Agree with most of this, especially the voltage sag point and the overall conclusion about the SCA chargers.
That fast charging should lower internal resistance felt wrong to me and I couldn't find anything supporting this either.
Can you point me to something where I can read up on this?
It is not that it
lowers it per se. It is not a linear single-property characteristic that is at play here. The compounds used to make the cell are amorphous for the most part, we seek to make them as directional as possible but they are not so entirely. It is more that it does not allow it to rise as fast. Any time a battery is charged/discharged the chemicals are "moved" slightly around. They do not stay /completely/ still as if in some type of a crystalline structure. The higher the temperature, the more they move randomly. The higher the potential difference /affected by the current at a given resistance/ the more they are "align" to it. Think of it like running water over a river bed. The steeper the incline, the faster the water goes, the faster the water goes, the smoother the river bottom as it abrades it, the smoother the river bottom, the yet faster the water goes. You will not see a fast river forming meanders in a homogenous substrate. Have the incline *too* much and the water will dig deep into the rock, destroying it. Basically there is a range of inclines that enforces a smoother bed and maintains a fast flow over time. Below it and stale water regions create, clogging the flow had the incline suddenly increase /causing river go out of banks/. Too much a gradient, the water will fast dig into the ground below it, destroying the river bed.
When you charge/discharge slow, the "pull/push" of the current(-defined-potential-differential-aka-voltage*) is less, allowing the non-directionality of structures to propagate inside the pack more freely. This over time limits the high-load current by increasing internal resistance ever so slightly *without* affecting capacity at low currents, it may even increase it.
When you charge/discharge (too) fast, the local potential difference becomes too strong and re-arranges the compounds in undesirable ways.
When you charge/discharge at too high a temperature, the worst happens, the high temperature messes up the structure and it even force a gradual breakup of the compounds involved.
Effectively, you want to charge a cell /for high performance usage like in power tools/ at as fast a rate as you can *before* it is too much to start degrading a cell.
Festool knows these properties of their cells, so they set the charge rates individually for each pack type to not exceed this threshold but get as close to it as possible given the used charger capability.
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I do not have a specific research on this at hand as this is not my field these days, and I am
HUGELY oversimplifying here. That said, recent research you will find covers the various micro-effects that happen. I have not seen a "macro" study on this for like .. a very long time. And I do not see a point. In a macro study you inevitably get so many variables moving, the results may be useful for marketing but useless scientifically. And not sure who to market this to. In the models/drones space, where they use high-performance 18650s extensively for decades, it is well known to not slow-charge high performance packs as a general rule.
That said. For most Festool packs, the TCL 6, the SCA 8 and the SCA 16 are
all fast chargers. It is just the Airstream ones are able to move the curve slightly more into the optimum territory by not being limited by the heat generation of a charging pack as the TCL line of charges are. The only pack type where the TCL 6 moves into the "slow(er)" charger territory is the 8Ah one. For all other packs the SCA 8 is about as fast a charger as you want/need.
*)
more precise wording, shall someone nit-pick
