 which is permanently attached to it is almost as warm.

View attachment 31720 View attachment 31721

However, I agree that they need the BMS to be more sophisticated that that. If there is a failing cell that can't reach or maintain the target voltage, that failure is exhibited as heat in the cell. Sooner or later it will go into thermal runaway.
[/OTE]
And that is exactly what I was saying and understand too. I know there is no way to bypass cells. The whole bank gets charged until a slightly higher than desired voltage is achieved based on maybe an average cell voltage (?) and then probably allowed to rest, then balance by bleeding off. If there is no way for the BMS to detect cells that are not coming up as fast then I give up and it is piece of junk.
 

·
Registered
Joined
·
593 Posts
Again, though, I think that the question might not be whether the BMS is sophisticated enough; the question might actually be, "Does it still function with adequate sophistication if improperly installed?"
Respectfully, I find it hard to believe that Weber caught on to this easily in making his video but the largest manufacturer of EV batteries in the world with a revenue of 21 billion dollars could not instruct the assembly plant to hook up the wires in the correct order. So are you saying it is every car or just a few? All they need to do is drop the batteries and re-arrange a few wires? Sorry, does not compute on my end. There are defective cells, plain and simple. Maybe the BMS is defective in that it can't safely dissipate heat for the period of time it needs to just gets a short in it and catches on fire. I am incredulous to believe the BMS does not have an abort function. If it does not then we may as well call it Willy Wonka's Chocolate Factory.
 

·
Registered
Joined
·
4,176 Posts
Respectfully, I find it hard to believe that Weber caught on to this easily in making his video but the largest manufacturer of EV batteries in the world with a revenue of 21 billion dollars could not instruct the assembly plant to hook up the wires in the correct order. So are you saying it is every car or just a few? All they need to do is drop the batteries and re-arrange a few wires? Sorry, does not compute on my end. There are defective cells, plain and simple. Maybe the BMS is defective in that it can't safely dissipate heat for the period of time it needs to just gets a short in it and catches on fire. I am incredulous to believe the BMS does not have an abort function. If it does not then we may as well call it Willy Wonka's Chocolate Factory.
In these cases, you also have to account for a number of performance factors, including linguistic barriers, training, workplace culture, etc. Production line employees don't always do exactly what they're told, and if they find that plugging 1, 3, 5, 7 is faster than plugging 1, 2, 3, 4, they might do the former, especially if they aren't aware of the ramifications.

Keep in mind, it looks like Hyundai was subject to some of the same LG QC issues after GM had already corrected those issues with their own LG partnership.

*Edit: And frankly (I'm sure I'm not the only one here who feels this way), I do find your post disrespectful. Anyone who's followed Professor Kelly's channel knows how assiduous, meticulous, and knowledgeable he is. To assume that assembly line employees have the same knowledge, skills, and attitudes as an industry expert, master mechanic with decades of hands on experience is a bit insulting.
 
81 - 100 of 175 Posts
Top