What Exactly Does The "C" Rating Mean?

[The Mighty Volt]

Batteries are sold as being "C" rated at a certain figure, i.e. 1, 5, 15 etc.

What does this really mean, in layman terms?

Thanks.

 

[SilverSurfer]

its a discharge rate, I bleieve its the c number times the number of ah in the cell. ie. For a 1c cell that is 10ah, it can push 10a. 2c for same ah is 20a.


Edit = add ---- You raise your c rate by wiring in parallel, cause that raises the amp hours. So the end equation makes more amps available. Wiring in series, keeps the amps, but raises the volts. In general, the higher c rating will also take a faster (higher amp) charge.

 

[wasp]

c=capacity as in you have 5ah batteries running them at 1c is 5amps 2c would be 10amps ect
if running at 5amps lasted 1 hour then running 2c 10amps(same 5amp battery) would last only 30min

 

[SilverSurfer]

c=capacity as in you have 5ah batteries running them at 1c is 5amps 2c would be 10amps ect
if running at 5amps lasted 1 hour then running 2c 10amps(same 5amp battery) would last only 30min

I think ya need to be carefull. If a battery is rated at 1c, and you draw 2c you might have an issue.

 

[dak664]

C is the amp-hour storage capacity of the cell and I think universally 1C means a 1 hour charge or discharge rate, 2C a half hour, 3C 20 minutes, 10C 6 minutes. However manufacturers can and do quote the maximum C at some optimal discharge rate, without telling you what that is. A 10 amp-hour capacity at 2C should mean 20 amps for a half hour, but that 10 amp-hour rating probably came from a .1C discharge, and is really more like 9 amp-hours at 2C. So for fair comparisons you have to drill down to the actual amp-hour capacity at the given charge or discharge rate.

 

[dumbass]

its a discharge rate, I bleieve its the c number times the number of ah in the cell. ie. For a 1c cell that is 10ah, it can push 10a. 2c for same ah is 20a.


Edit = add ---- You raise your c rate by wiring in parallel, cause that raises the amp hours. So the end equation makes more amps available. Wiring in series, keeps the amps, but raises the volts. In general, the higher c rating will also take a faster (higher amp) charge.

Nope! You do not increase the C rating by paralleling packs. You increase the AHs of the pack but the C rating stays the same. In other words; if you started with a 10AH pack that had a 1c rating it would be 10a. If you then paralleled 2 of these packs you would have 20AHs at 1c or 20a. Yeah, I know this sounds like the same thing and in simple terms it is but not really. The reason is if a person parallels packs of different values (and they do it all the time).

 

[katou]

First a bit of Background:

An amp represents 6.25 x 10 exponent 18 electrons (or a COULOMB) flowing through a wire.

An amp hour means that one coulomb's worth of electrons is passing through a wire, for a period of one hour.

a battery that is rated for 10 ah can produce 10 coulombs of electrons, over a period of 10 hours.

Problem here is that due to the Peukert Effect, batteries do not produce what they should, when discharged over shorter periods of time.

Batteries are rated by discharging them over a really long period of time, like 20 hours or longer.

However, in the real world, we discharge them in a period of 5 hours. The shorter the period of discharge, the more pronounced the loss of power.

So, our 10 ah battery was tested for the 10 ah rating by discharging over 20 hrs, 1/2 amp load, voila, 10 ah.

We discharge it in 5 hours, and we find we get 8 ah out. Hmm, Peukert effect sucks.

We discharge it in 2 hrs, and we find we get 6 ah out. Peukert really sucks.

We discharge it in 1 hr, and we get 5 ah out. Dang.

The C rating of a battery indicates how fast it can be discharged without damaging the battery, or showing wicked voltage sag.

10 ah batt with 2c rating = spikes of up to 20 amps
10 ah batt with 10c rating = spikes of up to 100 amps
10 ah batt with 30c rating = spikes of up to 300 amps

Motors require power in uneven patterns. They are not constant load devices. They need power, a boatload of of it to combat the slight rise in the road, and a slight headwind, and all of a sudden, they don't need it anymore because the road flattened out. The spike or surge capacity of batteries is very important because of this.

Perfect example is here: http://endless-sphere.com/forums/viewtopic.php?t=15704#p233590

He's got a monster motor, but it is doing poorly because the batteries are SLA and can't handle the 300A spikes that the motor needs. Performance was very poor, realizing only a fraction of the massive power that the Etek is capable of.

C rate is very, very important. If you use a larger battery, you can compensate for low C rate:

10 ah 15C A123 pack = 150a surge capacity
30ah 5c Emoli pack = 150a surge capacity

The capacity of the packs aren't equal, but the surge capacity is.

Hope this helps, I've learned from this place, just trying to give some back. If you want more about Peukert, check this site out:

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

Katou

 

[The Mighty Volt]

c=capacity as in you have 5ah batteries running them at 1c is 5amps 2c would be 10amps ect
if running at 5amps lasted 1 hour then running 2c 10amps(same 5amp battery) would last only 30min

Thats great, thanks. I had a lot of suppliers throwing various C rates at me and I needed to be sure of my interpretation of "C". Thanks.

 

[The Mighty Volt]

First a bit of Background:

An amp represents 6.25 x 10 exponent 18 electrons (or a COULOMB) flowing through a wire.

An amp hour means that one coulomb's worth of electrons is passing through a wire, for a period of one hour.

a battery that is rated for 10 ah can produce 10 coulombs of electrons, over a period of 10 hours.

Problem here is that due to the Peukert Effect, batteries do not produce what they should, when discharged over shorter periods of time.

Batteries are rated by discharging them over a really long period of time, like 20 hours or longer.

However, in the real world, we discharge them in a period of 5 hours. The shorter the period of discharge, the more pronounced the loss of power.

So, our 10 ah battery was tested for the 10 ah rating by discharging over 20 hrs, 1/2 amp load, voila, 10 ah.

We discharge it in 5 hours, and we find we get 8 ah out. Hmm, Peukert effect sucks.

We discharge it in 2 hrs, and we find we get 6 ah out. Peukert really sucks.

We discharge it in 1 hr, and we get 5 ah out. Dang.

The C rating of a battery indicates how fast it can be discharged without damaging the battery, or showing wicked voltage sag.

10 ah batt with 2c rating = spikes of up to 20 amps
10 ah batt with 10c rating = spikes of up to 100 amps
10 ah batt with 30c rating = spikes of up to 300 amps

Motors require power in uneven patterns. They are not constant load devices. They need power, a boatload of of it to combat the slight rise in the road, and a slight headwind, and all of a sudden, they don't need it anymore because the road flattened out. The spike or surge capacity of batteries is very important because of this.

Perfect example is here: http://endless-sphere.com/forums/viewtopic.php?t=15704#p233590

He's got a monster motor, but it is doing poorly because the batteries are SLA and can't handle the 300A spikes that the motor needs. Performance was very poor, realizing only a fraction of the massive power that the Etek is capable of.

C rate is very, very important. If you use a larger battery, you can compensate for low C rate:

10 ah 15C A123 pack = 150a surge capacity
30ah 5c Emoli pack = 150a surge capacity

The capacity of the packs aren't equal, but the surge capacity is.

Hope this helps, I've learned from this place, just trying to give some back. If you want more about Peukert, check this site out:

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

Katou

WOW. Thanks for taking the time Katou. Thats a very lengthy breakdown. I appreciate the time.

 

[The Mighty Volt]

its a discharge rate, I bleieve its the c number times the number of ah in the cell. ie. For a 1c cell that is 10ah, it can push 10a. 2c for same ah is 20a.


Edit = add ---- You raise your c rate by wiring in parallel, cause that raises the amp hours. So the end equation makes more amps available. Wiring in series, keeps the amps, but raises the volts. In general, the higher c rating will also take a faster (higher amp) charge.

Nope! You do not increase the C rating by paralleling packs. You increase the AHs of the pack but the C rating stays the same. In other words; if you started with a 10AH pack that had a 1c rating it would be 10a. If you then paralleled 2 of these packs you would have 20AHs at 1c or 20a. Yeah, I know this sounds like the same thing and in simple terms it is but not really. The reason is if a person parallels packs of different values (and they do it all the time).

So simply put, if I have cheap 1C lifepo cells and I want a pack that can provide 20Amps, or which works well with a 20Amp rated controller, then would need a 20Ah battery made from 1C cells?

 

[dumbass]

its a discharge rate, I bleieve its the c number times the number of ah in the cell. ie. For a 1c cell that is 10ah, it can push 10a. 2c for same ah is 20a.


Edit = add ---- You raise your c rate by wiring in parallel, cause that raises the amp hours. So the end equation makes more amps available. Wiring in series, keeps the amps, but raises the volts. In general, the higher c rating will also take a faster (higher amp) charge.

Nope! You do not increase the C rating by paralleling packs. You increase the AHs of the pack but the C rating stays the same. In other words; if you started with a 10AH pack that had a 1c rating it would be 10a. If you then paralleled 2 of these packs you would have 20AHs at 1c or 20a. Yeah, I know this sounds like the same thing and in simple terms it is but not really. The reason is if a person parallels packs of different values (and they do it all the time).

So simply put, if I have cheap 1C lifepo cells and I want a pack that can provide 20Amps, or which works well with a 20Amp rated controller, then would need a 20Ah battery made from 1C cells?

You got it. A 20ah would be the bare min. I would recommend. Personally I wouldn't buy a 1C pack. the cost of a much better (but not the best) is a ThunderSky pack. I personally like the small 20ah cells sold by Elite Power Solution. they sell them as a 12v pack but contain 4 individual 3.2v cells that can be easily removed from the pack and configured in any voltage you need.

 

[The Mighty Volt]

Thanks for that.

I had made up an 11Ah pack from A123 M1 cells. I knew it wouldn't last long under strain but I couldn't understand why it was lasting for as little as it did. Somebody here joked that it would last 10 minutes but you know what, that figure is not far off.

The packs being sold by Volgood, Cammy, Vtac etc are all 1c-2c packs. Not worth a damn then.

Cheers and thanks again!

 

[wasp]

one other thing i would add is keep a eye on how the battery it tested eg over 1 hour or 20 hours big difference

 

[dumbass]

Thanks for that.

I had made up an 11Ah pack from A123 M1 cells. I knew it wouldn't last long under strain but I couldn't understand why it was lasting for as little as it did. Somebody here joked that it would last 10 minutes but you know what, that figure is not far off.

The packs being sold by Volgood, Cammy, Vtac etc are all 1c-2c packs. Not worth a damn then.

Cheers and thanks again!

There's more to determining a battery's quality then just the C rating though. For example the ThunderSky 20 ah cells are rated for 2C constant discharge and 10c intemitant. For my use this is perfect because I run 35a controllers. So my 24v 20AH packs are providing me 40 amps which is right inline with my max needs and I am not carring more battery then I need nor am I depending on high C rating to offset low AHs. So for my application these packs are perfect and they are cheaper and better quality then the crap Cammy or even Ping are selling. But that's just my opinion and other may disagree. The great thing about TS cells is they are prismatic with bolted ocnnectors. So if you have a cell failure it can be replaced quickly and easily.

 

[The Mighty Volt]

Thats a good point.

when my duct-tape 36v 20Ah battery was in use I was constantly perturbed by how heavy the mother was

I would have been happy to pay for a smaller {physically} battery made from better quality cells, rather than lumping the 36v 20Ah around all the time.

My whole thing is beating hills. In that case I would have been better off with a 5c 10Ah battery and the appropriate controller.

Lives and learns, I guess.

 

[tostino]

If that is what you want, learn more, and use Lipo.

 

[katou]

Mighty Volt, what happened to your pack? Would you like to discuss the problem?

A123 cells rock, way high C, durable as hell, and apparently very tolerant of oopsies during charging. I'll be getting myself some in a few days if I get lucky (may get some toolpacks from a fixit place near here).

The explanation is all math. The math explains it all. I just wish I was good at math, I'd understand it all much better myself.

Katou

 

[cell_man]

High C rate cells also make for a much more pleasant experience on the bike. If all your cells can do is just about deliver the required current according to the spec they will be sagging a lot. Manufacturers are not generally known for being conservative with their ratings, very optimistic/BS is often closer to the truth. This will impact on the power being delivered and will also likely lead to shortened life cycle of the pack.

A high C rate pack is also generally much less effected by the current level it is discharged at. It might deliver 95% or more of the it's capacity at 10C that it does at 1C, Whereas a 2C pack might only deliver 80-90% of it's capacity at 2C as opposed to 0.5C. High C packs might seem like a waste of time and money until you've actually tried a high power bike with a high C rate pack. The power required to give good acceleration is much more than to maintain a reasonable cruise speed but you only use that high current for some of the time. It also means that you don't need to carry round a pack twice as big as required to get the required power. IMO good cells are pretty high C rate, cheap cells are just cheap

 

[The Mighty Volt]

Mighty Volt, what happened to your pack? Would you like to discuss the problem?

A123 cells rock, way high C, durable as hell, and apparently very tolerant of oopsies during charging. I'll be getting myself some in a few days if I get lucky (may get some toolpacks from a fixit place near here).

The explanation is all math. The math explains it all. I just wish I was good at math, I'd understand it all much better myself.

Katou

Hi Katou....nothing happened to my A123 pack. I just fooled myself into thinking that 11Ah of A123 was as long-lived as 11Ah of 1c 18650. I now realize that this is a false economy, and not the proper way of thinking at all.

"Better" batteries are capable of discharging themselves faster. The drawback of this is that it means exactly what it says on the tin: they drain themselves faster.

In a way, all batteries are the same. Some just plod along for longer, doing less work. Others do more work, faster, and for less time. It is easy to trick yourself into thinking that 1Ah= 1Ah.

Of course, A123, as you correctly point out, has certain serious advantages which go beyond C rating issues. Hence the price.

 

[The Mighty Volt]

High C rate cells also make for a much more pleasant experience on the bike. If all your cells can do is just about deliver the required current according to the spec they will be sagging a lot. Manufacturers are not generally known for being conservative with their ratings, very optimistic/BS is often closer to the truth. This will impact on the power being delivered and will also likely lead to shortened life cycle of the pack.

A high C rate pack is also generally much less effected by the current level it is discharged at. It might deliver 95% or more of the it's capacity at 10C that it does at 1C, Whereas a 2C pack might only deliver 80-90% of it's capacity at 2C as opposed to 0.5C. High C packs might seem like a waste of time and money until you've actually tried a high power bike with a high C rate pack. The power required to give good acceleration is much more than to maintain a reasonable cruise speed but you only use that high current for some of the time. It also means that you don't need to carry round a pack twice as big as required to get the required power. IMO good cells are pretty high C rate, cheap cells are just cheap

Precisely. That is exactly my rationale.

I am pretty decent flat cyclist. I dont need a 20Ah or 30Ah battery made up of 1 or 2C cells.

I need a pack that will provide a phenomenal wallop for maybe 20 seconds.

Then, with the hill out of the way, I come off the power, and settle into the straights on pedal power.

Needless to say, no battery-pack retailers go into this in much detail with their customers.

A 10Ah battery made out of 1C cells is as good as useless. But a 12Ah pack made from A123 could do the job required.

It might not "last as long" as the 10Ah 1C variety but that's a misleading concept, as I have learned to my cost.

 

[katou]

I get what you are saying. It's like a car with a V8 (or LFP's 4 cylinder that makes what LFP, 400 hp?) If you have the acceleration on tap, it's oh so tempting to use it. And when you do, mileage per tank drops like a stone, in a vacuum, on a high-gravity planet, you get the idea.

The better way to do this with batteries, is to use the controller or a secondary device to amp-limit the system. I don't know how to do this, but others do. Apparently the Throttleizer will do this, but it might be as simple as leaving the throttle control in set positions, avoiding Wide Open Throttle (WOT). I haven't got there yet, so I don't know how to do it.

I guess it's like, hey, don't let the batteries amp-limit you, that's permanent. Let the controller amp-limit you, that can be changed or over ridden anytime you want to!

I know that you know some (or all) of this already, but I'm also writing for the lurkers who get lost with acronyms and miss the context of certain ideas. I was one of them, and WOT confused me for about 2 weeks.

Katou

 

[The Mighty Volt]

A lot of what I read here when I first joined confused me Katou!!

I genuinely thought that duct-tape was cutting edge. Not even close. It is to ebikes what leaf-spring is to suspension. Basic.

 

[cell_man]

A 10Ah 1C rated pack will probably give you maybe 9Ah if used at it's limit, whereas a high C 10Ah pack will give you it's rated 10Ah. Added to that the low C rated pack will deliver those Ah at a lower voltage. So the real picture is even worse when you consider the Whrs. A high C rated 36V nominal 10Ah pack would give you pretty much 360Whrs. A 36V 10Ah 1C pack might only maintain 32V nominal and supply say 9Ah, so total is 288Whrs or 20% less. To make the situation even worse the low C pack will trip the BMS very easily for the last 20% of the discharge as the voltage sag will be very bad and will not be able to deliver it's rated C rating. Better to use less throttle or have some way to set the controller to an economy mode.

Duct tape has got nothing to do what is inside, Cylindrical cells are not inferior to prismatic or pouch cells. Just because you put middle of the road prismatic cells into a fancy case with a fancy brand name doesn't make them better than a pack of A123 duct-taped and wrapped in a cardboard box

 

[The Mighty Volt]

Well actually a 10Ah 1C rated pack will probably give you maybe 9Ah if used at it's limit, whereas a high C 10Ah pack will give you it's rated 10Ah. Added to that it will lower voltage whilst supplying those Ah. So the real picture is even worse when you consider the Whrs. A high 36V nominal 10Ah pack would give you pretty much 360Whrs. A 36V 1C pack might only maintain 32V nominal and supply say 9Ah, so total is 288Whrs or 20% less. To make the situation even worse the low C pack will trip the BMS very easily for the last 20% of the discharge as the voltage sag will be very bad and will not be able to deliver it's rated C rating.

Duct tape has got nothing to do what is inside, Cylindrical cells are not inferior to prismatic or pouch cells. Just because you put middle of the road prismatic cells into a fancy case with a fancy brand name doesn't make them better than a pack of A123 duct-taped and wrapped in a cardboard box

Ah yes, that is absolutely true. When I say "Duct" I mean Cammy_CC and Volgood.....and they are most definitely not using A123. To the best of my knowledge they are using a 50g 1.5Ah 1C cell in the 18650 format.

 

[dogman dan]

Lately it seems to me that the c rating may mean a number you cut in half, just like you cut a manufacturers claims for range in half.

So a 5c headway is best used at 2.5 c, a 2c ping is best used at 1 c, 5 c nicads are best used at 2.5c etc. Sure, the battery can do the rated c rate, but for long lifespan, low voltage sag, etc figure on getting enough battery size to supply the amp rate at half the c rating they rate the cell at.

This can get to be meaningless though, once into really high c rates, like lipo that might be 30 c cells. Cut that in half and you can still get 75 amps out of a 5 ah pack.