SamTexas said:
It's best to work with real and reliable data. Here's the datasheet for Panasonic CGR18650E cell. It tells you the voltage sag at various discharge rates. I have another datasheet from either Sony or Panasonic that show even more dramatic voltage sag. But it's buried somewhere in my hard drive at this moment.
Hi, I should have stated in my first post which cells I'm using, my bad; They're marked Samsung ICR18650 26C and I believe the original capacity was 2600mAh. The only datasheet I could find was this one: http://www.meircell.co.il/files/Samsung%20ICR18650-26C.pdf Which did not contain any discharge graphs, but it did say that the maximum discharge current is 5200mA, so that's better than nothing.
The CGR18650E datasheet you posted is helpful however, as I'm guessing the voltage sag characteristics should be pretty similar as they're similar capacity and both 2C capable. i'll use 3.5v as a low voltage limit for now, as I'm planning to run 12 cells in series then if my voltmeter goes below 42v on a fully charged pack then I know I'm pulling too much current.
amberwolf said:
HypnoToad said:
I have the option to run 6 cells in series, or 12 if I really need to.
6s is only about 22V-24V or so, depending on charge level. 12s is closer to 48V, though I'd use 14s myself.
Most likely you'll need to run quite a few parallel cells. DrkAngel has a nice thread full of data on how to make packs like this, with good pics and data, which you will probably want to read--it also has good info from other contributors.
Current provided under specific loads, as well as voltage sag, will depend on the specific cell, and what it's rated for, as SamTexas pointed out.
I have a bunch of various cells that I someday intend to build a giant pack from, but have yet to have time and materials at the same time to do it. I expect I can get 30 or 40p, at 14s, with what I have right now, though I doubt all of them are very good cells. Maybe 20p if I get a rather picky about which cells to use. The more in parallel, the less load on each cell, and thus the less sag. At 20p, I'd give myself a max of 20A burst current, and 10A constant. Even that is probably a lot for these old cells. :lol:
Doh, I made a typo in my first post, I was supposed to say "I have the option to run 6 cells in
parallel, or 12 if I really need to."
I'm almost done wiring up a 12s 6p pack full of Samsung 18650's, and if this turns out to not supply enough current then I can make a 2nd pack and run them in parallel. However it takes quite a while to build these packs, and quite frankly I'd rather be out on the bike rather than cutting and soldering cables!
Also, I'm guessing this is the battery building thread you're referring to;
http://endless-sphere.com/forums/viewtopic.php?f=3&t=26383&hilit=DrkAngel
?
I'm on the 3rd page right now, it is full of good info, and it seems like I'm on the right track with my first pack, thanks.
You should use your cells, I have a little motivation as I have an e-bike with no battery right now so the sooner I get the battery done the sooner I can be out riding, also li-ion's lose capacity over time weather they are used or not, so I figure you may as well use 'em while they're still good. 20p will be huge, but who said overkill was a bad thing?
etriker said:
Laptop cell packs for ebikes work fine for extender packs and don't need to be so large. 40 cell laptop cell extender packs work fine and are small, light and pack a lot of power.
Using them as extender packs you can keep them cool, barely sag them and make them last.
It may sound complex at first but believe me it is less complex than building and troubleshooting large laptop cell packs.
Hmm, I've never heard of this until now. Would you use a li-po paralleled with a li-ion extender pack? Would you parallel the balance cables also?
SamTexas said:
amberwolf said:
I have a bunch of various cells that I someday intend to build a giant pack from, but have yet to have time and materials at the same time to do it. I expect I can get 30 or 40p, at 14s, with what I have right now, though I doubt all of them are very good cells. Maybe 20p if I get a rather picky about which cells to use. The more in parallel, the less load on each cell, and thus the less sag. At 20p, I'd give myself a max of 20A burst current, and 10A constant. Even that is probably a lot for these old cells. :lol:
aw, go ahead and build the pack or at least play with your cells. You will soon discover that it's not as bad as you think. Most cells are 2.2 to 2.6Ah new. If your old cells still have 70% of the original capacity (conservative), a 20p group will have 31 to 36Ah. So 36A burst and 18A continuous is still conservative.
90% of the time I use 0.5C or less on my pack. But I have no worry pulling 1C for a few minutes climbing hills, or pulling 2C for a few seconds when I'm accelerating hard from stop.
My 21s10p, 18Ah pack is the one I used to push my dual 9C 2807 bike to 40mph. That's around 2,300W. Of course, I would hurt the pack if I keep that speed for more than a few minutes. But a 21s20p pack will have no problem sustaining that speed at all.
It's nice to see some numbers.
My pack right now is 12s6p. Original cells 2600mAh. My RC charger thinks they're between 2200-2300mAh each after balancing, so they seem to be around 85% the original capacity if my charger is to believed (Imax B6).
Now assuming 6p and capacity is 2250mAh, combined capacity = 13,500mAh:
0.5c = 6.7A, 1.0c = 13.5A and 2.0C = 27A, so I may be pushing it a little with my 30A car fuse here, I'll have to order a 40A one.
However my setup is a far cry from your dual 9C, I'm using a 250W Ezee motor that's a little overvolted (44v instead of 36v), and it struggles to do 20mph let alone 40. I think I may be ok, in fact I doubt that this motor will ever use more than 20A unless I find some pretty steep hills.
JimW said:
Depends on the C of the batteries.
Hot off the charger, the voltage will drop like a rock (what, about half a volt guys?) at maximum C discharge and then continue to slowly fall based on the total capacity (Amp hours) of the cell.
If, however, you exceed the maximum discharge C, the voltage will fall further and you run the very real risk of damaging the pack. The chemistry can only supply power so fast. You never want to drop below the minimum voltage for the cell either.
Pretty sure this is right, but will yield to others on the forum more knowledgeable than I.
Thanks,
Jim.
I don't really have a BMS, just a voltmeter on the handlebars, but I'll be sure to make sure it doesn't go below 36v (3v per cell).
