999zip999
100 TW
I mean I don't understand the whole compression thing. Where did you get the cells and which ones.
A123 20AHr Pouch Cell Battery Build & Info Thread
- Thread starter bigmoose
- Start date
megacycle
100 kW
Nice looking builds. 8)
Just a note on the cell insulators.
If you search way back, it was proved by Luke (live for physics) that the outer foil bag can conduct and track to adjacent cell, or would expect to an earthed frame of a motor bike of similar, if the ends cells were sandwiched against metal.
So i'd be carefull there.
Just a note on the cell insulators.
If you search way back, it was proved by Luke (live for physics) that the outer foil bag can conduct and track to adjacent cell, or would expect to an earthed frame of a motor bike of similar, if the ends cells were sandwiched against metal.
So i'd be carefull there.
megacycle
100 kW
You nearly ready to road test Zappy 
.
.wb9k
10 kW
Coming in late here, and I haven't read the whole thread, but I'm seeing questions on compression, a mysterious topic to many.
Shoot for 10-12 psi evenly distributed across the flat surfaces of all cells. You want light compression.
Some nice looking packs on the last couple pages here (and maybe elsewhere), but I would not pile up cells right on top of one another with no material in between. At the very least this will likely lead to hot spots in the pack and possible premature death for the innermost cells. I see some mention of current flowing through the foil pouch, but this should not happen normally. That pouch is supposed to be isolated from the cell electrically, in contrast to our cylindrical cells which have the cans strapped to the cathode. However, breaching the pouch insulation is not too difficult, especially in a cell that has been removed from a module and subjected to unknown handling procedures by God-knows-who. Additionally, any debris that may get in between the cells can become an issue when cells expand. Our modules have thin foam sheets or heat sinks between each cell. These together mitigate both potential issues.
dh
Shoot for 10-12 psi evenly distributed across the flat surfaces of all cells. You want light compression.
Some nice looking packs on the last couple pages here (and maybe elsewhere), but I would not pile up cells right on top of one another with no material in between. At the very least this will likely lead to hot spots in the pack and possible premature death for the innermost cells. I see some mention of current flowing through the foil pouch, but this should not happen normally. That pouch is supposed to be isolated from the cell electrically, in contrast to our cylindrical cells which have the cans strapped to the cathode. However, breaching the pouch insulation is not too difficult, especially in a cell that has been removed from a module and subjected to unknown handling procedures by God-knows-who. Additionally, any debris that may get in between the cells can become an issue when cells expand. Our modules have thin foam sheets or heat sinks between each cell. These together mitigate both potential issues.
dh
oatnet
1 MW
wb9k said:
Have you found some special non-insulating fireproof foam, or are you using the regular petroleum-based foam that will torch the pack at the first spark (aka Frozen Napalm)? You mention concerns about "hot spots" in the middle of the pack, I would expect the insulating properties of foam would exacerbate the issue if it exists. What c rate discharge are you using on your prisimatics that is generating the thermal load in excess of normal dissipation?
Also, while the "pouch is supposed to be insulated"', real-world testing showed microvoltages on virgin cells - btw none of the hundreds of single-cell pouches I have used was ever in a module, or likely even out of the factory packaging, so handling is not the cause.
999zip999
100 TW
When putting togethere a pack keeping a clean surface and a backup clean surface if you have take apart and to refit right. Keeping all the dirt out including the small solder balls jumping off a large hot iron.
I can understand compression like keeping the boiling point down in a radiator with a pressure cap ? And not changing to a gas. ? Is that what going on.
I can understand compression like keeping the boiling point down in a radiator with a pressure cap ? And not changing to a gas. ? Is that what going on.
wb9k
10 kW
Microvoltages on the pouches is the result of a capacative effect between the bag and the electrodes and that's normal. The numbers you see will vary depending on your measurement method, but this phenomenon will not lead to any sort of failure. I'm talking about pouches that have had their poly layers compromised on at least the interior side. You'll see real voltage on the bag in this case. This is a real possibility with the cells that had the famous welder alignment issues (grey market stuff). If the isolation fault is between the anode and the pouch, the aluminum pouch will decrepitate as Li alloys onto it, leading to black spots around the perimiter of the cell and electrolyte leakage. In reality, for this to be a hazard in a pack, multiple faults have to be present--both adjacent cells would have to have interior AND exterior isolation faults for anything bad to happen. But it could be really spectacular. A thin layer of Mylar or some such between cells should also be effective against this sort of thing.
I'm not sure exactly what type of foam we use between cells, but I highly doubt that it is very flammable. It's more like a foam rubber. And yes, without heat sinks, it may not help your thermal problems at all, but that's not why it's there--it's for evening out stack pressure, providing proper spacing, and some electrical isoation. Thermal management has to be addressed in other ways.
As far as C-rates, remember this is automotive and there's much more to be considered than just that. Automakers have to plan for worst-case scenarios and benign neglect by owners. A smart design will assume at the outset that cells will see some nominal level of abuse in their lifetime and plan accordingly. So we expect a certain amount of gas generation. The question is how to best handle that. Compression forces any gas to the margins of the cell, outside the electrode stack where it does the most harm to the cell's performance traits. If the electrodes are allowed to develop space between them, that area becomes basically dead in the cell and performance takes a hit. Shoot for 10psi, evenly distributed. Trust me.
dh
I'm not sure exactly what type of foam we use between cells, but I highly doubt that it is very flammable. It's more like a foam rubber. And yes, without heat sinks, it may not help your thermal problems at all, but that's not why it's there--it's for evening out stack pressure, providing proper spacing, and some electrical isoation. Thermal management has to be addressed in other ways.
As far as C-rates, remember this is automotive and there's much more to be considered than just that. Automakers have to plan for worst-case scenarios and benign neglect by owners. A smart design will assume at the outset that cells will see some nominal level of abuse in their lifetime and plan accordingly. So we expect a certain amount of gas generation. The question is how to best handle that. Compression forces any gas to the margins of the cell, outside the electrode stack where it does the most harm to the cell's performance traits. If the electrodes are allowed to develop space between them, that area becomes basically dead in the cell and performance takes a hit. Shoot for 10psi, evenly distributed. Trust me.
dh
megacycle
100 kW
That's probably the first time i've heard any real technical explanations of cell issues.
They're like black boxes (grey bags) . Sophisticated gear attatched to them, but noone up to now has particularly provided any technical input on cell mechanics. It would be good to have find some 2nd order info from the manafacturers, saying this is exactly what you have and need to do with them and this is how you would kill them.
I have been presuming that under heavy load, heat in the cell might migrate to the tab, so i've been heat sinking the tabs, using copper and aluminium over with thermal paste between the Cu & Al, hoping thismight draw of the heat.
I've got 0.5mm formica between cells, not just insulating, helping keep the light compression uniform at cell level.
At the moment one of my packs is running kids gokart at around 100A@24V,several months later, keeps on driving it.
They're like black boxes (grey bags)
. Sophisticated gear attatched to them, but noone up to now has particularly provided any technical input on cell mechanics. It would be good to have find some 2nd order info from the manafacturers, saying this is exactly what you have and need to do with them and this is how you would kill them.I have been presuming that under heavy load, heat in the cell might migrate to the tab, so i've been heat sinking the tabs, using copper and aluminium over with thermal paste between the Cu & Al, hoping thismight draw of the heat.
I've got 0.5mm formica between cells, not just insulating, helping keep the light compression uniform at cell level.
At the moment one of my packs is running kids gokart at around 100A@24V,several months later, keeps on driving it.
megacycle
100 kW
That's probably the first time i've heard any real technical explanations of cell issues.
They're like black boxes (grey bags) . Sophisticated gear attatched to them, but noone up to now has particularly provided any technical input on cell mechanics. It would be good to have find some 2nd order info from the manafacturers, saying this is exactly what you have and need to do with them and this is how you would kill them.
I have been presuming that under heavy load, heat in the cell might migrate to the tab, so i've been heat sinking the tabs, using copper and aluminium over with thermal paste between the Cu & Al, hoping thismight draw of the heat.
I've got 0.5mm formica between cells, not just insulating, helping keep the light compression uniform at cell level.
At the moment one of my packs is running kids gokart at around 100A@24V,several months later, keeps on driving it.
They're like black boxes (grey bags)
. Sophisticated gear attatched to them, but noone up to now has particularly provided any technical input on cell mechanics. It would be good to have find some 2nd order info from the manafacturers, saying this is exactly what you have and need to do with them and this is how you would kill them.I have been presuming that under heavy load, heat in the cell might migrate to the tab, so i've been heat sinking the tabs, using copper and aluminium over with thermal paste between the Cu & Al, hoping thismight draw of the heat.
I've got 0.5mm formica between cells, not just insulating, helping keep the light compression uniform at cell level.
At the moment one of my packs is running kids gokart at around 100A@24V,several months later, keeps on driving it.
Update on my earlier post below. Aliexpress have proved to be excellent. Victpower tried to weasel out of providing a refund with irrelevant information that didn't address any of their fraudulent behaviour and refusing to provide a return address so Aliexpress refunded all my money. The interesting thing is that this actually makes them a safer service than Ebay or Trademe which are just auction sites and don't hold the money until the client is happy like Aliexpress does.
The cells had all the usual problems - tabs cut off then welded, missing or irregular insulation layer around tab base, bashed corners, pouch foil cut away then covered with a sticker. Plus a few soft bendy ones. I've now switched to new Enerdel power cell modules which Evolve Electrics have on sale. They are twice the price of Victpower but half the cost of new A123 cells and almost as good (15Ah and 15C peak discharge):
http://evolveelectrics.com/Enerdel.html
I'll post an update when these arrive if there are any problems.
Edward
The cells had all the usual problems - tabs cut off then welded, missing or irregular insulation layer around tab base, bashed corners, pouch foil cut away then covered with a sticker. Plus a few soft bendy ones. I've now switched to new Enerdel power cell modules which Evolve Electrics have on sale. They are twice the price of Victpower but half the cost of new A123 cells and almost as good (15Ah and 15C peak discharge):
http://evolveelectrics.com/Enerdel.html
I'll post an update when these arrive if there are any problems.
Edward
Mr Ed said:
bigmoose
1 MW
wbpk just to be clear since the cells are approximately 6.4 in x 8.9 inches at 10 psi we want a compressive force on the stack of about 570 lbs. Is that correct?
johnrobholmes
10 MW
I built up half a pack with the OSN termination kit. Turns out the blocks are about 2mm wider than the cells, so we need 2mm of spacers between each cell. Of course the battery box was already made based on the cell dimensions, so it won't work with an extra two inches added to the pack. I got the kit to reduce my labor, and it just increased it over just spot welding the damn things.
Anybody else run into this? The very "detailed" building instructions from OSN make no mention of the spacers. I'm about ready to dump it all and just get some NMC packs.
Anybody else run into this? The very "detailed" building instructions from OSN make no mention of the spacers. I'm about ready to dump it all and just get some NMC packs.
wb9k
10 kW
bigmoose said:
No, it's just a simple 10psi of compression force applied to the module end to end. Then band snugly. It's not really that much pressure, but it needs to be evenly distributed. I've recently learned another reason this is important with pouch cells is that without compression, the electrolyte distribution inside the cell becomes uneven. This leads to local gradients of current (and heating) in a cell. Local temps can get hot enough to cause gas formation. So, lack of properly applied compression can be a contributor to "puffed cell syndrome", even if Voltage and current limits are never exceeded. A pack with no mechanism for sinking heat from the deep interior of the stack will have a harder time yet dealing with this. The pouch cell spec assumes proper pack design and assembly, something we don't need to worry about with cylindrical cells because the cylinder takes care of electrode stack pressure.
bigmoose
1 MW
Thank you wb9k, your detailed information is priceless! It is filling many of the voids we had in the engineering of the battery pack.
www.recumbents.com
10 kW
The NMC cells sold by http://evolveelectrics.com/Enerdel.html do not appear to be LiFePo4 formulation, it looks like they are closer to LiPo. Does that mean that they are not as safe as A123?
Edit - nevermind - looked it up and read the wikipedia page: Lithium iron phosphate (LFP), lithium manganese oxide (LMO) and lithium nickel manganese cobalt oxide (NMC) offer lower energy density, but longer lives and inherent safety.
-Warren.
Edit - nevermind - looked it up and read the wikipedia page: Lithium iron phosphate (LFP), lithium manganese oxide (LMO) and lithium nickel manganese cobalt oxide (NMC) offer lower energy density, but longer lives and inherent safety.
-Warren.
yokneamcity
1 W
- Joined
- May 9, 2011
- Messages
- 57
Doctorbass said:April said:Hi Doc,
This is April or Nancy from OSN Power.
About the kit, I reported your worries to our technicians.
For the kit, only the general postive and negative end carry the high current, other matal plates mainly to create the clamping pressure, so no worry about the soften of the plastic plates.
And we have tested the clamping pressure of the kit, there is no problem.
The material of the kit is a kind of special epoxy resin, not common plastic, and the studs and screws are copper.
On the general postive and negative end, there are two special matal bars sandwiched together, which is used for high current.
By the way, we are now developping a new special matal bar on the general postive and negative end to update the kit especially for high current.
Hi April,
I understand your explanations but the problem is that the flat bar you use and the only two screw with one on each end make the flat bar to bend and not have the pressure to all the width of the surface.
I made a drawing to explain what i mean. Please consider that you might not see the bending wby eyes but in high electrical current, a simple variation of pressure mean alot of diference and your proposed desing can not guaranty that pressure along all the width of the cell tab
I also encountered this problem. I'm make a battery for Vectrix scooter.
Added another screw in the middle of each plate.
But there is another problem, several packages were swollen. Packages bought from OSN company.
Attachments
999zip999
100 TW
Yokneamcity nice build can you give us more info at what we are looking. I started counting and guessing but I thought it easier to ask.
wb9k
10 kW
bigmoose said:Thank you wb9k, your detailed information is priceless! It is filling many of the voids we had in the engineering of the battery pack.
Awesome! Very happy to help.
Cheers!
cwah
100 MW
yokneamcity said:
What are the small transistors on your last picture inside your battery? Are these mini BMS?
wb9k
10 kW
It's impossible for us to say what the root cause is without a lot more information. It may not have been the cells' fault at all. It could have been, but it's not a common suspect....even among the stuff culled from Fisker modules, BTW. Relatively deep analysis of a number of "campaigned" Fisker modules showed that the number of actually defective cells in these modules was not that high--less than 10%. Now, I say that with some trepidation because I've seen tab "repairs" from Victpower in particular that make my hair curl. Their handling made a lot of cells sick with decrepitation of the Al pouch that spring leaks. That population (the "campaigned" cells) is more prone to this decrepitation than normal---but that's it. There is no elevated risk of puffed cells here--that happens, to my knowledge, from the following types of abuse:
1. Overcharge. Any extended time above 3.8 Volts will generate enough heat and electrochemical activity to puff a cell, especially one that is improperly compressed.
2. Overdischarge followed by charge. Any A123 cell that has been pulled low enough to come to rest at <300 mV should be immediately scrapped. The published number for that is 500 mV, but the real figure is closer to 300, so that's a "safety buffer" if you will. Below this Voltage, the Cu electrodes start to dissolve into the electrolyte. When charge is applied, the Cu forms dendrites that puncture the separator layer, forming an internal short in the cell. This can puff a cell in a hurry---the more charge current on tap, the worse it's prone to be.
3. Driving a cell negative. I've neglected to mention this before, but it is a possibility. I don't know much about the specific mechanism at this time.
4. Malfunctioning or misinformed electronics. This is the most common cause of all of the above in my experience. At this stage of the game, it is critical for YOU to understand how your BMS functions on at least a cursory level. Choose your BMS very carefully and periodically verify that it is operating properly. They're not all created equal. Make sure V sense lines are securely connected and free of corrosion. Just because your BMS says there was never a problem doesn't necessarily make it so. Avoid harnesses or ribbon cables between multiple modules if possible--they are problematic wherever they are used in any mobile electronics.
5. Exposure to or generation of sufficient heat. I don't know exactly at what temperature gas formation begins in the electrolyte, but we spec a max storage temp of 80 (or 85?) degrees C and I suspect this is the reason. The hotter, the puffier--to a point. This is why soldering tabs poses a real hazard to cell health. If you feel you must solder, sink or blow the heat away from the body of the cell. Use a big iron that can make sufficient local heat quickly, before the whole mass of the cell gets hot. You might even get the cell warm enough to melt separator if not careful.
6. No compression, not enough compression, improperly distributed compression. This is a pack/module design issue. Apply 10, maybe 15 psi to your cell stack end to end and then band snugly and evenly. Use hard endplates of some sort--never wrap cells directly or allow their shape to become distorted. Protect all areas of the pouch from impact damage.
I can't think of any other causes right now, but that's a longer list than I've put up here before. Bottom line is, cell defects are rarely the cause of puffed or vented cells, at least in my experience with A123 cells.
1. Overcharge. Any extended time above 3.8 Volts will generate enough heat and electrochemical activity to puff a cell, especially one that is improperly compressed.
2. Overdischarge followed by charge. Any A123 cell that has been pulled low enough to come to rest at <300 mV should be immediately scrapped. The published number for that is 500 mV, but the real figure is closer to 300, so that's a "safety buffer" if you will. Below this Voltage, the Cu electrodes start to dissolve into the electrolyte. When charge is applied, the Cu forms dendrites that puncture the separator layer, forming an internal short in the cell. This can puff a cell in a hurry---the more charge current on tap, the worse it's prone to be.
3. Driving a cell negative. I've neglected to mention this before, but it is a possibility. I don't know much about the specific mechanism at this time.
4. Malfunctioning or misinformed electronics. This is the most common cause of all of the above in my experience. At this stage of the game, it is critical for YOU to understand how your BMS functions on at least a cursory level. Choose your BMS very carefully and periodically verify that it is operating properly. They're not all created equal. Make sure V sense lines are securely connected and free of corrosion. Just because your BMS says there was never a problem doesn't necessarily make it so. Avoid harnesses or ribbon cables between multiple modules if possible--they are problematic wherever they are used in any mobile electronics.
5. Exposure to or generation of sufficient heat. I don't know exactly at what temperature gas formation begins in the electrolyte, but we spec a max storage temp of 80 (or 85?) degrees C and I suspect this is the reason. The hotter, the puffier--to a point. This is why soldering tabs poses a real hazard to cell health. If you feel you must solder, sink or blow the heat away from the body of the cell. Use a big iron that can make sufficient local heat quickly, before the whole mass of the cell gets hot. You might even get the cell warm enough to melt separator if not careful.
6. No compression, not enough compression, improperly distributed compression. This is a pack/module design issue. Apply 10, maybe 15 psi to your cell stack end to end and then band snugly and evenly. Use hard endplates of some sort--never wrap cells directly or allow their shape to become distorted. Protect all areas of the pouch from impact damage.
I can't think of any other causes right now, but that's a longer list than I've put up here before. Bottom line is, cell defects are rarely the cause of puffed or vented cells, at least in my experience with A123 cells.
wb9k You have stated 10 to 12 (psi) for stack compression, to me that means 10 to 12 lbs/inch.
You had responded to big moose with the same question and said 10 to 12 lbs total for the stack. But I keep seeing 10 to 12 psi. mentioned Please clarify
The one thing that keep me from buying those grey market cells, is not knowing the shelve life, and knowing my emoto build will take time.
That said what is the shelve and maintenance required.
BTW I really enjoy your efforts to inform and educate people on this endeavor. I learned much from your sharing.
Pete
You had responded to big moose with the same question and said 10 to 12 lbs total for the stack. But I keep seeing 10 to 12 psi. mentioned Please clarify
The one thing that keep me from buying those grey market cells, is not knowing the shelve life, and knowing my emoto build will take time.
That said what is the shelve and maintenance required.
BTW I really enjoy your efforts to inform and educate people on this endeavor. I learned much from your sharing.
Pete
999zip999
100 TW
Yokneamcity I would put a set of 8s sense wires on those packs for easy checking with cheap cellog.
megacycle
100 kW
wb9k think there must have been a typo. Must be seeing who's on the ball :wink:
Yeh, if it was 10 psi i think there'd be a problem, can't see 500lb+ on end plates, that would squash the guts out of them.
Must be pack total, 5kg in the metric stuff, would be like light firm compression.
Yeh, if it was 10 psi i think there'd be a problem, can't see 500lb+ on end plates, that would squash the guts out of them.
Must be pack total, 5kg in the metric stuff, would be like light firm compression.
wb9k
10 kW
OK, well mech stuff is not my forte. Apparently the verbiage comes from the air pressure driving the stack compressor, not the actual measurement of psi in the stack. My bad. So what's the proper verbiage...10 lbs of end-to-end pressure?
bigmoose
1 MW
That is the clarification I am after also. We need to know the total compressive load on the cells in lbs. To get that from the psi number on an air cylinder we need to know the area of the piston in square inches to multiply by.