Hillhater said:
The discharge "trace" is defined by voltage and capacity.
If you parallel the cells as suggested the end capacity will be much better matched (9.6 & 10.?). So the slope of the trace will be very different.
Try it and report back .!
Now I understand your comment, and yes that makes sense.
I have the cells and the test equipment, and empirical trumps theoretical data in most cases, so I will run the test myself and see if 3P GA or 2P 48G performs better. You will need to give me as many as two weeks though, life is too busy... sigh.
Hillhater said:
Also, you could theoretically calculate the sag from the internal resistance of the 2p and 3p configurations.
Both cell sizes have approx the same 0.040 ohm DCIR ,
So a 2p config of 21700 would have an effective DCIR of 0.020 ohm (in theory) and a Vsag of 0.4 v @ 20amps
And a 3p config of GAs would have an effective DCIR of 0.015ohm (in theory) and a Vsag of 0.3 v @ 20amps
Theory suggests there would be less voltage sag using the 3p of GAs ? :wink:
IR is difficult to measure accurately especially for high-drain cells, is your source saying both GA and 48G are 0.040 ohm reliable? HKJ data lists GA IR as 0.050 ohm, and 48G IR as 0.040 ohm, but I take that information with significant reservation and do not make decisions or comparisons based on that.
For example, (again, HKJ data because it is well organized, thorough, and respected):
Sanyo GA IR is listed as 0.50 ohm
Sony VTC5A IR is listed as 0.40 ohm.
but look at traces of 15, 20, and 30 A draw for the two cells.
GA with "0.050 ohm" IR is not capable of 20 or 30A, and I would argue not suitable for 15A without significant heating and damage to the cell in short order.
VTC5A is fully capable of 30A and we know it is one of the best (possibly THE best, currently) high-drain-high-capacity cell available - but it has IR of 0.040 ???
VTC5A IR is essentially the same as GA? I think not.
IMO IR as measured by our methods is not a reliable metric for these high performance cells.
Hillhater said:
PS: There are vendors on this site selling new GAs for $3 or less.
So your 14s, 3p , pack could cost $126 or less
That 48G pack could cost you $80+ more, and have more sag ,!
What is the source for $3 GA? That is very good, especially if it is a trustworthy source.
I chose a popular US source that sells both cells, so it could be compared apples-to-apples (same-same). Price for 48G from the first (trustworthy) Chinese supplier I checked is $4.75/cell, so again - more costly, but only 5%. Not 50%.
Hillhater said:
......but yes, it would be 84gms lighter !
Not sure if you are mocking me or not? :?
- note: it's not 84g, it's 99g just for the cells
- After 50% additional ni/cu mass for 3p instead of 2p (ok, make it 40% due to larger 21mm vs 18mm C-C distance), longer balance wires, potentially longer discharge wires (depending, for the pack shape I prefer the discharge wires can be shorter, but not always), and more massive wrapping, the difference will be closer to 200g.
- 200g may not matter to you or most people on this forum who seems to build 50kg ebikes that are more similar to small electric mopeds than bicycles and consider a bike "light" at 25kg. But my ebike weighs under 30lbs (~13.5kg) without lights and it makes the bike so much more fun to ride. It feels almost like a real pedal bicycle, which is my goal. The way I accomplished this is by choosing lightweight components whenever possible. So if you say 200g doesn't matter - well it does matter to me, and possibly others.