# is strapping REQUIRED for Li Cells?



## dtbaker (Jan 5, 2008)

It seems the Li cells come with different forms of strapping to hold them in blocks and compress the 'long' face of the cell... presumably so it doesn't bloat too much during charging, or fail from flexing during thousands for cycles.... but is this really required?

I am wondering if the cell 'block' strapping is required in addition to normal hold-down, or if the strapping is just an extra thing and the cells would be happy flexing a little during charge/discharge with a little gap between each cell which might even help with circulation. As long as the hold-downs kept the cells from shifting and bus-bars from flexing and straining terminals.... right?

Has anyone actually measured the case swelling between 'empty' and 'full'? does it get more with age?

On a related topic.... does anyone know what the cell casings are? is it polypropylene like FLA, or something different?


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## TX_Dj (Jul 25, 2008)

If you're talking about cells like the ThunderSky style cells, then yes. Strapping is required.

These cells expand when charged, and the casing is not strong enough by itself to keep them from swelling beyond tolerance.

Charging them without the cells being tightly bound with plates on the end and straps holding them tightly, they will swell to the point where the plates inside are too far apart to work properly. The end result is a bunch of expensive paper weights.


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## TX_Dj (Jul 25, 2008)

These cells were both not-strapped and overcharged.


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## dimitri (May 16, 2008)

TX_Dj said:


> These cells were both not-strapped and overcharged.


This topic is not so simple and somewhat controversial. These cells were damaged by overcharge, if they were strapped, they would still fail, by venting, smoking, melting, swelling on other sides, etc. My point is that damage caused by severe abuse will not be contained by strapping as much as you think. It might help somewhat, but its hard to quantify precisely. Its certainly not as simple as you stated.

I know of many conversions without strapping, and not having any issues. Some of my own cells aren't strapped as much as others and some not at all. As long as I keep them within operating range, they seem fine. I don't know if this is going to reduce their lifecycle, ask me again in 10 years  , but so far I see no evidence of any issues.

I would say, if you can design the battery box to contain the cells tight enough so they don't bounce around, you are probably OK, as long as you don't push their limits. If you plan to regularly run at max C rates, you should probably strap them tight.


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## dtbaker (Jan 5, 2008)

TX_Dj said:


> ...the casing is not strong enough by itself to keep them from swelling beyond tolerance.


this just seems odd.... what if you had just one cell? The user is expected to add a secondary casing?! 

I was sort of hoping that if the charge/discharge was held within parameters of voltage and <3C to control temp, that all would be well.... If the ends are constrained, it seems that pressure has to go SOMEWHERE, so I was thinking it might be BETTER for the cells to be held down, but even to have 1/8" spacers between cell faces on purpose. Then, they could swell a little without building internal pressure, but not too much?


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## dimitri (May 16, 2008)

dtbaker said:


> If the ends are constrained, it seems that pressure has to go SOMEWHERE, so I was thinking it might be BETTER for the cells to be held down, but even to have 1/8" spacers between cell faces on purpose. Then, they could swell a little without building internal pressure, but not too much?


You don't want them to swell at all. Any swelling is bad, since it means less contact area between sheets, hence increased IR and reduced capacity. However, under normal use there should be no swelling. Swelling is indication of abuse, in which case pressure helps avoid separation of sheets in the cell, but at the cost of venting gas and increased heat buildup. When energy has no place to go, something has to give somewhere. Since the cell is going to take a hit anyway, letting it swell is no better than letting it vent gas, but there is a chance of less damage if its under pressure.


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## dtbaker (Jan 5, 2008)

dimitri said:


> You don't want them to swell at all. Any swelling is bad, since it means less contact area between sheets,


hhhmmm, Explanations of the LiFePO4 chemistry I remember are that there is some 'swelling' as the paste picks up electrons under normal circumstances. If the case can't swell, then the pressure HAS to go up. right? I am just trying to establish whether or not the cell casing swell under normal charge/discharge, or if they are designed stiff enough to hold up UNLESS they are overcharged or discharged too fast generating too much heat and softening the cases. ?


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## dimitri (May 16, 2008)

Maybe on a micron scale there is some "swelling", but there should never be any visible swelling on the outside. Even minor visible swelling indicates some damage done. I have a few of such cells, they still work, but they are now weaker than they used to be.


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## dtbaker (Jan 5, 2008)

dimitri said:


> Maybe on a micron scale there is some "swelling", but there should never be any visible swelling on the outside. Even minor visible swelling indicates some damage done. I have a few of such cells, they still work, but they are now weaker than they used to be.


ok then, just to beat this into the ground.... were the cells that swelled 'mistreated', or did it happen under normal recommended charge/discharge use?


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## EVfun (Mar 14, 2010)

I have been told that if they swell the internal resistance increases. That will increase discharge heating and limit discharge currents. I've been told that swelling is most likely to happen if charging over about 3.8 volts or discharging fast enough to have noticeable heating.

Since I like regular discharge bursts to 5C (and planning more) I consider clamping tight to be mandatory. I don't plan to find out what happens without it.


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## dimitri (May 16, 2008)

dtbaker said:


> ok then, just to beat this into the ground.... were the cells that swelled 'mistreated', or did it happen under normal recommended charge/discharge use?


That depends on your definition of "normal" 

Some people claim that max C rate advertised on datasheets is "normal", even though the voltage drops to 2V and cell is cooking hot and hissing at the end. I personally can't call such usage "normal", that is why I always say that Chinese datasheets are made up baloney. In my personal opinion 2C-2.5C is max "normal" usage for TS and CALB cells, at least that is how I set my battery current limits in my EV. My swelled cells were a result of Soliton1 controller testing gone a little out of hand , where those cells were run at just a tad over 3C for 15 minutes.

So, I guess there is no clear cut answer, all depends on your "normal" usage, I know what mine is, so I don't strap my cells too tight, but I wouldn't run them loose either. Reasonable restrain should do them good in a long run.


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## dtbaker (Jan 5, 2008)

dimitri said:


> Some people claim that max C rate advertised on datasheets is "normal", ... My swelled cells were a result of Soliton1 controller testing gone a little out of hand , where those cells were run at just a tad over 3C for 15 minutes.


ahhhh, NOW we are getting somewhere. 

I am upping my nominal pack voltage from 96v to 120v in the move to Li... and with my 'suburban' driving I see 300amps only briefly during accelerations, mostly in the 200s with some rest time and coasting as I don't do much highway driving and don't have a lot of extended uphills. I expect that upping voltage to 120v and dropping 500# will result in significantly lower average amp draw than I have now....

so, and now feeling good about not stressing cases too much, and not worrying about binding cell blocks too much... I am thinking that good top-mount hold-down bars with cells packed 'hand tight' *should* be just dandy.


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## dexion (Aug 22, 2009)

If it helps, some cells on paper anyway are better than others. Mine arent supposed to need strapping in any usage (but do have a spec that says max cc is 3c and max pulse is 10c (10s) When I asked about not having to strap them the engineer snickered and said ours dont swell we make the cases well. They are 15% heavier and about 15% larger/thicker/more expensive than the same size cell from ts/calb. Strapped ts cells actually are smaller/lighter. So its hard to say which is a better method strap and lighter casing or heavier casing and no straps. I dont strap them but I dont really abuse them either acceleration is about 1.5c-2c(rarely) the car is very light about 2300lbs or so. At this point theres only about 100 cycles on them so its too early to tell if they will hold up. But i do look for any signs of swelling and I cant see anything or feel anything.


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## dtbaker (Jan 5, 2008)

dexion said:


> If it helps, some cells on paper anyway are better than others. Mine arent supposed to need strapping in any usage


...and yours are? SkyE/CALB ?


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## dexion (Aug 22, 2009)

They are blackhawk cells from a company called powerforce in california. They call them powerforce cells now and the specs may have changed in the last year.


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## Harold in CR (Sep 8, 2008)

Dimitri
How many of those swelled cells do you have, and, would you consider selling them ??

What capacity are they ?? I am looking for a 12V set or larger.


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## dimitri (May 16, 2008)

Harold in CR said:


> Dimitri
> How many of those swelled cells do you have, and, would you consider selling them ??
> 
> What capacity are they ?? I am looking for a 12V set or larger.


I am not selling them because they are still driving my EV just fine, after I paired them in parallel.

However, if you need 12V for stationary application, like solar bank or house bank, I have 4 new 200AH HiPower cells, which are a little too big for the car. Send me a PM if you are interested in those.


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## JRP3 (Mar 7, 2008)

In his latest video Jack discharged 100ah CALB cells at 400 amp pulses of 2.5ah each with 60 second rests and got 99ah out of them. They started venting near the end but I think were still functional. They were strapped of course.


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## dimitri (May 16, 2008)

JRP3 said:


> In his latest video Jack discharged 100ah CALB cells at 400 amp pulses of 2.5ah each with 60 second rests and got 99ah out of them. They started venting near the end but I think were still functional. They were strapped of course.


Those cells were cooked and voltage drop to 2.1V is far from "normal", so as usual Jack's conclusions are very strange to say the least  He also needs to learn Celcius to Farenheit conversion better.

Very convenient for CALB to state 4C discharge, but not provide the graph for more than 1C in their datasheet, so how are you supposed to say if 2.1V at 4C is "normal"? I don't think its normal at all, I think its beyond abuse.

TS sheet does have a graph for 3C, but their voltage drop on the graph is science fiction compared to real life drop at 3C, a fact that Jack and everyone else who claims "normal" 3C usage seems to ignore in their infinite wisdom 

How can you call it "normal" when you lose over 30% of energy from the cell to heat, cooking it in the process?


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## JRP3 (Mar 7, 2008)

I don't think Jack considered what he did to be normal at all. It was an abusive test that no one would see in the real world, just showing what the cells are capable of. Even after that abuse he was able to recharge the cells and I think all but one are still functioning. If he had stopped after about 95ah they would probably all be fine.


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## dimitri (May 16, 2008)

JRP3 said:


> Even after that abuse he was able to recharge the cells and I think all but one are still functioning. If he had stopped after about 95ah they would probably all be fine.


Just because they take charge doesn't mean they are fine. I think its irresponsible to say that just because we don't know how much this abuse reduces their lifespan "they are probably all be fine". Maybe Jack can afford to replace his pack sooner than planned, but most other people can't.

I know from my own "testing" that deep voltage sag under heavy load accompanied by heat buildup DOES damage the cells, even though they still take charge and still work, they don't work as well as they should and they significantly reduce entire pack's useful capacity.

My "testing" already cost me a $1000 in replacing cells and I hate to see people with little experience follow this so called "expert" and drive their cells at "normal" rates listed in "datasheets"


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## dtbaker (Jan 5, 2008)

ok, back on topic.... if we stick to qualifying 'conservative use' as not charging above 3.65v, not using to below 3.0v, and only occasional 30 second bursts of 3C use... then individual battery casings *probably* will not be expanding much, and strapping is a mostly precautionary measure....
agreed?


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## dimitri (May 16, 2008)

dtbaker said:


> ok, back on topic.... if we stick to qualifying 'conservative use' as not charging above 3.65v, not using to below 3.0v, and only occasional 30 second bursts of 3C use... then individual battery casings *probably* will not be expanding much, and strapping is a mostly precautionary measure....
> agreed?


I'm not sure how to quantify this. This implies that someone should run their pack with no strapping for a while and see what happens, since we have no concrete data on the subject. Strapping definitely does not hurt, but I wouldn't go crazy clamping cells in a vise, and I wouldn't run them loose either. Some reasonable restrain is good, but I don't know how to quantify it.


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## dtbaker (Jan 5, 2008)

dimitri said:


> I'm not sure how to quantify this. This implies that someone should run their pack with no strapping for a while and see what happens, since we have no concrete data on the subject. Strapping definitely does not hurt, but I wouldn't go crazy clamping cells in a vise, and I wouldn't run them loose either. Some reasonable restrain is good, but I don't know how to quantify it.


yeah... it is hard to come to conclusions without tests. 

What I am trying to get at is whether packing the cells in 'hand tight' and then top-clamping them in place with normal restraint system is enough as all except the end cells would just expand against each other, or if I should take the extra time to strap or clamp the cell block with 'endcaps' in addition to top-clamps.

An interesting data point I have no way of measuring since I don't HAVE cell in hand would be to discharge a cell to 3.0v, measure the thickness of the middle of the casing face as accurately as possible, charge to 3.65v and re-measure... just to see if a single unconstrained cell flexes much under 'normal'  charging cycle.


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## JRP3 (Mar 7, 2008)

A cell isn't going to suddenly have a problem just because it's discharged to 3.0 volts. I've pulled mine to 1.77 under load, probably around 2.5 resting. I wouldn't do it all the time but an occasional excursion in that range probably isn't going to do much. I think a "snug" fit in a box would probably be fine, I know that's how some people are running them.


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## MN Driver (Sep 29, 2009)

One of the guys on the UK BVS site ran his HiPower cells without strapping one got high internal resistance to the point where it was useless and was swollen, when put in a vice and compressed, it was slightly better, at least enough to be measurable but still useless for the application. Of course it is hard to tell if strapping would have prevented this but I'm pretty sure it would as the layers wouldn't separate. The cell was replaced under warranty and the new one and all of the rest of the 50 cells seem fine but they aren't being discharged any quicker than 40 minutes to where he cuts off the discharge.


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## dtbaker (Jan 5, 2008)

MN Driver said:


> One of the guys on the UK BVS site ran his HiPower cells without strapping one got high internal resistance to the point where it was useless and was swollen,


I am sortof remembering some tests showing that the HiPower cells had about 3x the internal resistance of TS or CALB cells.... I wonder if this was a victim of overheat from over-C use?


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## newbcake (Aug 5, 2010)

dtbaker said:


> ok, back on topic.... if we stick to qualifying 'conservative use' as not charging above 3.65v, not using to below 3.0v, and only occasional 30 second bursts of 3C use... then individual battery casings *probably* will not be expanding much, and strapping is a mostly precautionary measure....
> agreed?


Yup, that's true. However, I find the actual "swelling" to be quite minimal. What's causing the battery to expand is the gases emitted by the electrochemical reaction if you do several cycles without rest. I know TS has a bad rep for this especially.


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