Peukert number for lithium battery technologies

[jimmyhackers]

hello all.

been googling trying to find a peukert number for lithium battery tech, li-ion, li-po and lifepo4.

my sla's are somewhere in the 1.1 to 1.2 range

im still incredibly dubious about lipo. then what makes me even more dubious is a complete lack of peukert ratings for any new lithium battery tech out there. (dont tell me its not susceptable becasue it is, wiki says so)

like SLA and conning you about their ah rating by giving it a 20hour test period. with new battery tech (probably being sold by the same battery salesmen) comes new ways of manipulating the numbers to make lipo look better than it is.

i.e. the C rating. its like your car salesman saying this car can do 200mph and 40mpg. however he wont tell you it cant do 40mpg at 200mph unless you ask him about it.

the only number ive found is on the wikipage for peukert effect, saying sla range from 1.1 to 1.2 and that lithium ion batteries are worse at 1.2857.

and thats it for my googling.... the only number i found was for lithium ion and its worse than a SLA rating. so any help on that front would be appreciated.

before people get on their defensive lithium high horses.......unless your charger specificall tells you in mah how many mah its put into your battery on each charge. you dont know about the actual usable AH in your battery.

i have a skyrc imax b6 charger and it does tell me how many ah it puts in my rc lipos on each carge......and as far as it tells me, my (30c) 11.4v 3300mah down to 3.8v a cell from 4.2v have about 1600mah of usable juice in them. my 7.4v 600mah (25) have about 330mah of jucie in them. the same results have been found by my firends usung a wide variety of rc helicopter lipos etc

so thats about %50-60 of their rated capacity. its like when maccies give you a half litre of coke in a half litre cup, thats half filled with ice :S

bit of a rant there but what im basically asking is the 1.28 figure correct for lithium ion? and what are the numbers for lipo and lifepo4?

thanks in advance
jim

 

[Teh Stork]

Peukerts number are not used for lithium ion chemistries. I'm not very familiar with it (peukert number) myself, and I've mostly dealt with li-ion.

Lithium ion batteries are mostly rated at C/5 discharge rate, or 1C. Furthermore internal resistance or discharge capacity are specified at different discharge rates.

4,2V to 3,8V is typically 40% of capacity for a NCA lithium cell. For a NMC cell it is ~50%. Li-co: ~50%. NMC can be discharged to 2,8V. NCA down to 2,5V.

Batteryuniversity is not perfect, but offers a lot of information on lithium batteries (and batteries in general). There is not one single lithium-battery, there are many different chemistries with different characteristics.

Lead is dead. Li-ion is the new king.

 

[wesnewell]

Yep, if you're stopping at 3.8V resting, you're only using about half the capacity. If you want to use it all, you'll need to go down to ~3.3V resting.
http://endless-sphere.com/forums/viewtopic.php?f=14&t=47294

 

[Punx0r]

AFAIK, the only reduction in capacity due to high discharge rates in a li-ion cell are due to capacity wasted in ohmic heating in the cell. In cells with low internal resistance the effect seems to be slight.

Apparently: "Peukert's effect is partially a result of slow electrolyte dispersal/diffusion and partially a result of an internal resistance that varies as a function of the discharge rate. This is what makes it so difficult to calculate."

http://www.smartgauge.co.uk/peukert2.html

I wouldn't be surprised if the effect was particular to the LA chemistry.

 

[jimmyhackers]

thanks for the info.

i stop at 3.8v because my friend who goes to 3.3v ends up with a lot of broken,puffed,burnt lipos. (rc cars n helicopters).

i get the C discharge rate.....but i cant help but think they are overstepping the mark. my lipos are (30C) 3s1p 11.4v lithium polymer 3300mah fluereon brand lipos.

30C x 3300 = 99amps continuous. i cant help but think if i ran it at 99amps it wouldnt last for 0.03 of an hour or 2minutes. not sure if my maths is correct there.

id imagine itd either explode or last about 30seconds if that.

the loss of heat through resistance....seems to make sense. the faster you discharge, the hotter the cells get, the extra heat increases resitance, all my lipos get warm.

this may not excatly be the peukert effect, but the overall effect is the same. i.e. the more current you take the less overall amp/hours there are in the battery. so i guess what i should what is the safe C rating then that means i will still get the full specified ah out of the lithium tech batteries?

on another tangent here....thought about this while daydreaming....... but i wonder if lead acid batteries had balence leads and used balence chargers like lithium. they too would have a longer service life?

 

[Ohbse]

The Peukert effect does not apply to lithium chemistry, it's almost 100% efficient with the obvious exception of resistive losses.

The energy you're getting from your lipo's is pretty much exactly as expected. You're complaining about marketing and not getting what you pay for, but your analogy about the Maccas cup is probably more like you're getting a full cup, drinking half of it and throwing the rest away. You're actually on the right track with regards to shallow discharges leading to longer lifespan, by not utilising 100% of the energy available you can make a pretty dramatic difference to lifespan. That's just part of the fun with lithium chemistry. Even using half the available capacity the power density, energy density, lifespan, efficiency etc are all dramatically better than lead, even cost is directly comparable depending on the application.

 

[Hillhater]

Jimmy, you could clear your mind and answer your own questions by doing a few simple tests.
You have the kit to measure charge input, so why don't you buy a $25 watt meter and measure the discharge usage also ?
Then compare the LiPo and the SLA performance !
Much better than speculating , or 2nd hand information.

 

[whatever]

seems the puekert constant is applicable to lithium batteries,
this article has some good information, starting at bottom of page 7 in the paper ( or page13 of pdf viewer)
http://drum.lib.umd.edu/bitstream/1903/12381/1/Williard_umd_0117N_12841.pdf
It doesn't give any values for lithium batteries, but does go into detail of what it means and how its used to calculate battery capacity for a given constant discharge rate.

 

[whatever]

this paper has lots of peukert values for many different lithium battery types
http://www.mdpi.com/1996-1073/6/11/5625/pdf
range from 0.99 to 1.09

 

[dogman dan]

Re c rates for lithium batteries. I've been saying for years, "cut all c rates in half". What I mean by this, is though the battery can put out the rated amps, It will stay balanced longer and perform with less sag under load if you discharge a 1c rated battery at .5c.

With the RC batteries, at least the Turnigy and Zippy brands, cut the c rate to more like 1/4. So a 30c pack is not happy to be discharged at much more than 7c. And to be honest about it, less than 5 c performs even better still. Performs better means less sag under load, and a not so hot pack at the finish.

Depth of discharge. I used to think much better to never discharge below 3.8v too. But now I think you just need to lower the C rate, once you get below about 3.65v. Riding watching a watt meter and volt meter, it's clear that the sag increases dramatically when you get below 3.65v. But if you need to ride them down to the bitter end, you can do it, just slowly, and the batteries seem to not get so out of balance.

I'm not sure about the lifespan effect. My own climate seems to degrade lico in 2 years regardless of how it's treated. It's just hot here, half the year. But I do try to stop at around 3.8v for most rides by carrying plenty. But when I need it, I never hesitate to do a deep discharge. Just slowly the last miles.

Obviously, this kind of slow way down for the last bit cannot be done with a drone helicopter. So stopping early does make sense for that.

 

[Punx0r]

A lot of folks use their RC lipo between 4.1 and 3.6V per cell. This gives about 80% of rated capacity, long pack life and minimal imbalance. The other big factor is not to let the pack sit for extended periods at full charge - it's time at full charge (and time at high temperature) that has a big effect on calendar life.

And yes, a 30C discharge should deplete the pack in 2 minutes.

 

[Hillhater]

thanks for the info.

i stop at 3.8v because my friend who goes to 3.3v ends up with a lot of broken,puffed,burnt lipos. (rc cars n helicopters).

i get the C discharge rate.....but i cant help but think they are overstepping the mark. my lipos are (30C) 3s1p 11.4v lithium polymer 3300mah fluereon brand lipos.

30C x 3300 = 99amps continuous. i cant help but think if i ran it at 99amps it wouldnt last for 0.03 of an hour or 2minutes. not sure if my maths is correct there.

id imagine itd either explode or last about 30seconds if that.
?

I don't think those Fluereon brand lipo's are a good representation of LiPo performance

 

[jimmyhackers]

i throw away the ice because i dont want watered down coke

in that furtherment of your furtherment of my analogy, watered down coke is a naffed up abused battery.
my point was they advertise it as a 500ml cup of coke, not a cup of 250mls of coke and 250mls of frozen water

thanks for the info....with lipos etc, looks like if i want what it says on the label amphourwise im going to have to abuse them : (

thanks aswel for the c rating....i pretty much figured 0.5 to 1c seems acceptable before losses are enncured.

i have ran my 4x 12v 20ah lead acids down to about 12.3ish volts and then a full charge at 2amps back upto 14.7v takes about 15,000mah.
my motors 48v 1000w or roughly 20amps max. that lasts me around an hour and half of runtime (no pedaling, i dont have pedals).
i could go lower but i dont wanna damage them so id say there is at least another 2000mah in there.
thats pretty good performance id say.

if i plug in my joke 4x 3300mah lipos instead of acids (the bike weighs a load less )
i literally get 2 maybe 3minutes of runtime untill the cells are at 3.8v aparently this is %50 capacity
so if i did run them to their real "low" "damaging" limit i get 6minutes max of runtime.

3300mah divides into 20000mah about 6 times. so if i make a 20000 amphour lipo i can multiply 6 minutes runtime by 6 and i get a total runtime of 36minutes

an hour and a half on leadacid compared to my "theorised/extrapolated" lipos half an hourish runtime. and that being generous

my real world numbers on lead acid compared to my extrapolated idea of my real world lipos has me wondering if i am missing something in my calculations like a peukert rating for lipos.....hence me asking/starting this thread.

0.99 to 1.3.......id like a 0.99 number. actually means i can use a 1C discharge rate and get more than the specified amp hour from the battery

i dont think flureons using that tech

thanks again for the help

 

[Hillhater]

Guessing how much Ahr /Whr you use in a 1.5 hr ride, and then comparing to the recharge capacity,.. Is not very precise !
You need to hook up that Watt meter to see real consumption data.
And maybe compare to some reputable LiPo. ,

 

[whatever]

to use the puekert constant to estimate remaining capacity on a lithium, only works for constant current and constant temperature. So not the most useful for lithiums.
It might be possible to write a formula for a specific battery to estimate remaining capacity, using the peukert number, but would be complex to take into account variable current and temperature, not worth the effort I would think.
It might give some indication how a cell will handle high currents.
The a123 20ahr pouch cells, theres not much difference in capacity between low and high current draw, so its peukert number should be close to 1 ( I have no idea what the exact value is).
The manufacturers dont seem to publish the peukert numbers anyway, so its probably not very important to diy ebikers.

 

[dogman dan]

IMO, if you abuse them, you'll never get the stated capacity. Only by running them at about 25% of the stated c rate do I see full capacity. Run at that rate, you can take them down to 95% empty without really abusing them.

It's not half a coke. It's just taking the time to taste it, vs chugging it down.

It could be a good thing to not have too much run time, at high wattage rates. For the motors sake.

One of my rules of thumb is the inexpensive and common 28 mm wide hubmotor lasts 10 miles at 40 mph. After that, it's in danger of melting down. This is not a lot of time, 15 min, you are done. Not that much battery to do that, 750 watt hours worth of 72v. or 10 ah of 72v. or 8, 5000 mah 5s packs. you'll stop at about 3.6 to 3.7v from a 4.2v start. In this example, the rate of discharge each pack sees is 4c. 72v 40 amps controller on the motor.

You just need more battery bro, to avoid abusing them is all. Right now, you got a micro pack. And you need a watt meter.

 

[jimmyhackers]

i just took one of my lipos down to 3.2v a cell using my imaxb6 charger. and got just over 3000mah capacity.
so it looks like they can go that low and are rated to do so and i was wrong in some respects...but maybe not when at the 20amps when im discharging them on my bike. they get hot going this low in voltage on my ebike, and i dont like hot lipos.
in the charger it was at a 2amp draw rate. and they didnt seem to get that hot.

ive had another thought. C rating is the number multiplied by the batteries capacity. and there in might lie the battery salemans play on words sales-tactic

so for instance in my 3300mah (30c) batteries "when full" provide 3300x30 or 99amp max. but then lowered down to say 300mah'sleft its 300x30 or 9amps max draw current.

at 9amps its well below my possible max of 20amps or more of my bike hence i might be damaging them.

 

[burningwings]

CycleAnalyst installed ,questions answered !

 

[liveforphysics]

All the cells I've worked with in the last 5 years have a Peukert number more often below 1 than above it, and when above it it's by a couple percent at most during extreme situations.

Your cells suck because you bought over-rated garbage cells, not because pouch cells have to suck. Some nicely over-deliver beyond all the claimed specs.

Also, you lack the instrumentation to even make a judgement on the cells performance.

 

[dogman dan]

If your packs whole capacity is only 3.2 ah, then it's too small, even for use at 20 amps. Your problem is pretty simple, too small a pack really. You are correct, if they get all hot, more than just warm, you are not making them happy.

If you have paralleled some, and have 6.2 ah, then you are fine. But do ride slow once you are below about 3.6v per cell. Real slow. There is capacity below 3.6v, but not much, and you need to milk it out at .5c by then. Pulling 20 amps when they are empty will definitely heat em up.