Here's the theoretical answer, because any real-world answer runs up against questions of what the manufacturers are actually doing today.

If you change from a 4s pack to a 5s pack and you adjust the per-cell capacity down by a factor of 4/5 to match the 5/4 increase in voltage, you'll keep the total pack energy the same.

Assuming that your ESC and motor efficiencies remain largely the same, the increased voltage means your current will go down by a factor of 4/5.

So your capacity will have dropped by 20%, your current will have dropped by 20%, and -- all else being equal -- your battery heating will remain the same.

Only if you go to higher "C" rated batteries, or keep the same cell capacity (thus making your pack 20% heavier), will you pull less current compared to the cell's "C" rating -- so I wouldn't anticipate less heating.  If anything, unless you go to a 2 + 3 pair of packs with airflow in between, I would expect that you'd see worse cooling rather than better.

If someone was running 5s packs and they were running cooler, it was either because (a) the batteries had better real-world discharge performance than yours did (and manufacturers "C" ratings vary wildly.  I can't quite say they're all lies, but I wouldn't depend on them much), or (b) they had better cooling than you did.

A higher cell count will probably result in lower dissipation from the ESC, if its power loss is more conductive than from switching losses (switched amplifiers, like ESCs, suffer from switching losses that go by voltage).  But maybe not (I'd believe what Castle or Shultzi, and probably Jeti, says on this, but not any Chinese ESC manufacturer).

So I wouldn't expect a change from 4s to 5s to drive a change in pack temperature to any extent other than any other random battery change would.  It may do other good things, but to drive the pack temperature down I'd go for better cooling, and maybe buy higher "C" rated batteries from the same manufacturer, or look around for a different manufacturer with a more conservative "C" rating.

Here's the theoretical answer, because any real-world answer runs up against questions of what the manufacturers are actually doing today.

If you change from a 4s pack to a 5s pack and you adjust the per-cell capacity down by a factor of 4/5 to match the 5/4 increase in voltage, you'll keep the total pack energy the same.

So your capacity will have dropped by 20%, your current will have dropped by 20%, and -- all else being equal -- your battery heating will remain the same.

"Total pack energy"   and   "capacity"  are they not the same thing ?   So how can one stay the same and the other drop by 20% ?

I think you have to look at the internal resistance of the cells.  Smaller cells seem to generally have proportionately more resistance(the smaller plate area that the current has to flow through).  If the smaller cells have the same resistance as your current ones the higher voltage and lower current will result in less power loss in the batteries- less heating.

So look for the cells with the highest base current capacity.  That normally indicates a lower internal resistance and better efficiency.  Lower internal resistance, higher discharge rate to capacity, and lower overall weight all come at higher cost.

So look for the cells with the highest base current capacity.  That normally indicates a lower internal resistance and better efficiency.

I was using the wrong terminology last night, at least partially.  It's discharge rating in "C", not just "C".  But what you say above is true -- just keep in mind that there isn't an industry standard as to what the discharge rating means, so a cheap Chinese pack with an "80C" discharge rating may be no better than a name-brand "40C" pack.

"Total pack energy"   and   "capacity"  are they not the same thing ?   So how can one stay the same and the other drop by 20% ?

Pack capacity is the total pack charge in amp-hours or milli-amp-hours.  Pack energy is proportional to the capacity times voltage.

Haha....So was everybody!!!! ......40c .......Yuk!