Ok, I am normally pretty good with my packs, have a fairly set way I look after them, they are stored with around 40% storage charge, and I charge at 25 Amps in a Venom Stronghold charger (Balance charge) as I run blinky classes. My Graphene 6000 packs (90C) have been flawless with this for months, and my 21.5 and 13.5 tourers are happy.

But then I got a mod car...

and I put in one of my packs, car ran find, but it came off pack was luke warm but a slight bit of swelling (maybe 1-2 mm)

for a perfectionist type like me, I have to ask, WHAT THE?

Tested packs IR when cold the next morning was 4 milliohms on one cell 2.5mm on the other, so that seems ok.

Packs are 6 months old and done maybe 50-60 cycles each (always break in new packs at 6A charge / 2A discharge to 35% for 3 cycles before racing)

Tested with an ESR meter, all good there (same values as a new pack)

so I have to ask, why'd it give me a bit of puff? Do these packs get conditioned to a particular charge / discharge cycle, and by subjecting it to something at a higher amp discharge it was not used to, that was an issue?

Do we have to treat modified packs differently to blinky racing packs?

Curious on this. Suffice to say, not letting my other blinky packs near the mod car for now, bad mod car!

The heat generated from mod racing probably caused the gas inside the cells to expand a bit more than usual hence the puffing - did it come back down in the end? 4 milli ohms on graphene cells also sounds a bit high - more IR more heat.

Well, when I say puffed, maybe 0.1-0.2mm enough so it doesn't sit flat, it came down fine. 4 Milliohms I don't think is that bad, most people take an IR reading during a charge so see values like 1-2 milliohms on the charger, I take a reading on a pack that has been sitting a while at room temp as I find it's more accurate.

With the age and cycles, puffing is not that surprising. In our experience the things that lead most to puffing are discharging too far and storing a battery fully charged for more than a few days. What do you have your cutoff set to?

Cut off I usually set to 3.2v/cell, if they had puffed from storing charged I can handle that, my error, but I hadn't. Just wondering if I do want to dabble in mod, do I need a set of packs specific for that? It appears so....

Yes storing charged really does cut down on the cell life and accelerates puffing. As far as different batteries for stock/mod it is not necessary, 90% of it is battery maintenance.

Interesting, I Know cells degradation happens when stored at full charge. Do you have anything other than anecdotal evidence to back up the puffing part?

I also wouldn't worry about slight puff to your cells, as others have said cells puff slightly when you pull more amps through them. Your also running 6k mah batteries which leave very little room in hard cases for natural expansion and contraction of lipo's. If you get your hands on some 5k mah batts, you can squeeze the case a little bit and feel the extra room from the smaller cells. This same kind of stuff happens to short course trucks all the time, its a non issue.

Nothing scientific, just based on our experience. In addition to the cars I also personally fly a lot, and planes can suck down a battery in a few minutes so I have about 25-30 LiPo's at any given time that are in rotation for planes. On packs where I have not stayed on top of putting them in storage mode or ones that are used in high-current draw planes I can see they are more prone to puffing than other packs that have been put into storage mode when not being used for more than a few days. My point is that it's not any one specific thing, but overall battery care helps extend their life and prolong their pre-puffed existence.

Thank you for the response. I'm sure some lipo manufacture has done the research, but the data may never come to light.

run it and enjoy
I used to fret over the puffing and now they all puff at some point.

being it a hard pack it will swell also, supposedly soft packs are less prone.

Supposed by who? Soft packs are what lies within the case of any hard pack, so they (should) react exactly the same way. Maybe some people tend to notice it less (or later) on their soft packs, since there's no prior perfectly straight hard case that suddenly develops some funny bulge.

Besides, I'd strongly recommend to only use hard packs in RC cars.

Excessive draw ?