18650's... Less juicy than lipos!

brumbrum

100 kW
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Oct 30, 2010
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Location
Wales U.K
Just built a 20s 9p pack (72v 27ah samsung 30Q cells). This replaces a multistar lipo pack 20s 30ah (x15 4s 10ah).
Yes there is a 3ah difference but yesterday on the first full drain of the 30Q's my bike went no where near as far and ran out of juice due to a 4-5v voltage sag and hitting about 2.9v per cell. I never saw the lipo pack ever run out of juice and go lower than 3.7 nominal voltage when at rest. I know that the two types of cells discharge comlpetely differently, and i am sure i got around the same mount of sag from lipos but i was very surprised to hit rock bottom before the end of a ride,.. 63v once rested.
Some stats..
22.4ah used
26km /50wh per km
Steep hill muddy off road terrain, stop start riding.
Pulling 85amps peak. (About 9.5amps per cell)
I was able to still ride once on 63v but only at about 40amp.

I spent a lot building the new pack , and was a little disappointed.

Is this to be expected??
Or do i need to lower my coltage cut-off limits? Samsung data says 30q cell is empty at 2.5v equaling 50v on my pack.
 
With 18650 drained to 2.9v with that load you have about 2.6ah per cell. That's total of 9x20x3.6vx2.6ah = 1684wh

On lipo you had 20x3.7vx30ah = 2220wh and you are using about 95% of it. So 1684/(0.95x2220) = 0.798

So you should have about 20% smaller range, much higher voltage sag. And your speed is way lower than on lipo almost half off charge. But you have a tap lighter and safer pack. Tradeoffs between lipo and 18650. You should always make 1,2x watt hours on 18650 pack than lipo to have same range.

My personal preference is on lipos but they need much more care. It's nice to have pretty small max speed range.

With 18650 on 20s you have example 58-84kmh top speed (4.2v>2.9v)

But with lipo it's like 72-84kmh, much steadier. (4.2v>3.6v)
 
redline2097 said:
With 18650 drained to 2.9v with that load you have about 2.6ah per cell. That's total of 9x20x3.6vx2.6ah = 1684wh

On lipo you had 20x3.7vx30ah = 2220wh and you are using about 95% of it. So 1684/(0.95x2220) = 0.798

So you should have about 20% smaller range, much higher voltage sag. And your speed is way lower than on lipo almost half off charge. But you have a tap lighter and safer pack. Tradeoffs between lipo and 18650. You should always make 1,2x watt hours on 18650 pack than lipo to have same range.

My personal preference is on lipos but they need much more care. It's nice to have pretty small max speed range.

With 18650 on 20s you have example 58-84kmh top speed (4.2v>2.9v)

But with lipo it's like 72-84kmh, much steadier. (4.2v>3.6v)

Thanks, that is really informative. I never realised i would be losing so much performance due to Wh differences and chemistry. So although i have a lighter and safer pack, i now have to factor in more sag which really takes hold when on the last part of the discharge. And in reality i am getting about 23ah real usage. This is all bad news. I could possibly add another 3ah in parallel. Looks like i should have gone the other way and added more ah rather than going less.
10% less ah has cost me 20% less watt hour :roll:
 
redline2097 said:
That's total of 9x20x3.6vx2.6ah = 1684wh
20x3.7vx30ah = 2220wh and you are using about 95% of it. So 1684/(0.95x2220) = 0.798
Might be a silly question but why 3.6v used in the first caculation of 18650wh?
 
Rube said:
redline2097 said:
That's total of 9x20x3.6vx2.6ah = 1684wh
20x3.7vx30ah = 2220wh and you are using about 95% of it. So 1684/(0.95x2220) = 0.798
Might be a silly question but why 3.6v used in the first caculation of 18650wh?

Because nominal voltage of Li-ion is 3,6v. 0,1v lower than lipo.
 
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