C-rate

[sacharlie]

From reading the posts here I get the idea that a LiPo listed with a 3C rating is better than one with a 4C...is that right?
I can't keep it in my head so always refer to this little cheat sheet.
If it displays in order the center is the 1C/1hr.
rate:****4C*******2C******1C*****0.5C*******0.25C
time:***0.25hr****0.5hr****1hr*****2hr********4hr

A calb 70ah from a dealer shows 4C continuous and 12C pulse. I don't know what to derive from this but the following. It will put out ~70amps for 1hr which is 1C. At 4C it will put out 4x70=280amps for 15 minutes and at 12C it will put out 12x70=840amps for 5 minutes. Am I understanding that right?

[Fulliautomatix]

Yes...but there will be a burst time limit.
The calb cells that I see...eg - http://www.thunderstruck-ev.com/calb-70ah.html all say 0.3C with a 10C burst?
This seems like a 'probably would work but not the best solution' battery.

[sacharlie]

Funny that you linked to the same website I looked at. I didn't see the info you posted just this.
http://www.thunderstruck-ev.com/batteri ... m-ion.html

[Burgerman]

You need the highest C rate you can get.

A 70Ah cell, that is 1C continuous CAN do 70Amps until discharged. Why not more? Because its internal resistance / surface area means it will get hotter than is safe for the battery. A 10C cell can do 700 Amps. So to keep away from this damage/volt drop we are better off using the 10C cells. So hi C rate cells generally are lower resistance, and better for the sort of use we put them to. Prismatic cells are less good here since one big block has less cooling external area than a bunch of round cells such as the headway ones that are spaced apart.

The burst current is for very short times, based on resistance and instant cell damage. Again the damage is caused by oxygen being created in some reaction I dont understand - pushing the many very thin plates apart. It makes the battery swell. The plastic case and flat construction of the prismatic cells allows this more easily. So round cells with a metal can are better here. No expansion possible, so internal plates stay in tight contact.

Some of the newest lithium battery packs I use in my hobby stuff are 50C and70C. This means you could fully discharge ine in 1 minute! Now we dont need this. But occasional bursts of super power (zap a helicopter up to 500 feet in a few seconds) does need this. And it means you suffer less battery damage at low c rate use.

The smaller the wheelchair battery Ah is, the higher the C rate you need. Also the higher the C rate the less voltage drop under load.

[ex-Gooserider]

The cheat sheet is right for Lithium batteries, but the rest isn't quite...

The higher the "C" rating, the better, regardless of how fast you are actually going to be discharging / charging. The higher the rate, the lower the internal resistance of the battery, which means less heating at any given discharge rate, and less concern about voltage drop at high rates....

Thus a 4C rate battery is better than a 3C rate, though the difference isn't going to be huge with just a one digit difference... OTOH, a 10C battery is going to be MUCH better than a 5C one.

Also, it is important to note that you should always avoid discharging at a higher C-rate than the manufacturers rating. Thus

A calb 70ah from a dealer shows 4C continuous and 12C pulse. I don't know what to derive from this but the following. It will put out ~70amps for 1hr which is 1C. At 4C it will put out 4x70=280amps for 15 minutes and at 12C it will put out 12x70=840amps for 5 minutes

Should not be discharged faster than the 4C rate, with the exception that one can draw at up to 12C for BRIEF surges, no more than a couple of seconds - i.e. charging a capacitor or spinning up a motor. Exceeding the specified max will lead to excessive heat generation and possible battery damage.

Also you are probably not going to get the full rated capacity out of the battery if you try to discharge it at the constant high C rate, though lithiums are much better about this than lead-acids.

This is why BM is using Headway cells, and so many of them, in his pack for the BM3. The headways have a higher C-rate, and by putting so many cells in parallel, he avoids ever getting even close to that higher number.

ex-Gooserider

[Burgerman]

So calb is OK on BIG packs where the capacity in very large since C rate is low because of the pack size compared to the draw.

On a powerchair high rate cells are better because we have high amp demands and small batteries compared to say a car. The headway cells are 10C and 20C pulse. So a 100Ah pack can do 1000 amps. But this means a 240 amp wheelchair controller such as the R-Net 120, will be just 25 percent of the batteries life shortening limit. A good same place to be, with little volt drop. A 50Amp hour pack, at 10C would still have enough headroom for safety/longevity. But I would hesitate to go smaller without higher C rate cells.

[sacharlie]

Yes...but there will be a burst time limit.
The calb cells that I see...eg - http://www.thunderstruck-ev.com/calb-70ah.html all say 0.3C with a 10C burst?
This seems like a 'probably would work but not the best solution' battery.

Thanks to everyone for the replies, I am getting a better understanding of the battery.

Looking at the graph in the above link and not wanting to draw DoD below 3v. With a 7amp load is that graph showing a time of roughly 70/7=10hours?

[Burgerman]

Yes. There is no peukert problem with Lithiums.

But we dont do 7 amps unless coasting. So while under load the cells voltage will fall much lower, then recover instantly as you take away this load. This is mostly its resistance.

Remember that we can take 200 amps with a 100 amp controller. 240 amps with a 120 amp controller. (each motor is seperate). So under a "ramp" condition, low C rate cells may fall below 3v even if charged well.

So with just 70 Ah worth of cells, that 200 / 240 amps is 3C. And right on the very limit of what the cells are "safe" with. They will work. But voltage drop under load will be high (for a lithium) and go below your preferred 3v per cell more easily. And the closer you are to the very upper limit of the cells max C rate the more you damage them. Best cycle life is when you take much less. Like half.

Now you have a max 10 seconds at higher C rate, but its best not to go there if it can be avoided... And a steep hill, for eg may mean 3C for considerably longer than 10 seconds.

This was the primary reason I used 10C cells, and 15C peak. To stay away from the volt drop, life shortening affect of drawing the max rated C output from the cells.

That said, you should not be looking at 70Ah cells! The whole point of going to lithium, is that you can get far more Ah in the same space, and with less weight. When you do this, then you dont need such a high C rate, as the whole pack is bigger anyway. So if you fit say 100Ah of calb batteries then you are down to approx 2C discharge, so less volt drop, and greater headroom/safety margin and better cell life.

Again though, they are bulkier than the headways. So that isnt so easy to do. It is easy to fit 108Ah in the same space as the pair of group 24s if you use 10C Headway cells though. And then you are miles away from any limits, can charge faster, and draw over 1000 amps without exceeding their C rate and get better range or a lower DoD. Result? Better range, cycle life, no danger of cell damage etc.

[sacharlie]

BM thank you for this latest post. Its funny I was reading over some posts here on Calb vs Headway and your reason for using Headway but was not quit picking it up. My thought was I was not going to question the guy that I have learned so much about powerchairs from as he is just seeing something that I can't see but this post really explained it all for me.

[Burgerman]

Its easy when you get your head around it.

[Burgerman]

http://www.wheelchairdriver.com/pdf-files/battery.pdf

sacharlie, In reply to your battery and what voltage is the 80 percent point (junkie forum) I posted the file from MK above. That covers their AGM and Gel batteries. Its a very correct and interesting read. Note also how charging at .7 of a volt too high, shortens service life by half! If you look at the gel charge voltage for a typical 70 degrees C house, its about 14.1v MAX.

The answer you are looking for (80 percent discharge voltage) is about 11.8v - After 24 hours wait - but it wont help you!

A FULLY charged lead battery has some strange affects, which are a complex mix of Peukert (surface charge/discharge on the "face" of the plates) and internal resistance, acid stratification (acid strength gets "used up" next to the plate surface on fast discharge) etc.

So a fully charged battery drops to say 10 volts if you pull 200 amps from it as your powerchair does at times. And keeps falling fast if you keep it up. (Thats the same as a wheelie/ramp) - so it cant "cut off" at a fixed point. This can be 8 volts per battery if already a bit discharged...

Thats LESS than the open circuit voltage of a 100 percent dead one... After the load is removed, the voltage rises over time, and takes some 5 to 24 hours to fully recover. The reverse happens on charge. Which is why you cannot tell the state of charge without disconnecting the battery, and leaving it for 24 hours. As such there isnt a cut off voltage as such on a power chair, they just throttle amps back, so as to maintain a safe control function working voltage. So you lose torque.

See http://www.wheelchairdriver.com/pdf-files/gel.avi Reversed to a wall, gel battery fully charged. Drops from 13.x to 9.8v under load.

[Fulliautomatix]

So with just 70 Ah worth of cells, that 200 / 240 amps is 3C. And right on the very limit of what the cells are "safe" with.

But it says 0.3C.
21A recommended dis/recharge current.

[Burgerman]

I was talking about max continuous rate, 3C.

Most of the day your wheelchair will be taking about just 200ma sat at a PC or in a bar...
As you move, the brakes take .7 amp, the motors anything from a few to a maximum possible amps depending. A few amps and in normal use on a flat surface, means 20 to 40 amps turning, 10 to 15 amps travelling at full speed on the flat, but spikes of more when correcting or turning.

So in the main, you are taking less than 20 amps. If not your batteries would last 3 hours to 100 percent dead... (Thats what 1/3rd of a C means) I never used a chair where the batteries were dead in normal use in 3 hours. Thats 0.3C

But average is very different to say a ramp where you may need full power (3C), or a steep hill for maybe a couple of minutes.

Average daily use is under 1/3rd (0.3) C. Sport may be 1C. Average, with many huge peaks. So a battery capable of high amps, is still required. This is an area that lead fails dismally, and where Odyssey is as good as they get.

[Fulliautomatix]

I was talking about max continuous rate, 3C.

Ahh! A completer spec would tell me this!
http://liionbms.com/pdf/aleees/SE70AHA.pdf
I thought 0.3C was very low and quite useless...

[sin85]

i am confused, is this a real 10c or pulse. is this good for us
http://www.made-in-china.com/showroom/d ... -Cell.html

[Burgerman]

Yep, headway are 10C quite low resistance, and even 5C charge rate.

See www.evassemble.com

[sin85]

this is an email i got from from evassemble:

Hello Sinisa,
Thanks for your qeury. It's our pleasure to serve you.
Capacity is depend on the Ah you require. C rated is about how big current the battery pack can discharge. For electric vehicle application, long cycle life is one of the most important factor. To keep the battery pack in good cycle life, we suggest to make the battery pack 1-2 C rated discharge and 3-3.5C max.
Regarding the dimension, please provide dimensions with highxlengthxwidth. And we will provide you pack dimension solution accordingly.
You can also let us know the power rate of your device so that we can check the BMS setting it will requires.

Best regards,
Michael Cheung

is he wrong or what?

what does he mean by power rate?

[Burgerman]

And saying exactly what I have done already. So no, absolutely right.

He is saying make the discharge LOW C and lower than the battery C rate...

Its like this.
C rate, of the CELL, is its load (amps) capability. A 10Ah cell that is 1C can be discharged at 1C continuously. Or some other higher PULSE or short term figure for short periods.
This C rate depends on many things.

a) max peak amps before damage is caused. Theres no "point" where this happens, the lower the current, or the higher the C rate the better. And the longer the cell will last.

b) resistance (internal) causes volt drop under load. And heats the cell and wastes power.

c) heat. Depends on cooling and resistance. Less cooling, or greater resistance is worse. It lowers the cont. load possible. (so round metal shell cells, spaced apart with free air space are better than monoblock or prismatic cells).

d) construction and chemistry, some are much better (tougher) and can stand higher charge/discharge rates, and huge pulse currents.

So higher C rating is always preferable.

So if you use say 3C cells, and build a 100 Ah pack, you COULD take 300 amps from it. But life will be short. And depending on IR voltage drop may be high. A typical 240 amp maximum controller (like the DX2 oir R-net) can take a 240 amp load from the batteries for a few secs, and 200 amps for quite a while on a steep long ramp for eg. So you are almost at the MAX the 3 cell allows.

So to stay away from its limits, its better to use 5 or 10C rated cells. These will last longer, have lower internal resistance so lower volt drop, and offer better performance and less energy wasted as heat in the batteries. Hence the 10C headways I am using.

[sin85]

in a 140 Ah configuration and 150Ah controller would these be at risk of damage
Model name SE70AHA Alternative product marking CSE70AH
Nominal voltage 3.2 V Operating voltage under load is 3.0 V
Capacity 70 AH +/- 5%
Internal impenetrableness <1 mOhm 1kHz AC
Operating voltage min 2.6V - max 3,6 V At 80% DOD
Discharging cut-off voltage 2.5 V The cells is damaged if voltage drops bellow this level
Charging cut-off voltage 3.65 V The cells is damaged if voltage exceeds this level
Recommended charging - discharging Current 20 A 0.3 C
Maximum short-time discharging current 700 A 10 C period=10s
Life cycles 2000 0.3C, 80% DDC
Operating thermal ambient - charging 0°C ~ 45°C The battery temperature should not increase this level
Operating thermal ambient - discharging -20°C ~ 55°C The battery temperature should not increase this level
Storage thermal Ambient -20°C ~ 45°C The battery temperature should not increase this level
Shell Material Plactic flame retardants
Dimensions 113 x 60,5 x 206 mm Millimeters (tolerance +/- 1 mm)
Weight 2,5 kg Kilograms (tolerance +/- 150g)

[Burgerman]

Recommended charging - discharging Current 20 A 0.3 C

Recommended charge or discharge 20Amps. .3 C? Seems very very low.
PEAK (Pulse) 700 amp. Like when blowing a fuse or charging a capacitor etc. For an instant.

140Ah battery, at .3C is (rounded) 50 Amps. But that battery is 70Ah??? http://en.calb.cn/product/show/?id-599
140Ah battery, at 1C is 140 Amps. But half that for a 70Ah batt. But only in a shorted situation most likely...

150Ah controller would these be at risk of damage

Controllers are rated in amps. A 140 Amp controller will pull a maximum of 280 Amps. (2 motors two seperate channels) but nobody makes a 140 Amp mobility controller. So yes.

[Burgerman]

I have to add, that in automotive use, the low C rate of these cells is not a problem because you have room for a huge stack of them. In a powerchair thats not so easy. But the usual quoted C rate for these is 3.

Compared to the headways that I chose to use see specs here for a small 12Ah cell: http://www.evassemble.com/index.php?mai ... ucts_id=21

[sin85]

sorry they would be connected in 24V so 8s 8p =140 H. i have 1 motor, 150A controller

[sin85]

140Ah

[Burgerman]

24V so 8s 8p =140 H

That will give 560Ah... (8x 70Ah in parallel). And 24v.

How many actual cells will you use? Because that would power my van.

[sin85]

it is late and i am making mistakes 16 cells in total
8s for 24V

[sin85]

2p

[Burgerman]

OK lets guess. Either 1 set of 8 in series. Or two strings in parallel?

8 cells in series will give 70Ah and take slightly less space than a set of powerchair grp 24 batteries.
16 cells will not fit any powerchair but might fit some big scooters?? Dont know.

But I wouldnt have expected that to be the case. If it will then I expect that 140Ah and 150Amp controller would be fine, since these cells were rated at 3C by most suppliers.

If its 70Ah (8 cells in series) then its a bit borderline. But should be OK, with possible voltage sag under heavy load or shortened serice life. They quote 2000 cycles at 20 amps load.

[Burgerman]

You replied while I was typing!

See here. The 70Amp discharge curve is what you will get at 1C (140Ah pack at 140 amps discharge or 70 at 70, same thing). You will lose a few volts on a hill compared to the headways all else being equal.

These used to be rated at 3C its now "officially" been reduced to 0.3C. A TEN TIMES REDUCTION!
Same with Headways. These are actually 10C but official line is now 3C. A THREE TIMES REDUCTION

In both cases its for warranty reasons, they dont like taking dead cells back and people use them in too small a packs... Or charge with dumb chargers and BMS systems and kill them.

So I would ignore the "new" figures for the Calbs and the Headways... EV and Bycycle people have been using these CALBS at 3C and Headways at 10C without issue for years - If controlled conditions. Its bad basic cheap BMS systems that kill the cells...

[sin85]

but i am not sure i want to use those. i can fit 104of these and have 20 C and cca 200 Ah A123 Systems Prismatic Pouch Type 20Ah Automotive Class Lithium Ion Cell AMP20M1HD-A

[Burgerman]

For about 3k. And a mass of soldered connections and cell balancing wires...

Already looked at those years ago. Soft plastic bag fragile cases worry me when sat over them.

http://www.youtube.com/watch?v=EM6V2h5uto0 punctured plasic bag. With a nail admitedly. But a tiny tear and you get big problems with smoke if not fire.

Theres a guy on this forum with 200Ah of them in a chair, but he got hold of some used ones.