Poey50
Well-known member
I hadn't heard that, Vas. Do you have a link?
Lithium 304Ah Build
- Thread starter gregcope
- Start date
I hadn't heard that, Vas. Do you have a link?
vas
Well-known member
link to this guy J5GURU video taking apart a DOA prismatic cell
there's a part 2 where he unfolds the lot
to me (at least) it looks like nothing's going to break if you pull or push the terminals for 2mm or 3...
there's a part 2 where he unfolds the lot
to me (at least) it looks like nothing's going to break if you pull or push the terminals for 2mm or 3...
Poey50
Well-known member
Ah ... I have seen those videos before. Quite reassuring that these are not cells that will fall apart in your hands! He didn't spend much time on the terminals and I didn't get the impression there was the level of flexibility of movement that you mention. However they don't look fragile in any way.
vas
Well-known member
It's my assumption based on the fact I've seen in the video. Clearly having an origami of some sort with soldered (or somehow connected) terminals on a case that deforms up to 2-3mm to my eyes it's clear that it's not a rigid thing that terminals will break loose or short or whatever.
Looks rather forgiving as a construction which is a good thing!
now see when they become even cheaper ?
V.
Looks rather forgiving as a construction which is a good thing!
now see when they become even cheaper ?
V.
Poey50
Well-known member
I see you are basing that on the idea of the case deforming. That could work.
vas
Well-known member
thin ali sheet cannot do anything but deform surely!
Nothing rigid within this casing...
Nothing rigid within this casing...
Poey50
Well-known member
But I doubt it will deform back so it should prevent a one-off catastrophe but not to be relied on for any regular movement.
vas
Well-known member
not a structures engineer, don't know about you so the answer is probably, and definitely wouldn't rely on it flexing back and forth all the time!
it's unclear to me if our cells bulge when charged and shrink when discharged. If so then yes go flexbar if you cannot secure them v.tight.
but *I think* that they bulge in use and sort of stay like that if not restrained. Maybe I got that wrong...
it's unclear to me if our cells bulge when charged and shrink when discharged. If so then yes go flexbar if you cannot secure them v.tight.
but *I think* that they bulge in use and sort of stay like that if not restrained. Maybe I got that wrong...
Poey50
Well-known member
The cells expand and contract a few mm under normal charge and discharge cycles. When overcharged they bulge and never recover. There is some evidence that constraining cells during normal expanding and contracting increases longevity (number of cycles). I'm not sure if it prevents permanent damage from overcharging but I doubt it. The way I approached this was to take all the cells down to 3.2 volts (12.8 volts for the pack) and then tighten the nuts on the threaded rods that held the end-pieces until they were just 'nipped up'. I made sure the length between the end-pieces was the same so the cells are compressed evenly. I don't know how much this corresponds to the recommended torque setting but it seemed about right to me and has been fine for 18 months since I made the pack.
vas
Well-known member
oops, that's very different to what I did, cells were as received (more than half empty I guess) and I tightened them hard as in HARD HARD, cannot turn the 13mm nut any more with a spanner. but I have 2mm rubber between all of them and thought that I need to take some of that "slack". Oh well, we shall see.
Poey50
Well-known member
Cripes!
Kelpie
Well-known member
I just used M5 threaded rod and did it up tight enough that the end plates held the pack together firmly enough that I could move it around without any fear of the cells moving out slipping out the bottom.
Not very scientific but made it a bit easier to get the assembled pack on to the boat!
Not very scientific but made it a bit easier to get the assembled pack on to the boat!
gregcope
Well-known member
interesting, btw Greg Amy's supplied busbars are 2.2mm thick (just measured them)
I agree that they are v.useful when you fit your cells in a box like you did and the only means of packing them is straps.
However, having checked how mine work with 20mm endplates clamped with 4 M8 bolts, I doubt they are really necessary on really secure setups.
Slightly worried on their temp under stress, saw on some video (don't think it was the one you mention above) some flex ones that were heating up, didn't like that. Have you checked them tempwise?
Further if you see how the battery is constructed (there are some videos of guys taking prismatic cells apart) it's obvious that there is a fair amount of give in both posts. I can see why there's a trend not to bother and not to clamp them.
What is indeed necessary is insulating the bus bars! got Φ14/7 thermoshrink tube to get them in.
Then also need to find rubber 10mm nut cups (for the M6 studs on the posts). I'll have a perspex cover on the whole thing, but would really like to make sure nothing is going to make a firework out of this battery bank!
Ah. Will need to remeasure mine 10% out is poor accuracy on my part. Apologies!
I have not checked under stress as I do not have a high 12V load at home. I must resist the Victron 3000kva inverter purchase... Must resist more blue victron stuff.....
I tried thread caps and gave up as there was too little thread and they fell off. Nut caps may work well. I agree with the concern on a short. Yours has 8kwh mine 4kwh which is allot to short circuit! I want to learn welding but not now.
gregcope
Well-known member
Not sure this helps much. But this is what I have from the latest datasheet...
Poey50
Well-known member
OK there is quite a lot of confusion about what kgf means. This is what Ghostwriter (from DIY Solar) says. She is responsible for huge orders of these cells and is very knowledgeable.
"OK ... talked to the senior design architect for EVE. He was very specific and short response. So these are NOT all his answers since I don't want to ramble BUT these are the ones that I think are important to us. Let me preface that he started out that although there are straightforward chemistry (liFePO4) involved -- there is still allot of mystery when it comes to LFP and why it does certain things a certain way ...
SOOOO - he said - if he was a DIY'r and doing this for an off-grid cabin or RV he would do the battery virtually the same way that the junior engineers said last night. (see my previous posts earlier) (post #215)
Any compression is better than no compression. BUT NO compression is better than compressing it above 18 PSI.
Anywhere between 6-17 are good numbers - but those numbers will change on SOC and temp.
SOOO In a perfect world - when your SOC is at 50% - your PSI would be 12 and your temp is 72. He would do the adjustments at 50% SOC and then assume that 100% SOC will be a few PSI higher and 0% a few pounds lower. As long as your high and low is between 17-6 you are fine but a constant 12 is what you are looking for.
For temp - and I know this was not one of the questions -- BUT 72 is perfect -- and actually its better to be at 52 than 82 .. although 72 is perfect - when temp starts going above 72, the UNHAPPY FACE scale starts going up exponentially as the temp climbs and when the temp lowers its more linear ... (if that makes sense). That also will affect the PSI.
In fact nearly everything affects the PSI -- and over the life of a LFP you will get a couple more years from a battery at 12PSI CONSTANTLY then one going between 6-17 -- BUT that's NOT the real world unless you have a spring or mechanical controlled clamping system.
BUT technically 12 is the magic number and if you CAN KEEP it at 12 then you will probably get ALMOST 1.6X the cycles out of these batteries as you would if you do nothing."
12 psi is the kind of pressure that you would feel raising a 5 litre can of fuel off the ground with your thumb. So I think we are talking about constraint rather than compression.
"OK ... talked to the senior design architect for EVE. He was very specific and short response. So these are NOT all his answers since I don't want to ramble BUT these are the ones that I think are important to us. Let me preface that he started out that although there are straightforward chemistry (liFePO4) involved -- there is still allot of mystery when it comes to LFP and why it does certain things a certain way ...
SOOOO - he said - if he was a DIY'r and doing this for an off-grid cabin or RV he would do the battery virtually the same way that the junior engineers said last night. (see my previous posts earlier) (post #215)
Any compression is better than no compression. BUT NO compression is better than compressing it above 18 PSI.
Anywhere between 6-17 are good numbers - but those numbers will change on SOC and temp.
SOOO In a perfect world - when your SOC is at 50% - your PSI would be 12 and your temp is 72. He would do the adjustments at 50% SOC and then assume that 100% SOC will be a few PSI higher and 0% a few pounds lower. As long as your high and low is between 17-6 you are fine but a constant 12 is what you are looking for.
For temp - and I know this was not one of the questions -- BUT 72 is perfect -- and actually its better to be at 52 than 82 .. although 72 is perfect - when temp starts going above 72, the UNHAPPY FACE scale starts going up exponentially as the temp climbs and when the temp lowers its more linear ... (if that makes sense). That also will affect the PSI.
In fact nearly everything affects the PSI -- and over the life of a LFP you will get a couple more years from a battery at 12PSI CONSTANTLY then one going between 6-17 -- BUT that's NOT the real world unless you have a spring or mechanical controlled clamping system.
BUT technically 12 is the magic number and if you CAN KEEP it at 12 then you will probably get ALMOST 1.6X the cycles out of these batteries as you would if you do nothing."
12 psi is the kind of pressure that you would feel raising a 5 litre can of fuel off the ground with your thumb. So I think we are talking about constraint rather than compression.
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Poey50
Well-known member
And this .... "300KgF corresponds to 660 pounds of force.
The Eve 280Ah cells have a width of 173.6mm and a height of 200.0mm for an area of 34,720mm^2 or 53.82”^2.
Applying 660 pounds of force over an area of 53.82”^2 translates to 12.264psi (pounds per square inch).
Eve has recommended to stay within 50% of this pressure at both extremes, meaning that any clamping fixture should be designed to stay under 18.4psi at 100% SOC (and above 6.2psi at full discharge, though since the only impact of under-clamping is reduced cycle life versus the possibility of damaging the cells from too much applied pressure, this is much less critical).'
Vas take note!
The Eve 280Ah cells have a width of 173.6mm and a height of 200.0mm for an area of 34,720mm^2 or 53.82”^2.
Applying 660 pounds of force over an area of 53.82”^2 translates to 12.264psi (pounds per square inch).
Eve has recommended to stay within 50% of this pressure at both extremes, meaning that any clamping fixture should be designed to stay under 18.4psi at 100% SOC (and above 6.2psi at full discharge, though since the only impact of under-clamping is reduced cycle life versus the possibility of damaging the cells from too much applied pressure, this is much less critical).'
Vas take note!
vas
Well-known member
thanks Poey, just had two beers, read it (sort of carefully) didn't like it. Will sleep on it and think about it tomorrow.
What makes my situation more complicated is that the 4 65cm long M8 rods are there to keep the whole thing together, OK, but also to make sure that they all stay in one line. If I have the bolts any looser, the thing will flex/bend and the middle couple of cells will reach the floor and you definitely don't want that. So if I'm to loosen the grip on the endplates, I'll have to devise a way to support all the cells underneath so they stay in line and don't sort of bow in the middle.
Had another full discharge test and now started charging them. I'll only charge to circa 20%, let them rest overnight and tomorrow have a look at the tension.
Problem is all these 12psi or 300kgf or whatever are impossible to actually measure! If someone would come up with a formula fe saying "well lubricated M6nut to 20Nm, or M8 at 14Nm" it would be something feasible. 12psi, yeah right, come and measure it ?
tbh, I'll probably end up undoing the nuts to the point just before the whole contraption starts flexing when you try to lift it off the floor...
cheers
V.
What makes my situation more complicated is that the 4 65cm long M8 rods are there to keep the whole thing together, OK, but also to make sure that they all stay in one line. If I have the bolts any looser, the thing will flex/bend and the middle couple of cells will reach the floor and you definitely don't want that. So if I'm to loosen the grip on the endplates, I'll have to devise a way to support all the cells underneath so they stay in line and don't sort of bow in the middle.
Had another full discharge test and now started charging them. I'll only charge to circa 20%, let them rest overnight and tomorrow have a look at the tension.
Problem is all these 12psi or 300kgf or whatever are impossible to actually measure! If someone would come up with a formula fe saying "well lubricated M6nut to 20Nm, or M8 at 14Nm" it would be something feasible. 12psi, yeah right, come and measure it ?
tbh, I'll probably end up undoing the nuts to the point just before the whole contraption starts flexing when you try to lift it off the floor...
cheers
V.
Poey50
Well-known member
Yes, you definitely need a redesign! I think the end pressure I apply wouldn't reliably support my 4 cells let alone your big collection. Mine rest on a stiff perspex base and are lifted with nylon straps under the base so no demands are made on the end plates to keep the cells sandwiched.
geem
Well-known member
A constant 72F (22degC) seems to be optimum. Batteries will change temperature with ambient conditions and charge/discharge. I suspect the implications of high ambient temperature are far less onerous in the UK than the Med or Caribbean. At the moment daytime temperature here is 28 dropping to 21 over night. Add in charge and discharge and my lead batteries are hitting 30degC some days. From what I have learnt recently, lifePO4 is less tolerant of high temperature than lead. How do you keep the batteries cool? You could charge and discharge them less but that isn't really the idea.
Poey50
Well-known member
At least 30 degC is not 40deg C ... and so on. I think people generally take the hit on longevity caused by higher ambient temperatures. As you say, it is the combination of high ambient temperature, high rate of charge and discharge and the percentage of time at high SOC which cause degradation. I have a fan that is thermostatically controlled but that is only of value with high C charging or discharging when the pack exceeds the ambient temperature but the most I charge at is 0.3C which raises the temperature fairly minimally so I've never used it being UK based. A fairly common solution is to have the pack adjacent to the main cabin so it shares the temperature that is tolerable to the human occupants who will do their best to keep the air cool by whatever means.
Will Prowse has a good summary of the issues in this video ...