battery box questions

[nechaus]

I was searching but could not find a good solution.

How much pressure is optimal to have on regular HobbyKing lipo ?
I am about to rebuild the battery area on my ebike

How much pressure will a cell put out when it discharges?
Does it need to expand?
Should the battery box compress the cells?
Will the cell explode if you over discharge it quickly? ( I have over discharged packs in the past and they got fat, but once charged they settled down a bit.)

On my thundersky batteries they came with nice metal straps, God I love the robustness of them



I have always used my lipos freely in bags with tape and heat shrink, The most sophisticated bag I have used it falcon ev bags or backpack.
I wanted to make a tight fitting wood container.... I was thinking I could easily compress the cells down with clamps when making the box..

With the way I use them, sometimes they get discharged quick and other times not so quick... There has been rare moments in the past where I have taken cells down to 3v or less and they expand heaps, Im wondering if I had them compressed if it would be good or bad on those rare occasions

 

[docnjoj]

Check dnmun's later posts. He has a neat way of using Masonite on the ends of his packs after he removes the Shrinkwrap. It seems to work and I use a similar technique on my Amp20 A123's.
otherDoc

 

[friendly1uk]

Hobby king don't sell batteries expecting you to compress them. They are ready to use. If you want to do something else with them, they won't perform to specification.

I'm not saying they will under perform. You could make them better.


I'm not interested enough to do it, but you could work out the pressure applied by a fixed container as the packs try to swell and are not allowed. Or would you clamp them in a fully swollen state, allowing them to shrink? How much will they expand today, and will it be different next year. It seems a lot of variables to me. People talk of 2-3psi but without a bladder I feel I couldn't do that consistently. It seems like an unnecessarily complicated project to me. One I have not seen tackled in a manner I would follow.

HK LiPo is cheap. It's not made in clean factories. Debris is a known cause of failure for both new packs and old. It can light up a pack at any time, it is why lipo is dangerous even with all external protection methods in place. Do you really want to squash one then expose it to vibration?


Gaffer is no good, it's not elastic. It will give a bit. A gaffered pack won't be under pressure. It might be when applied, but the strands soon sink in to the pvc and glue. Glue which shrinks. Barely any movement is required to loose all compression.


I really wouldn't bother unless your going to make it work. So far, I have not seen anybody do so. Lots of talk by people that understand, but no action.

 

[nechaus]

Hobby king don't sell batteries expecting you to compress them. They are ready to use. If you want to do something else with them, they won't perform to specification.

I'm not saying they will under perform. You could make them better.


I'm not interested enough to do it, but you could work out the pressure applied by a fixed container as the packs try to swell and are not allowed. Or would you clamp them in a fully swollen state, allowing them to shrink? How much will they expand today, and will it be different next year. It seems a lot of variables to me. People talk of 2-3psi but without a bladder I feel I couldn't do that consistently. It seems like an unnecessarily complicated project to me. One I have not seen tackled in a manner I would follow.

HK LiPo is cheap. It's not made in clean factories. Debris is a known cause of failure for both new packs and old. It can light up a pack at any time, it is why lipo is dangerous even with all external protection methods in place. Do you really want to squash one then expose it to vibration?


Gaffer is no good, it's not elastic. It will give a bit. A gaffered pack won't be under pressure. It might be when applied, but the strands soon sink in to the pvc and glue. Glue which shrinks. Barely any movement is required to loose all compression.


I really wouldn't bother unless your going to make it work. So far, I have not seen anybody do so. Lots of talk by people that understand, but no action.

thanks man that is helpful.
Well I wont even bother doing it.
I guess I will use like a rubbery foam material to keep them from moving loosely then.
Yeah I have noticed people talking about it, was not sure if its the new thing to do lol

 

[parabellum]

Actually all my un/wrongly compressed Lipos swelled and were severely under performing compared to compressed ones in same pack. Most damage goes to firs and last cell in every pack, because they puff more (no support from 1 side) and no good conduction is present between layers, resistance is higher, they heat, sag a lot, degrade faster, it is like avalanche. I disassembled few packs, week ago, because of this issue and recombined them to get 3/4th of the pack size. Now, after compressing them and adding few, they almost behave like other parallel side of the pack that was compressed from start on.

P.S. Pouch can tear apart if puffed to much. I found one with aluminium foil broken under the polymer layer.

 

[dogman dan]

Definitely compress them as much as can be conveniently done. My favorite way is to make coroplast boxes for no more than 500w per box. But you could also do the Masonite trick or other methods.

The shrink does not compress them enough, and the outer edge cells will puff most first, as they age. Some of my older packs, before I made the boxes, would stick in the battery box I used then if I discharged them too deep. Charging some, they'd relax and then I could get them out again.

Same sort of thing happens with my coroplast boxes. I make them very tight when the pack is brand new. Then six months later, if I want the pack out of the box, I have to cut the tape to get it back out. It's not compressing them a lot, but it sure has kept my current set of cells from swelling as much as the fist batch did. Back then I had compression in use, but not in storage. They'd puff up just a bit when fully charged.

I have wondered about going to something harder, perhaps outside the coroplast, then cranking down some hose clamps. But I like what I have now, and will see how just the coroplast does for this years cells.

To clarify, what I am advocating is just keeping the cells the size they are when new. Not trying to press them down even more. So a very rigid box that fits tight when the cells are new should do it. That will keep the outer cells from ballooning.

 

[friendly1uk]

Unhappy with my last post, I have given this deep consideration and decided I was tied up with the idea of uniform pressure increasing performance. I don't think that is happening. The subject here is more like preserving what you have. Maybe I'm just a bit late to realise it. Stress relieving the corners of soft packs and offering some support can only be a good thing. Some stout foam memory foam should offer continuous support while allowing for some expansion and contraction. Which might still be over thinking it if peeps are ok just clamping them in a fixed space.

Foam introduces thermal problems unfortunately. You wouldn't want it between each cell. It would just be used as a spring. It's thermal insulation abilities may explain why thin strips of it were used in earlier hardcase packs. Rather than covering the entire wall.

I do like the blissful ignorance that comes with hardcase ownership. It's great to just use them. The soft packs need an enclosure though, so you should relieve any bad corners and you should consider orientation as a minimum, and some support would be ideal.

I have unsupported cells stood on their edge. I have seen them stood on their end with little lateral support. Some bags offer velcro straps to band round them. They do work as supplied but we should also try to do better.

 

[Punx0r]

I think some people are hung up on the idea that the pack will swell whatever you do to it, and compressing it too much will cause huge, damaging internal pressure.

My suspicion is that compression prevents, not contains, swelling.

It makes sense if it avoids the cirle of swelling = increased IR = more heat = more swelling etc

Am I right in thinking that the swelling is due to decomposition of the electrolyte to gas? If so, static pressure will definitely help prevent that.

 

[dogman dan]

I can only relate what I have seen with my own stuff. So my opinion is not science, or complete.

I have seen both brands swell slightly over time if you just let them have the shrink. This was compressed on the run but left naked while charging and stored. They never puffed like hell, but now over three years old, they definitely have obvious deformation at the corners of the outer cells. Worse, you can see chafing happens worst at the corner. They were lightly puffed by the end of the first year, but never puffed noticeably more.

My current one year old pack has not puffed noticeably in a years use. Similar cycles about 50-60. Similar c rate, spikes to 60 amps on 10 ah, but mostly 20 amps running. I don't think normal use requires you to strip them of the shrink, or crank them down under a vise. but I will always advocate boxing up naked pouch cells to prevent dings at the corners or other damage.

I broke that rule "just this one weekend" last summer, and dinged hell out of a brand new 6s pack. It had to get taken apart, and new cells added.

I would guess that if you pack in the cells, then if you overdischarge it will puff enough to pop the foil, or to severely distort the mild compression I have put them in. When I was running the other box, the compression never popped a really puffing pack. But I did have a case where I had to quickly disassemble the box to get the pack back out. No way was that overdischarged pack coming out of the box. Later on when other packs had swollen some, I would have a tough time getting them to slide back out of the box if I discharged deep. But letting the cells cool, they would usually be able to be removed.

The least puffed, was when the packs were at 50% dod.

 

[nechaus]

interesting interesting..

I am thinking perhaps if you use the cells like a nanny, you would probably benefit from them being compressed?
However, I am thinking perhaps having them loose might be a safety buffer if they do need to blow up like a German sausage ?

 

[dogman dan]

That is worth considering. The time I really puffed a pack with a mistaken discharge to 0v, it was loose in a bag, no box. It never caught fire, but turned into something resembling a blown up rubber glove. It was HOT, the shrink dripping, and the cells could expand all they wanted.

Another time the same thing happened, I caught it sooner at about 1.5v. That time I just had trouble getting the pack out of the tight box, but it also did not pop cells. The way this happened twice was parallel connectors unplugging. So I had a section of the pack with 5 ah, and the rest 10 ah.

I don't approve of battery boxes that make it near impossible to remove a hot pack from the bike. Both times I was not on fire, but it could have been.

 

[friendly1uk]

As we are talking hk lipo, We have to take on board the advice given by them that their cells will expand and contract during use. Not just as they age, but each cycle. We see that when they put them in solid boxes, they leave room for this to happen.

I would have to do some serious research to know more about them than the manufacturer.



I guess a bathroom scale with a pack G-clamped to it could measure 3psi of compression. Plus hold it's shape while you cycled the battery watching the scale deflect. I still wouldn't feel comfortable ignoring the manufacturer, but the results might interest someone.

 

[dogman dan]

There is a change in size every cycle.

But the trouble seems to be, the outer pack swells the most, then the corner gets crushed by the shrink.

So it has seemed to make sense to me, to confine the outer cell on the flat side some, by using a mild compression.

My one year old pack, 14s 10 ah has definitely swollen now. I cannot get the packs out of that coroplast box without cutting the tape. Perhaps it's time to cut it open, and see if the corners look better or worse at one years use. It has not puffed enough to round the sides of the box. Still very much flat and square. I hope that this is helping to avoid the rounding of the corners on the outermost cells.

If I cut it, I will not get it packed as tight again. So I think I will leave it be as long as all the cells behave, and do not stink.

 

[parabellum]

Little data about compressed vs uncompressed.
http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=58210&start=50#p875103