Swelling

[Vab]

Hi Everyone,

Shortly: I am in the conceptual design stage of my electric scooter project. The most issues to date emerged mainly around the battery design. I already outlined a hub motor with around 5kW cont. power for reasonable hill climbing capability, and around 4kWh battery capacity.
In the end, I would like to have a cube shaped Al box (~5-8 mm walls), with around 300-350 mm edge length, stuffed with the controller, charger, bms, water pump ... and the highest amount of Wh-s possible (~4). I decided to build the battery myself, since this seems to be the most effective both financially and space wise.
In the current stage of my market research, I found pouch cells the best suited for this goal (18650s would be a good purchase too, but I don't want to deal with 500 cells, really). However, there is still one open question and possible show stopper I couldn't research deeply enough which is concerning the expected swelling of the cells.

My top candidate for battery chemistry is LiFePO4, but I wouldn't sort out Li-ion neither. The only paper on this subject I found were claiming that Li-ion pouches are likely to expand by around 10% of their original size through their lifespan in any case, which is normal. I was wondering whether anyone could back this statement with real life experiences? Also, I would really appreciate if someone could tell me about the swelling behaviour of LiFePO4 pouch batteries. I am aware that this may strongly depend on how well the batter is actually treated... So that I am hoping for a """normal""" users experiences, whose batteries are treated well in general, but sometimes suffer some suboptimal dis/charging parameters when unavoidable...

(It is my understanding, that cylindrical cells cannot swell because they are vented in most of the cases?)

Of course I tried to answer this question myself - the first thing coming into my mind are the many tightly packed and shrinkwrapped batteries around. But if I think about it, these wrapped batteries can expand almost unnoticeable (sure, it would be strange, but I did not notice any warning at all to leave some space around the batteries when enclosing them). At the same time, if I stuff my Al box with pouch cells, and they would grow by 10%, my box would be ripped apart... How much space should be left empty that is the question...

Any input would be much appreciated.

Thanks,

 

[dogman dan]

10% swelling sounds about right to me, for healthy cells. They definitely grow just a little over time. One problem some experience is that an unhealthy cell will swell a lot more. Then, if you have no way to remove the sides of a battery box, how do you get the thing back out when it's expanded that much?

Best approach can be a double box, or at least some kind of strapping to minimize swelling. Supposedly this helps internal resistance. But at a minimum, you need to be able to limit swelling if it goes past 10%. Then the inner box can at least be removed from the vehicle no matter what. Then once off the vehicle, the straps pressing the pouches tight can be cut to remove a puffed up bad cell.

Some kind of very stiff material needs to be used, so straps don't crush the corners or edges of the end cells. And also provide plenty of chafe resistant material around the cells. More than just one layer of shrink is needed.

 

[Vab]

Hi, thank you for your answer,

So in my understanding, the swelling of pouches can be physically limited (at least up to some point), without risking them to get popped, and this even affects the internal resistance in a positive manner?

Idea of rigid box inside the main electrical box sounds reasonable.

I am thinking of leaving like 20% extra space for 28 cells filled with some soft foam or something, this way the whole thing could be monitored with a few months intervals, sounds pretty safe.

LiFePO4 pouches acting the same way (~min. 10%)?

Thanks

 

[dogman dan]

I can't say exactly how much swelling is normal, but my limited experience was that both lifepo4 or lithium cobalt pouches do puff very slightly. Wrapping them tight will not harm them, if they start to pop, those were dying cells that were swelling more than normal. When a cell really fails, like when deeply over discharged, they can swell 100% or more. Nothings going to contain all that gas. So take care not to have that happen. Make sure any parallel connections cannot unplug during a ride.

Some of us have used various methods, but some kind of rigid board like Masonite, and fiberglass reinforced tape is the easy method. Strap them up good when new, then they will slightly expand in use to really fill the space tight. It doesn't really require mechanical compression like bolts IMO. Press the cells flatwise in a stack, then protect the sides and bottom of the pack too, but compression there is not crucial. Above all, don't let corners get crunched round on the cells on the end of the stack.