Hope this can be a place for all the knowledge that we all want Mooch to share with us. It is extremely important that we all understand battery cells and their useful discharge rates, especially as to how it pertains to our application. Realistic wh ratings at certain discharge/charge thresholds, the more we understand about the way different cells behave, the better equipped we will be with making a good decision as to which cells to put into our DIYs. Again thank you @Battery_Mooch for all the work that you do. We all quite often search the interwebs for what you have to say about certain cells before we buy them.
I will begin this discussion
A question for you @Battery_Mooch . What cell would you recomend for the following set ups
12s4p=for range maximization
12s4p=for discharge current maximization
12s8p=for range maximazation
12s8p=for dischatge current maximization
Keeping in mind that realisticly 90% of is dont need more then 60 amps continues.
Also as hard as it may be to answer this, what do you think is the best cell for esk8 apllication? Is it the good ol’p42a?
I sent him 4 tesla cells, 2 that came in the grey wrapping from bigbattery, and 2 that came in the blue wrapping from an ebay reseller. They should be at his place on friday. I am hoping that they atleast perform well at 15 amps
that’s interesting, wonder whatsup with the rewrap…I’ve seen the capacity specs varying between batteryhookup and bigbattery on these cells and I have 200 here waiting for , ummm, ya know, some purpose
Yes I’ve seen varying capacity ratings too, but all within a few 100mah, nothing crazy. Not sure what the rewrap is all about, only thing I can think of is the cells are probably bare when they are recieved from whatever model3 they came out of. They are for sure tesla cells tho. They have the same exact spotweld footprint on the negative side
I feel like these are impossible questions to answer. There are some cells that focus on discharge so much that their range is useless for us. Or vice versa.
You also didn’t specify the cell size (18650 or 21700 or other), so of course a 40T is going to beat a 30Q when you can choose either.
Here you can compare stats like energy per weight, per volume, power, cost, etc.
For example: you can see that an overall good cell like the P42A is very good for power per volume, but the 30Q still beats it in energy per volume.
Moral of the story: If you are building in an enclosure, measure all the dimensions, and calculate how big of a battery layout you could get with different cell sizes. Sometimes 18650s will fill the space better than 21700s, sometimes the other way round. Sometimes you can fit a staggered double stack in (double stack height is (1 + sqrt(3/2) * cell width), that’s 33.58mm for 18650 cells, and 39.19mm for 21700 cells)
Or, if you are not limited in size constraints, then just going with a cheap but overall good cell is usually the best option. 30Qs have become very cheap over the years, so if I was making a big battery for a mountainboard, I would probably use those instead of something fancy like Molicels.
Fun math fact: as the number of cells approaches infinity, no matter if you fill a space with small equally-sized circles, or big equally-sized circles, the ratio of space filled by the circles / not filled by the circles is the same. So choosing between 18650 or 21700 really doesn’t matter
One interesting idea I have is about mixing cells. If the P42A hadn’t come out, I would have done a build with 50% 40T cells, and 50% 30T cells to get a similarly performing overall battery pack. Might be worth mixing 30Qs and 35Es perhaps in some builds. (I believe the more powerful cells would take the current hit, due to how voltages even out, but I’m not 100% sure. Also important that each p-group has the same cell ratio, having 2 p-groups have different cell compositions would be bad )
Oh wow, I must have mixed up your sheet with someone else’s then, I definitely asked someone to adjust the stats for the P42A to not only use a more conservative current rating, but also increase the size to 21.8mm since they’re a bit thicker than others
Not sure what discharge current maximization means.
Always get down to the cell level. We do it for weight and size and it makes things easier when just thinking about one cell.
4P at 60A is 15A per cell ideally. Call it 20A worst case with typical unequal current sharing. I recommend the 30Q or VTC6 for 18650 or P42A for 21700. The 5000mAh 21700’s are really inefficient at current levels this high.
8P at 60A is probably about 9A or so per cell worst case. The Sanyo NCR18650GA is my 18650 choice. For 21700 I’d pick the Samsung 50G (Vapcell rewraps it as the G50) or Molicel M50A.
If the cells are being pulsed much above 60A then forget these recommendations as pulse are just short continuous runs as far as the battery is concerned and you need to choose cells that have higher efficiency to prevent the huge voltage sag you’ll get with lower current rated cells (with higher internal resistance).
Best cell? No such thing. It depends on how hard the cells are being used. The best for 5A will be very different than the best for 50A. Having said that the P42A is a great all-around cell that is a great choice for a wide variety of setups.
A lot of fakes can be spotted by comparing the physical appearance of the cell in question to one you know is genuine. You often need a 5x or 10x magnifier since it can involve details of the venting disk, tooling marks, bend radii, and small things like that.
Identifying if a cell is a rewrap or one from a smaller China factory is done the same way.
I often need to also do capacity, internal resistance, and performance testing though if the cells are very similar in appearance. Different factories making the same cell can result in slight differences in the metal cans and occasionally a cell changes its appearance over time.
and i am so exited about it!!!
I’ll do that now
It depends on how hard the cells are being used. The best for 5A will be very different than the best for 50A. Having said that the P42A is a great all-around cell that is a great choice for a wide variety of setups.