Cell cycle life vs "C" rate

[Hillhater]

If this has been debated please let me know..... A search revealed nothing relevant.

Question,... Cycle life.. how is it affected by discharge rate ??
specifically for HK type RC LiPo.
When cycle life is quoted, its often at 1C (sometimes even less ! ) discharge rate.
Now that is a useful "starting point" , but many of us are using our packs at 5C ,10C and even 20C rates frequently, and i cannot find any info on how that kind of discharge loading may reduce the expected cycle life.
Any data available ? or even just actual experience ?
IE :-.. a pack with an expected life of 500cycles at 1C , how many cycles might it be expected to last (to 80% capacity ?) if repeatedly discharged at 20C ??

[liveforphysics]

It ranges between, drastic, to roughly no effect.

A nano-tech cell is going to have roughly the same lifespan at 0.1C or 15C, even though they are 150x different in C-rate.

A laptop cell designed for a 0.5C application might have it's life reduced by 90% to merely run at 2C.

If temperature is kept below the points where the solvents have accelerated decomposition, and the discharge rate isn't near the current saturation limits, C-rate doesn't have much of an effect on cycle life.

If you are near saturation, then you get local concentrations at the surfaces of the cathode or anode (depending on if you're charging or discharging), which can result in formation of metallic lithium plating during charge, or local uneven expansion in the intercalation medium (cathode or anode coating) that causes physical cracks or faults. These cracked areas don't have as conductive of a connection to the current collecting foil, and there for the resistance of the cell increases, or if not connected at all, they become inactive and the capacity decreases along with resistance increasing.

If your cell has excellent low resistance ion pathways and good structure design, C-rate will have a fairly minor effect on cycle life. If your cell favors energy storage over having low resistance (ie, a phone battery or laptop cell), then it's going to get hammered by increasing C-rate.

[Hillhater]

Thanks Luke,
I am thinking of the typical HK 5Ahr 20-30C packs..Zippy, Turnigy etc. when run near their "Continuous" C rating
From what you say, the Nanotech packs are much more tolerant of that kind of use ?

[liveforphysics]

Thanks Luke,
I am thinking of the typical HK 5Ahr 20-30C packs..Zippy, Turnigy etc. when run near their "Continuous" C rating
From what you say, the Nanotech packs are much more tolerant of that kind of use ?

The C-rates they list most RC cells at only works for RC devices, which if you're an RC car, you only get maybe a 30% duty cycle of full c-rate at most, and if you're a plane or heli, you can get loaded with the full C-rate or more for your whole discharge cycle, but you've got a 50-150mph propwash or rotorwash blasting over you the whole time, and you're not sandwiched between a bunch of other packs in a bag or whatever.

If you wanted a rule of thumb for commuter bikes that want a long healthy life, I would avoid peaking over 1/2 of what they say as continuous, and perhaps 1/4 the continuous rating as your actual continuous draw in riding.

If you're racing or something, run them at the full rated C-rate or beyond. My last race bike used 20Ah of 30-50C cells, and we pulled over 680amps in continuous dyno pulls and 100amp charging, and didn't have a lick of battery issues or heating, stayed cool as a cucumber and perfectly balanced. I wouldn't do that with a commuter bike pack that I was counting on lasting years though.

[Hillhater]

Hmm ?
so are you saying that temperature ( internal resistance etc) is the main factor for cell life ?
Is there a "critical" temp to keep below ?
Even taking temp out of the situation with the heli example you suggested, with max 20 C rate and plenty cooling, would you still expect that pack to last the same cycles as a similar pack used at 1C ??
Has there ever been any data published, ? someone like A123 should have tested this presumably ?

[neptronix]

high internal temperature kills RC lipo quick. I've found that even drawing 1/2 the rated C continuous on hobbyking packs results in the packs getting warm-hot within 5-10 minutes.

Those people who complain about getting 50-70 cycles out of a lipo are running somewhere near the full C rate constantly.
Aaronski killed a bunch of lipo cells running the rated C constant..

The people who are getting 300+ cycles are running their lipo packs at 1/4th the C rating or lower, by running multiple packs in parallel.. As for me, i've got ~80 cycles on my lipos and i'm still getting at least 5ah out of each 5ah brick.

[Hillhater]

high internal temperature kills RC lipo quick. I've found that even drawing 1/2 the rated C continuous on hobbyking packs results in the packs getting warm-hot within 5-10 minutes.

Those people who complain about getting 50-70 cycles out of a lipo are running somewhere near the full C rate constantly.
Aaronski killed a bunch of lipo cells running the rated C constant...

Hmm ? Thats what is worrying me.
I have been thinking of building a Electric powered sprint /Autocross kart (with lipo batttery obviously !)
For minimum weight i keep the pack to the minimum required to feed enough power and last the distance, but that means using near the max C rate. But now i wonder exactly how long i can expect the pack to keep performing at that level.
I could add more cells but that just adds weight and hurts track performance.
I dont like the prospect of building a $2k pack that lasts less than 100 cycles
This is the senario facing most people building "race" EV's i guess, and some of those with bikes etc will have $5k - $10k in their packs

[neptronix]

Well... nobody said electric powertrains were cheap
Unfortunately it will be a while, if ever, before batteries can even touch the power density of gasoline.. but you do get a big head start on the weight savings with an electric motor versus a gas motor..

You could go halfway between nanotech and cheapo stuff and get 30-35C cells.
Or just deal with the extra weight and get some more AH of the cheapo 20C stuff. Mount it as low and centered as possible so that the effect on handling is minimal..

[liveforphysics]

You definitely want Nano-tech cells. 45-90C packs.

You don't want to have to be worrying about your battery while you're racing. Racing will bring enough things to worry about.

[dogman]

Good call. Buy the best c rate you can afford. My dogsense thinks the less sag you are seeing, the less each romp on the throttle is damaging the pack. My number one impression of kart track racing is that the battery really hates having the corners come so frequently.

I think racing did some damage to my lipo. Only running 4c, but nevertheless packs that were run at 8 c for some practice runs went bad first, and now I'm beginning to lose a few of the packs that went 4c max. Racing in Tucscon, just getting there the packs baked in the car some, then on the track the spiky discharge is never so good. So the packs would be hotter than normal by the end of the race. I never figured on much more than 100 cycles from packs used on the track, and that's not too far off from what I seem to be getting. These 20-30 c turnigys have puffed a bit, and now they seem to warm up a bit more than when new. Since the race they mostly get cycles on my dirtbike, at 4c maximum.

[999zip999]

Cycle life also depends on DOD and balance of the cells and match of IR. But if your looking for 500-800 cycles you are not racing. You want to be a winner. Hit it. Can't hit a home run if you don't swing the bat. Cycle life in racing ? A winner doesn't ask a question like that. Or how many miles will my tires last. It's racing. Leave it on the track.

[dogman]

Amen to that. All I wanted was some practice and a couple track outings from the raced lipo. A summer of dirt riding on the same packs was pure gravy.

[Hillhater]

... Cycle life in racing ? A winner doesn't ask a question like that. Or how many miles will my tires last. It's racing. Leave it on the track.

Well you should if you were sensibly planning to know how long to expect your batteries to last, when to plan a replacement, is pack capacity going to deteriorate rapidly, or noticeable affect performance etc etc,
Its like an ICE racer wanting to know how long his tires will last , so he can plan a tire change strategy for the rase ( if you follow any major race series ?) .. or how long a motor will run before it needs a rebuild to prevent it blowing !
Of course, you could learn by trial and error ( run them until they fail ?) but that is old school, these days winners plan to avoid failure.
Batteries are the same ..you need to know their performance and life expectancy under "Race" conditions.

[999zip999]

Just saying there is a lot of hope in racing.

[Hillhater]

Just saying there is a lot of hope in racing.

Sure is !, ..but the successful racers plan ahead to eliminate the potential problems !