voltage sag rc lipo vs Li-ion with heavy load

I'm considering either these two as choices for powering a tangent 6kw unit

With heavy load (full throttle) which of these would perform better and have minimal (significant voltage sag)

I'm considering going 18650 30q 8p (good for 20A in theory) vs 2P of 10c 10000mAh ( 200A continuous essentially)

Thanks!

12-C said:

I'm considering either these two as choices for powering a tangent 6kw unit

With heavy load (full throttle) which of these would perform better and have minimal (significant voltage say)

I'm considering going 18650 30q 8p (good for 20A in theory) vs 2P of 10c 10000mAh ( 200A continuous essentially)

Thanks!

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18650 sucks for high power setups to be honest lipo will get a better wh/kg in that power range

14s8p 3000mAh would give me 24Ah while the rc Li po would give me 20Ah.

The continuous draw would be significantly less than max continuous rating of the 18650 which is 20a per parallel string. And peak would be about 120a which is 15a per series so it seems like I would be running them under their potential but please correct me if this logic is wrong and real practice proves other wise.
Thanks

If you never exceed their spec discharge and charge C rate, the round cell are an excellent choice because they will last longer and safer. Then, if you need so much power that a battery made of round cell would have to be much bigger than your capacity requirements, you are better with RC Lipo to save weight and size. Generally, if 10 C RC Lipo is enough for you, round cells should be considered. If you can swell 25 C lipo in a short ride you should stay away from round cells, you need high C rate RC Lipo.

Few mid drives builds are needing RC Lipo to feed their power demand. You would have to build your own mid drive with a much bigger motor than those that are commonly used in kits. Most of those who are using RC lipo with mid drive bikes, are not using them because they have to. They use them to save weight and size, to benefit the fast charging time, or the simple assembly.

Mad rhino

The 10c rc lipos are only good because the capacity is 10000mah and I would use 2 in parallel so at 10c they can provide 200a continuous which is more than my projected 120a peak requirements. But also the round cells at 14s8p in theory should be good for 160a continuous again below my 120a anticipated peak. But are these projections of mine reasonable and enough buffer so that I might see inconsequential voltage sag ?

I just have no practical expertise with e bike and this is my first battery build so I'm not sure by how much one needs to consider over sizing by adding capacity in parallel so to decrease burden on the pack voltage when gassing it hard...

The other thing is that max continuous quoted value usually see voltage drop about .4V even if they are good for it at cc. The rating from what I understand its based on temps the cells reach and not the voltage out put/ stability for that given cc.

Thanks

As a side note the 112 18650 cells would be less expensive (here in Canada), 5.1kg( vs5.8kg for the rc bricks) and allow me to fit the batteries a lot better than any rc bricks.

12-C said:

Mad rhino

The 10c rc lipos are only good because the capacity is 10000mah and I would use 2 in parallel so at 10c they can provide 200a continuous which is more than my projected 120a peak requirements. But also the round cells at 14s8p in theory should be good for 160a continuous again below my 120a anticipated peak. But are these projections of mine reasonable and enough buffer so that I might see inconsequential voltage sag ?

I just have no practical expertise with e bike and this is my first battery build so I'm not sure by how much one needs to consider over sizing by adding capacity in parallel so to decrease burden on the pack voltage when gassing it hard...

The other thing is that max continuous quoted value usually see voltage drop about .4V even if they are good for it at cc. The rating from what I understand its based on temps the cells reach and not the voltage out put/ stability for that given cc.

Thanks

As a side note the 112 18650 cells would be less expensive (here in Canada), 5.1kg( vs5.8kg for the rc bricks) and allow me to fit the batteries a lot better than any rc bricks.

Click to expand...

my suggestion is to get of the 18650 cells you will be using discharge it at the various rates you will use them at and see what the capacity is temperature voltage sag and dont forget to remember you will have a lower average voltage then if you think they will work go for them if not look at your other options (here is testing for the cell you want to use http://budgetlightforum.com/node/42075 )

i would expect about 800 usable wh from the pack if you go for an 80% discharge so at 5.1kg (196wh/kg for 100% discharge ) that is at 10A / cell

12-C said:

As a side note the 112 18650 cells would be less expensive (here in Canada), 5.1kg( vs5.8kg for the rc bricks) and allow me to fit the batteries a lot better than any rc bricks.

Click to expand...

Yep, but the round cells do require a lot of assembly, making size and weight above RC Lipo once ready to use. Also, cheap round cells should be avoided, or carefully tested before assembly.

Then, about 10 C RC Lipo :
High impedance pouch cells are crap, and over rated. You should cut by half their rated discharge, to use them safely. Low resistance of RC Lipo cells is the safety factor. It is also what is keeping them from sagging voltage, and loss of performance in very low or high temp. For those reasons, you should inquire of the impedance of cells that are low C rate. Chinese manufacturers are not all crooks but there are no standards. Some of them are rating their cells conservatively, some other are over rating. Resistance measure doesn't lie.

MadRhino said:

12-C said:

As a side note the 112 18650 cells would be less expensive (here in Canada), 5.1kg( vs5.8kg for the rc bricks) and allow me to fit the batteries a lot better than any rc bricks.

Click to expand...

Yep, but the round cells do require a lot of assembly, making size and weight above RC Lipo once ready to use. Also, cheap round cells should be avoided, or carefully tested before assembly.

Then, about 10 C RC Lipo :
High impedance pouch cells are crap, and over rated. You should cut by half their rated discharge, to use them safely. Low resistance of RC Lipo cells is the safety factor. It is also what is keeping them from sagging voltage, and loss of performance in very low or high temp. For those reasons, you should inquire of the impedance of cells that are low C rate. Chinese manufacturers are not all crooks but there are no standards. Some of them are rating their cells conservatively, some other are over rating. Resistance measure doesn't lie.

Click to expand...

Thanks makes sense. The round cells are the Samsung 30q or the Sanyo 20700b. So they are well known and quality. The discount price is just bulk pricing~$5 per cell.

I quoted the rc lipo incorrectly they are 25c and are a well reviewed brand. What type of resistance is acceptable for C ranges.

Yeah, safer.

Seriously, while the 18650 c rate spec may not be as big a lie as the lipo, you definitely will see a lot less sag from 20 ah of 30c or better lipo, than any 18650 cells.

Bump to 40 ah, then you might have a big enough pile of cells to see less sag. But if you want it light and punchy, its still the lipo.

With a 1000 w setup, a 20ah Li-ion batteries will show a bigger drop under load and will heat up more than a 20AH Lipo pack. Unless the Li-ion pack is at least of the 30AH and better if it 40AH there is not much way of getting around. Because I test large RC aircraft and drones using Lipos, I find it advantageous in my 1000w setup to use lipos. I can carry enough batteries to go where I need to go. Using Lipos also offers me a bit of flexibility for shorter trips I need only carry 15Ah for longer trips I will carry up to 30Ah worth of Lipos. Unless you build an extra li-ion pack and parallel it in, you kind of stuck on your range, Because I have several 300 watt chargers I am able to charge and balance all of my lipos in a little over an hour. Over the years when carrying 20ah or more worth of Lipos, I've never had a problem mixing 20c to 50c rated batteries together as my bike does not draw more than what a 20c can put out. Also because I use Lipos, I can lower the center of gravity by mounting 4 of my packs lower in the frame.

He's talking about 6000w. that would take a crapton of 18650s, even the best type.

Of course it can be done, tesla does. but not with 20 ah of them.

dogman dan said:

He's talking about 6000w. that would take a crapton of 18650s, even the best type.

Of course it can be done, tesla does. but not with 20 ah of them.

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Holy Smokes 6000w, you are right at that level it would take a small suit case full of 18650 to provide enough power to get that going. That is a heck of a lot of weight on top of that.

Its rediculous to expect a technical answer if you dont state what cells you are comparing.
If you are serious you will invest in a few cells and test them yourself...or trust data that can be found on the net .
But if you understand battery tech at all, you should know the answer to your question !

There are some factors pro 18650 and some pro Lipo, when you think about a battery pack for ebikes: but almost none of the 18650 adavntages apply to the high powered bikes, while all the downside are more than a concern. I would resume the major downsides I experienced:

1: A 18650 pack under daily/usual heavy loads will deliver lot less capacity than its rates.

2: You will need IMO approximately 3x the capacity of the best high discharge 18650 (not the 30Q) to be in the sag spot of just a regular Lipo 20-30C pack, so SAG is much more even if you compare 18650 and LiPo with similar labeled rates (for instance Sony VTC5 and regular 10-20C stated Lipo pacs as Multistar, GraphenePro or regular Zippy/Turnigy Packs)

3: The ΔVoltage of the full SOC of a 18650 pack is much higher than the LiPo one and this means a 18650 powered bike will have a significative performance loss in the lower middle of the SOC compared to a LiPo powered one, plus, since the Δ-SAG is in my experience somehow related to the SOC ΔV, so to the chemistry itself, and not only to the IR, this could give us a partial explanation of point 1 and 2.
4: Since Ripples and wide Δ-SAG are serious enemies of the RC ESCs I would say that a LiPo pack is safer even in this field. I noticed also a much more stable IR in a LiPo pack along its life compared to 18650 ones, so a 18650 pack that was stable as a LiPo pack in the first part of its life, could be not so stable after some cycle aging, this means less capacity and performances, but also less ESC safety.