My limited understanding was if the controller is asking 40A from a battery pack made of 18650-25R (2.5Ah, 20A-max) with no BMS (or a BMS that is 45A or 50A, meaning the BMS factor is irrelevant in this case study) then a pack made of 10S2P is 36V-nom (42V-max) 5Ah 40A-max you are pulling 20A from each P group with 10 18650's in each series, it is really 2A from each cell, 10A from each Parallel string made up of 10 cells.
I think what you are saying is that you could build a pack that pulls 20A from the cell, so in the case above it would be a controller asking 400A from the battery pack with no bms or 500A BMS, again BMS being meaningless here, the battery needs 200A from each Parallel string with 10 18650's in each which means 20A from each cell.
The second situation the battery will not last long pulling 20A from each cell, the first situation the battery will last longer because you are pulling 2A from each cell. The lower the better, which is why building your own battery you must keep that in mind, along with the other factors, like draining the battery half way all the time then charging it compared to draining the battery all the way to minimum levels all the time. The biggest killer of batteries is storing battery at top voltage or keeping the battery at top voltage for to long before riding.
When I was making home made battery packs I would use 10awg solid core house copper wire for the series and the 14awg solid core house copper wire in the same cable for the parallel. There is a building design question where the Makita's were 5S2P so you would put two in series to make 36V 10S2P, then you either parallel the positive and negative ends to attach another 10S2P to make 10S4P or you parallel each Parallel group. I had 10S8P in 10S2P groups and did it both ways. Parallel each parallel group was not successful for me as it could kill the entire 10S8P pack rather then on block of 10S2P. That was what happened to me and I was not keeping track of balance then.
Reading SM statement again I understand more clearly. The series string carrying amps would be divided upon how many parallel strings if everything is equal and it all depends on how your battery is built. In the situation above, the first would be a current of 20A on each series link, second situation would be 200A on each series link. Some batteries can push out very high amps like A123 LiFePO4 26650 2.5Ah, 20C(50A-max), Lipo 4Ah 60C which is taken from Hobbyking which is exaggerated 60C is 240A-max, 120A-cont.
Design of the battery build needs to look at all factors including the resistance of the parallel and series link, positive and negative leads of the battery pack itself, connectors have resistance, mechanical or solder connections of connectors, cells themselves have internal resistances which all add up. Look at it from an over all view bottom up, top down. Crappy cold solder joints, some generic crappy connector, net enough tab welds on the tab, undersized series/parallel gauge conductor will not be good.
One last comment is to make sure you have good insulation protection to protect the end of the cell from touching the tabs.
Be sure the series connections are the right gauge for the amps running through them, controller rating, if your battery has a bms take note of those specifications. Always keep that in mind when changing the controller because you dont want the controller to ask to much from the battery and you dont want the bms to always kick in. The less stress on the battery the better :thumb:
Controllers can be cheap, batteries are not cheap even the cheap batteries are not cheap and why would you roll the dice on a cheap, back alley, basement built, unkown, p.o.s. someone bought on ebay, alibaba, aliexpress? Yes there are some reputable builders who sell on there and nothing wrong with buying from a known, reputable entity but it will not be cheap like the eye catching rock bottom blow out sale batteries made with who knows what, counterfeit, faked components. That is why people build their own :thumb:
spinningmagnets said:
The series connections flow the full amps per-cell, which can be as high as 20A per cell for fairly common cells (30A temporary peak per cell is possible).
If you put nickel on top of the copper, the nickel gets hot enough to melt in a tiny spot, from the spot-welder. The nickel-heat is hort enough to melt a spot of copper to the cell tip.