Best C Ratings for 18650 rechargable batteries question.

[boucaneer]

Hi, I'm moving from London to India with just a backpack and plenty of ideas.

I'm buying an Indian tricycle and fitting a geared hub wheel to help me over the hills. I could buy a battery from Em3EV but I would like to learn to construct my own battery pack for the future.

I have an Indian trader man who sells 18650's and he has asked me what capacity ratings I would like information and a quote on.

Now, this is quite new to me, yes, I am studying the subject after work but thought if there is anyone here who would like to offer the best advice concerning C ratings and even A rating. In fact any kind of prefered ratings for the construction of a 18650 battery pack.

I have just learnt that the C stands for capacity, although some sources say it stands for continuous. I will stick to capacity.

So a 1 Amp hour at 1C rating means it will discharge at 1 continuous amp an hour, a 1 amp hour battery at 2 C rating will discharge twice as quick at double the power, is this correct?

So you can comprehend my level of understanding with my above statement. Like I said I am just starting out.

Any support or advice to help me learn and gain an insight to building the battery pack would be greatly appreciated.

Just think if your ever in Kerala you can look me up and I will buy you a drink or a nice dinner.

Any links or posts would be great.

Cheers, Alex.

 

[wesnewell]

I have just learnt that the C stands for capacity, although some sources say it stands for continuous. I will stick to capacity.
So a 1 Amp hour at 1C rating means it will discharge at 1 continuous amp an hour, a 1 amp hour battery at 2 C rating will discharge twice as quick at double the power, is this correct?

That's true. but serves no practical purpose as to capacity. A batteries capacity is rated by amp hour (AH) or (WH) and C rating really has nothing to do with the capacity of the battery. C rating simply reflects the the rated discharge capacity of the battery. A 1C discharge rating means it's only rated to output 1 amp per AH capacity. So a 10ah 1C rated battery is rated to output 10A. C rating times AH rating = amperage out put rating. AH times nominal voltage = WH.
18650 cells were never designed for high power output and I'd never build a battery pack out of them. It just takes too many of them strapped together to get a decent output, which leaves many points of possible failures, and creates a nightmare for repair if the need arises. But if you're determined to use them, this may help.
http://lygte-info.dk/review/batteries2012/Common18650comparator.php

 

[wiredsim]

Hey you want to start here:

http://batteryuniversity.com/

C does not stand for Capacity or continuous, it doesn't really "stand" for anything. If you start with the question of the battery capacity you have to understand that batteries have two main measurements, their voltage (nominal voltage as the batteries voltage will be higher when it is fully charged versus discharged) and their Amp Hours. Voltage times Amps gives you watts so another measurement is Watt Hours as that is a total capacity measurement independent of the pack voltage

The Amp Hours is how essentially how BIG the battery is, the voltage is determined by the chemistry of the battery cells and how many cells you put together in series.

If we start at the "cell" level this is essentially the smallest a battery gets, a single AA Alkaline is a "cell" and the Alkaline chemistry is considered 1.5 volts. A Lead Acid cell is 2 volts, NiCd is 1.2 volts, Lithium Ion is 3.7 and Lithium Iron Phosphate is 3.2 volts.

If you have a 100ah Lithium phosphate cell then it will be considered 320 watt hours as the nominal voltage is 3.2 volts (so 3.2v x 100 ah). With that cell if you connected it to a 100 amps light bulb it should last for 1 hour, that is considered a 1C load, If you had a 50 amp light bulb it would last 2 hours (.5C load), if you had a 200 amp light bulb it would last 30 minutes (2C load).

Different batteries have different C ratings as a measurement of how many amps they can put out safely. Most batteries can put out far more then they are rated for but they may get very hot and damage the cells or worse.

The other MAJOR consideration is that there are lots of JUNK batteries out there. You can't trust the capacity values sellers tell you 8 times out of 10, at the least you need to buy from reputable vendors. If you look at eBay there are lots of "Ultrafire" and similiar junk 18650's that say that have 5000 mA hours (5amp hours), which is basically impossible for 18650's at this time. They may even tell you something that seems legit like 2500mAh, but will only ever give you 1000-2000 and die fast.

You need to do a lot more research before you buy anything in my opinion- 18650 batteries can be a good option, but there are others better yet for a beginner.

 

[boucaneer]

Thank you for your good information regarding this subject.
Yes, I'm not set on the 18560's, I just know that they are available in India and have seen a video by a young lad called Rinoa super genius, a young kid having fun with his projects.

If you may be able to suggest another battery which you think may be better suited I can research the availability of the battery cells and then study the construction. The necessity of the fuel cell is so important to me it actually energises me to study and digest the information and reading.

It's an answer to a freedom away from combustion engines and plus I havnt got an Indian driving licence, nor an English one either, just a boatmaster's licence as I work as a waterman. So to drive my trike in India would be a great freedom, and for that I need a good fuel cell that I can build and carry on building as the years get on.

So if you may have a suggestion I would be very grateful and appreciate it, then I can try to get my head around it.
Thank you and looking forward to seeing what it may involve.

I've just signed up to battery university and will begin my education.


Thank and Cheers, Alex.

 

[wesnewell]

All I use is rc lipo. My current 24s 10ah 88.8V pack is 20C, so rated for a 200A output. I can convert it to a 20ah 44.4V pack in seconds rated for a 400A output. Took me about 10 minutes to first assemble it. It's made up of 12 5ah 4s packs that just plug together.
http://www.rchelicopterfun.com/rc-lipo-batteries.html
http://scriptasylum.com/rc_speed/lipo.html

 

[friendly1uk]

It's fair to label ours high discharge rate. You can expect cells around 2.5Ah in size, and rates from 1.5c are useful. At 1.5c, 4 such cells in parallel could produce 15Amp for a small geared hub. Though running them on the limit is detrimental.

Cells I saw today just said 5A on them. Some others I had to look up and they were 10A. No C rating.

What do you need them to be?

 

[dogman dan]

Here's a bit of my battery rules for dummies. I need these since I'm such a dummy myself.

C ratings. a 10 ah pack, discharged at 1c, would be discharging at 10 amps. At 2c, 20 amps. C rate number x amp hour size is the theoretical max continuous amps.

Then, bear in mind that your battery doesn't exactly love running at it's max. What does? Ever had a car last longer if you drove it with the gas pedal floored all the time? Of course not. Peukerts effect is why batteries prefer a lower than max rate discharge.

So my rule of thumb is cut all c rates in half, to find the size battery you will need to cruise. Because electric motors are different from gas, cruise may be at full throttle.

I know this is getting complicated fast for the dummy version, so some more rules of thumb. About 400-600 watts will get you cruising at 20 mph/32kph.

Typical ebike controllers are 20-25 amps. so with a 36v battery, 20 mph cruise happens at about 15 amps continuous draw. Viola! now it just got simple. you need at least 15 amps to cruise on the flat.

Consiser your battery is 10 ah. Then to have 15 amps, AT, half the c rate the manufacturer lists, you need a battery with 3c rating. 3c for a 10 ah battery is 30 amps, and higher than controller amps if you have a 20-25 amps controller. Perfect. You are ideal while cruising, and not over when at max amps leaving a stop.

If your battery is 20 ah, then you'd need only 1.5c rating. and so on. The bigger your battery, the less it matters so much whether you have a great 10c rating, or a modest 2c rating.

Hope this helps you cut through to a practical answer for you a bit quicker. But do go learn about why these shortcuts make sense to me. The better you understand it the better. I'll never understand it all myself, but have learned enough from others here, to give reasonable practical advice.

Match your controllers amps, to the battery. If you go for big controller amps, either the battery size or the c rate has to increase.

 

[boucaneer]

Thanks guys, I will go through this tonight and then I will be back.

Very interesting and liberating.

 

[spinningmagnets]

http://batteryuniversity.com/learn/article/what_is_the_c_rate

Today, the battery industry uses C-rate to scale the charge and discharge current of a battery.

 

[agniusm]

...and one of the hihgest discharge rate has A123 APR18650M-1A but the capacity is 1100mAh.

 

[John in CR]

I have some 20C 18650's that are 1600mah ea of capacity. I used 4 strings of 5s7p to add 22ah of 10s to my bike when I went to high voltage and ran them for a year. They stayed in perfect balance that whole time, and while I only used them at 57% of their rating they never got the least bit warm. The 21ah of 20C and higher RC lipo that I used for the 20s portion of my pack did get slightly warm during my most extreme use. Unfortunately I had those 18650's for about a year before putting them on my bike, and now I can't get any more, because my guy went to work for a LiFePo4 factory.

Boucaneer,

The C rating tells you the current a battery is rated to be drawn from it. Simply take it's ah capacity and multiply by it's C rate, and that's the current in amps that it is rated for. Additional cells in parallel will add to that current capacity in direct proportion. eg The 20C 18650's I talked about above are 1.6ah of capacity, so they have a 20x1.6=32A rating. I ordered mine in blocks of 7 cells in welded in parallel, so each block had 11.2ah of capacity and was rated for 224A of current.

Those cells were quite cheap, just over $2/ea including the tab welding, but you want to be careful comparing batteries just on C rate, because there are much higher capacity 18650 batteries available that are rated for 20-25A that would have worked on my bike. Since their capacity is much higher their C rating is only 8-10C. For 18650's it's best to compare them based on the current rating instead of the C rating. C rates are more useful for RC lipo, where the capacity is quite similar for same size cells. With 18650's the cell capacity varies greatly, from the 1.1ah A123 18650 up to the latest and greatest cells that approach or exceed 3ah of capacity, and at a higher nominal voltage to boot.

 

[liveforphysics]

Sony VTC5 is the king of high power while still having excellent energy.

 

[Eclectic]

+1 on Dogman's recommendations.

One thing you might want to think about. In an ebike application, you cannot just pop an 18650 battery into a springy battery holder like you would in a flashlight or a toy. The way we use them, they require a much better connection. This usually means a specialized spot welder or something similar to connect the the individual batteries together to create a battery pack.

 

[liveforphysics]

+1 on Dogman's recommendations.

One thing you might want to think about. In an ebike application, you cannot just pop an 18650 battery into a springy battery holder like you would in a flashlight or a toy. The way we use them, they require a much better connection. This usually means a specialized spot welder or something similar to connect the the individual batteries together to create a battery pack.

Don't underestimate a pressure based contact done right. They can be extremely low resistance if designed properly.

 

[boucaneer]

Right I certainly have some good reading to do, that should keep me busy.
Thank you for the head start of current information. That's certainly put me on the right track and a good place to start. I will spend a few days digesting the information and try to come up with a good design with some spec's to view.

This information is so helpful to start me off, I am very pleased I have found this site.

Thank you all so very much.