will LI cells swell under non-abuse conditions?

2719 Views 13 Replies 5 Participants Last post by ElectriCar, Dec 26, 2010

dtbaker

Discussion Starter · Dec 26, 2010

I am wondering if the need to constrain blocks of cells and add plates at the ends is REQUIRED under 'normal' use, or only a safeguard against case deformation under extreme conditions involving overcharge and/or over-heat from extended high-C discharge?

any actual experience from people with 'lightly bound' cell blocks under normal use having problems with cell expansion?

I am have some space constraints in designing racking for replacing my lead w/ Li and need to make a decision in one area how much to worry about adding stiff end plates, or just thinner and less supportive strapping.

D

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Duncan

· #2 · Dec 26, 2010

Hi

I am have some space constraints in designing racking for replacing my lead w/ Li and need to make a decision in one area how much to worry about adding stiff end plates, or just thinner and less supportive strapping.

I was thinking about this, It seems to me that I need enough support structure to retain my batteries against 10G in all directions - this support structure should be able to squeeze and constrain my cells from swelling

If my battery support structure can't resist the swelling then it is too weak for road use

If it is the pre-compression that is the problem I am sure something can be done with low angle wedges tapped into place and then retained

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EVfun

· #3 · Dec 26, 2010 (Edited)

That is a difference between most of the USA and much of the rest of the world. I'm not sure about the rules in the state of New Mexico, but it is likely similar to Washington state. That is, no inspection will be required for modifications Dan is planning. There is nothing on the books that requires 10G protection anyway. We tend to have much more lax standards, especially in the west and south of the country.

I've been driving my buggy around for 10 years and some of the batteries are held down by nothing more than sitting in a 1 inch angle iron frame and a single piece of 1/2 inch by 1/16 inch wall aluminum tubing and 2, 1/8th inch J bolts. I've haven't been killed yet, not even once.

As to the question of, will the batteries swell," I cannot really answer that because mine are tightly restrained. Plus, my use might be considered abuse by some though the batteries seem to be fine so far. I'll really find out next year as I finish things up and start getting some real miles.

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· #4 · Dec 26, 2010

Properly charged batteries will not swell. Swelling is a result of over charging. Even if your cells are banded properly they will blow the band if they swell too much. They can bust the banding you have on the pack if they overcharge. Have a look. These were banded super tight and overcharged and blew. Not only did these blow but they blew the banding device apart into pieces. It was extremely tight and secure. It blew to bits. Restraints do not keep the batteries from swelling. They swell but you may not see it until you un-band them or they explode apart for super over charging.

http://greenev.zapto.org/prius/Prius/Prius_Pack.html

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Duncan

· #5 · Dec 26, 2010

Hi EvFun,

We don't NEED to have 10G restraint - unless the batteries are in the cab when we need to have 20G!

But I though it was a sensible ask to prevent a mild thump from turning into something worse! - it doesn't take much
100Kg of batteries at 10G = 1 tonne, mild steel is about 20 tonnes/ square inch
so 1/20 square inch = 1 off 1/4 inch bolt

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dtbaker

Discussion Starter · #6 · Dec 26, 2010

the area in question is enclosed in a polyp box with steel L edges, so unlikely to rattle around even in a rolling accident, which is also unlikely at the my normal 35-ish mph average in-town speed.

however, the question arises because in my particular case in the rear rack area, I am going to come VERY close on the flat ends of some of the cell 'blocks'. so close that I really don't have room for much stout end plates as the normal banded support plates and leaving any room for insulation and air gap.

hence the question.... whether the cell 'blocks' are or aren't likely to swell under normal well controlled charge/discharge.

heck, in NM, all I got for inspection was an 'eeeee, we don't inspect nutin honey'.

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Duncan

· #7 · Dec 26, 2010

Hi
leaving any room for insulation and air gap

I can understand needing insulation - but air gap?
I was intending to squash the cells using the battery box as a containment - I was thinking of using some thin dense foam and hammering in a pair of wedges to squish it all together

With a distributed clamping load from the foam I don't think rigid end plates are needed

We have a strict inspection process here - its run by the New Zealand Hot Rod Association!

Talk about poachers turned gamekeepers!

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EVfun

· #8 · Dec 26, 2010

Hey Pete, I think it was a pack of Nimh batteries that blew in those photos. They talk of Prius batteries and show lead acid batteries so I'm guessing that was the 2 incompatible battery types being combined (very bad combo too.) I also wouldn't call what they used as restraint.

I'm using 8 bands of 0.025 by 0.5 inch stainless steel banding and substantial aluminum end plates. I'm planning to blow the tops off first.

From what I have read, swelling is mostly caused by overcharging. It is accompanied with an increase in internal resistance. What has not been answered to my satisfaction is the cause-effect relationship. Does the swelling cause the resistance increase or does the overcharging cause the resistance increase? It is quite possible I was on an overkill mission with the banding.

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dtbaker

Discussion Starter · #9 · Dec 26, 2010

Duncan said:
Hi
leaving any room for insulation and air gap

I can understand needing insulation - but air gap?

more dead air space= better insulation. I was considering using the box walls as endplates too, but solid materials will almost always have high conductive loss than an air gap. So I would LIKE to have an air gap between cells and outer box if possible for the cold, and then it would help circulate air around to cool in the summer if I open some vents into the box.

I still might end up going with the squeeze-fit, I just wanted to know if anyone had actually noticed any bulging under 'normal' use that 'needs' compressive banding.

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ElectriCar

· #10 · Dec 26, 2010

From what I've seen, overcharging causes the electrolyte to gas, like water turning to steam when the temp gets high enough. Eventually the pressure will build to a point the vent opens and lets the gas out. I wouldn't want that to happen, but I wouldn't want swelling either.

Rickards testing shows for example in the Calb batteries that I'm buying that charging to the recommended voltage level only nets you a small amount of ah. Calb batteries "charge to" voltage is 3.6. His testing revealed that upon reaching 3.56V the voltage began climbing rapidly as the cell was full. Because of that he charges to just below the recommended voltage to 3.5V in the CC mode then switches to CV mode, maintaining 3.5V until the current comes to a trickle of 1amp or so. This is what I plan to do.

He has a pack he's driven quite a bit with no BMS and hasn't lost a cell. He bottom balances and keeps his charging just below recommended value and discharges to about 3V. Below that he said there was only a few ah left in the battery so why risk losing a cell? It seems the cell killing issues come from over discharging, frequently aided by top balancing, causing one cell to bottom first.

The solution to me seems to be three fold. First, bottom balancing so they all are operating at the same voltage near the bottom. Second, getting an oversize pack for the job so you're not pushing it to get you home. Third, getting an AH counter so you KNOW how much is left in the tank. Apparently you can't count on monitored/measured voltage to determine your SOC because SOC is not the same as terminal voltage and depending on the C value you're discharging at will change the terminal voltage. Depending on a measured voltage could likely cause you to toast a cell or pack if you're pushing the range limit of your pack.

Calb recommends discharge at 0.3C, I assume for long life though the cells will do much more than that. I'm sizing my pack to operate at 0.3C or so on flat ground. Hill climbing and acceleration will take it just over 1C but that's only for a few seconds.

With the cautious means of operations above I hope to maximize my battery life. I'm just getting started in the AH counter search. All suggestions appreciated!

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dtbaker

Discussion Starter · #11 · Dec 26, 2010

ElectriCar said:
I'm just getting started in the AH counter search. All suggestions appreciated!

lets start a new thread on this topic... I'd like to see some alternatives here too. I'd like to learn more about the pros/cons of shunt-based measure versus Hall effect, and then the best options available.

I have been using a 'cycle analyst', which is a shunt-based unit, and very inexpensive compared to others I see on the market capable of counting ah in/out, voltages, and an external rev counter for super accurate distance odometer calculating internal Whr/distance. BUT the display is digital numbers, which are a little hard to glance at because of size.....

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ElectriCar

· #12 · Dec 26, 2010

I think the new thread is a good idea. I'll research it some more but I'm planning on installing some radiant barrier today under my house. This stuff really does a good job of keeping heat in or out of your house, wherever it is. I did it in another house and afterward the crawlspace was VERY COLD if it was cold outside, unlike before when it would be much warmer than outside temp. And the stuff isn't expensive either.

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peggus

· #13 · Dec 26, 2010

In my experience Li cells swell in normal cycling. Of course, much more so when abused.

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ElectriCar

· #14 · Dec 26, 2010

peggus said:
In my experience Li cells swell in normal cycling. Of course, much more so when abused.

Well please share your experience with us. I started a thread in an effort to get people's experience here.

What brand are your cells?
What AH are they?
Do you charge them to full recommended voltage or stop a little short?
Use a BMS for balancing?
What is your normal discharge rate on flat ground?

I think in order to gather useful info those things need answering and probably more I didn't mention.

I certainly would appreciate your time answering the questions.

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