Samsung 40T and other cells like it?

[flippy]

the (automotive) industry keeps cells well below 60C wich is also the official operating range for the 40T.

for testing i would follow those temperature limits. and see how much performance remains.

 

[eMark]

Yes great power discharge rates.

But not great lifespan. 30Q is worse though, per Pajda 40T gets 1000 cycles at 100% DoD with 63% of initial capacity left.

50E is **much** lower power discharge, maximum continuous discharge 2C (9800mA) and 3C (14700mA) not for continuous discharge,

but if that is not critical, can build higher Ah capacity, then with the same test 1000 cycles leaves 85% capacity, may last twice as long![/u]

Anyone ever done a bench test (or have a guess estimate) on percent of cycle life increase between a 50E 2P bench test ("higher Ah capacity") vs a single 50E (2P 50E vs single 50E). Realize 2P would take twice as long to test same number of cycles ... so in effect isn't that the real advantage of "higher Ah capacity" and not an increase in cycle life. Right or Wrong?

Another way of asking same question as to whether or not there's actually an increase in cycle life with 3.4V to 4.1V charge and 4.1V to 3.4V discharge (cycling parameter) when comparing say a 10S5P 21700 battery to a 10S10P 21700 battery with bench test comparisons. Wouldn't a 10S10P bench cycle test take twice as long as a 10S5P cycle test whether over 250, 500, etc cycles?

My guess is that the main (only) advantage is twice as much ebiking mileage with no advantage in increased cycle life longevity with a build of higher Ah capacity (e.g. 10S10P vs 10S5P) no matter which same identical 21700 cells are being compared (2P vs single cell) whether 40T or 50E cells. Right or Wrong ?

 

[flippy]

you dont bench test multiple cells in series or parralel. you only test single cells, not whole batteries. so your question is not relevant.

 

[john61ct]

are you considering the 40T a 10amp 4c cell with 4000mah? im running 25rs also. if you are then isn't the 40T better than a 25r? you mentioned 5c destroys a 25r.

by weight a 30q and 40t are equal and you get less with a 25r

Again, designing for high power density (C-rate) is in opposition to high energy density (mAh storage capacity). Neither one is good or bad, just depends which is more important for **your** use case.

If I know that 2-3C will be my peak demand, then I get better range by buying a more energy dense cell.

If I need 10-20C in real life, then I need a bigger pack to get the same range.

And neither of those attributes says anything about longevity, many racers just want peak performance even if the pack is toast after 20 cycles.

And then of course price comes into it for most of us, so that's four factors that are all interdependent.

 

[john61ct]

My guess is that the main (only) advantage is twice as much ebiking mileage with no advantage in increased cycle life longevity with a build of higher Ah capacity (e.g. 10S10P vs 10S5P) no matter which same identical 21700 cells are being compared (2P vs single cell) whether 40T or 50E cells. Right or Wrong ?

You will get a **huge** increase in longevity by reducing your average capacity utilization.

IOW a lower HVC when charging - and sitting there for minutes rather than hours,

and stopping your discharge on average **long** before the LVC kicks in.

Like 1000+ cycles rather than the usual couple hundred.

A much greater factor than "quality" differences between these top vendors.

As for apples to apples testing longevity with the same 0-100% definitions but at different pack sizes, yes would likely not be practical, everyone tests one cell at a time.

 

[john61ct]

if that's the formula and 3c is max discharge for longevity of a cell. 3c is 7.5amps

so 4p of 40T at 3c is 30amps and 16ah and I should get a 1000 cycles if I keep the batteries below 50 but I could rig a fan or divert air to keep the core-temp of battery at 40.

as long as I don't overheat the battery why cant I run upto 12.5 amps through this cell without damaging it.

has anyone done a 10 amp discharge with a fan blowing on the cell?

You see, as long as the word "damage" is subjective it is pretty useless. There is no bright B&W line with longevity, it's a matter of balancing the many variables in each use case, all greyscale and sloping curves, owner's preferences and wealth just as critical as anything technical.

Personally I think a fan adds too much complexity, but a design to maximize passive cooling and natural airflow will always extend lifespan at C-rates high enough to noticeably raise temperatures.

But a design that simply lowers the C-rate and DoD by increasing Ah capacity will always raise longevity dramatically.

The ideal being a C-rate where temperature hardly rises at all.

Which of course needs to be balnced against weight and volume.

Which opposition a design to maximize the power density of the finished pack can help resolve.

 

[Papat92]

Thats very interesting!
Is that from the datasheet?
I would like to know the temps for about double these currents for about 1 minute constant

It´s my measurement.
1 minute is too short time to see the temperature change on the surface of the cell. Can you describe intention of such measurement more extensively ?

My most common use is high powered ebikes where nothing is constant current and it all works with bursts and even full throttle is not max current since it settles at max speed so about 1 minute at max current is the max my pack will see in addition to some cruising with full throttle bursts.

Where can I get that cell holder and what equipment do you use for testing to create the charts?

This is typically my usage as well and my experience is that those always varying operating conditions make a big difference wrt datasheet performance.
Before building last week a pack 21s6p with those 40T cells, I have been successfully using a pack made of basic Lipo Zippy in a conf 18s-20Ah for 5 years and 28.000km (~450 full cycles) with 160A peak. For those who are used to Lipo in RC model business, this an almost impossible lifetime as most of basic Lipo just die after 50 to 70 cycles.

I have reduced a bit the amp draw for the 40T pack (120A) and will regularly check its IR and capacity evolution with the Adaptto Max-E controller.
The only thing I can say for now is that the total pack IR is ~70mOhm meaning ~20mOhm per cell as indicated in the datasheet.
I would be surprised if the number of cycles is lower than the one of a basic Lipo but will keep you posted.

I forgot to mention that the measured pack capacity is for now ~21Ah between 4,10V and 3,20V (no load conditions) which are the absolute limits with my settings.