short question
a friend told me that people (in the flashlight scene) use magnets to make DIY a button top cell
I wonder, would this A) hold good enough under vibration and B) be acceptable in our Ampere range?
A) No
B) No
Magnets will slide around constantly, smashing against the top insulating ring and cell plastic wrap until it touches the negative part of the cell at the top, short-circuiting and destroying the cell.
The materials that magnets are made of have a higher resistance than pure nickel or copper and the contact points to the cell can be very small since the top contact of some cells is not flat. Performance will be terrible and possibly lots of heat created.
Doā¦notā¦useā¦magnetsā¦onā¦cellsā¦forā¦esk8.
Why try to create button cells for esk8? It just adds another layer of resistance and power loss. All button cells are just regular cells with another contact placed on top or spot-welded to the top. Sure, magnets give you the extra length to create a button top from a flat top cell but we donāt need that extra length AFAIK.
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as expected
thanks for clarifying 
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First time charging! All seems fine so farā¦ 
Is there a better app for the LLT BMS, or does it make any sense to go for the paid version of this one?
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I use Carplounge.
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Ahoy, first time BMS user here. Iām charging up a new pack for the first time and all groups have settled on 4.20V except one that started out 0.03V lower than the rest and ended at 4.13V. (my cells came in 2 shipments at slightly different voltages and couldnāt be divided perfectly evenly, so there was one odd duckling P-group)
Been plugged in for a few hours at full charge and itās now at mayyybe 4.14V. Should I be worried about the cells or is this BMS just reeeeeeeal slow?
Equipment is:
BAK 5000mAh cells
Daly dumb BMS
generic ali charger
checking voltages with a multimeter
many thanks
I think the daly is a top drain BMS, so it will drain all other p groups down to the lowest. Having it on the charger will interfere with that functionality, so not much is happening.
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This is the third battery Iāve built and Iām pretty happy with the results. After the theft of my daily driver and the build I was 80% done, Iām really moving on this build. Need to skateā¦ so fookin badly.
12s4p
Molicel P42a
Charge only bms
Loop key on main neg terminal
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Ok, Iāll leave it off the charger. Should I manually charge the single group to 4.20 or wait and see to confirm cell health?
I would expect that small of an imbalance to correct itself in fairly short order, a week at most and probably a lot less than that.
If leaving it off the charger doesnāt seem to be doing anything, try leaving it on the charger for several hours minimum and see if that improves things.
Most BMS we use are passive/static type, and can only balance by bleeding down the highest cells using small resistors, so the balance current is rarely higher than 150mA, usually quite a bit lower than that. As a result it takes a loooong time because our battery capacity is many thousands or tens of thousands of mAh.
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I was dismantling a battery pack and accidentally shorted 2 cells when I was cutting the nickel strip and nicked the wrap on the negative shoulder of the cells. It was really short time. I just saw a spark smelled some burning and that was it. The cells didnāt get hot or anything. Can I
still use those cells or should I Just trow them away?
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Thereās no way to know for sure the internal condition of them, but the odds of significant thermal damage are probably fairly low, both due to the rapid nature of the event, and also the fact that the cell shoulders are somewhat distant from the more sensitive internal bits.
That said though, best practice would be to not use them. Only you can make the judgement call whether or not to accept the risk of using anything but mint-condition cells.
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Well. I wouldnāt think of using them if I could get my hands on spare cells.
Nkon.nl is fresh out of Molicel P26As.
Anybody know of another store in the EU that has them in stock.
Iām going to use the cells with the NESE system and will use a smart BMS from LLT so i can keep an eye on the voltage, ectā¦ Does that make it safer to use them?
It would help a lot to see the cell itself. Could remove the top fishpaper ring?
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Iām at work nowā¦Iāll post a picture with the ring of when i come home.
Iām curious, what can you tell from the top view of the cell?
Whether you have pierced the metal down into the top seal or any other possible damage. Not to view the actual cell means we canāt possibly know if there is any damage that might be of concern. This is on top of not being able to know if there was any internal damage.
It would be like you throwing a tarp over your car after an accident and then asking someone how much repairs would cost. They would really need to see under the tarp to answer that. 
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Gotcha. Iām gonna post them later, and probably just wait to get some new cells.
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Fellas, is this an accurate way to calculate shrink wrap size? A formula that I see thrown around a lot is width x height + 10mm. This doesnāt seem to account for the total volume tho. Based on this formula, the 12s6p double stacked flat pack Iām building next would require ~200mm wrap. I just used 220mm on a 12s4p pack and it was about right.
Measure the perimeter of the thing youāre wrapping and make sure the shrink is larger than that. The perimeter of the shrink is (diameter * pi) or (flat width * 2)
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Also there are different kinds of shrink wrap out there, some advertise 20% shrink rate, some 50%.
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