Lipos

[Johnnie]

Bit confused about the designation of Lipo batteries. 3S, 2S etc. What does it mean. I see some advertisements for Lipos that dont mention that designation.

Johnnie

[Phil Meredith]

3S 2S is the number of cells in a pack,
ie a 3S is a three cell pack at 11.1 volts,
3.7 volts per cell,
there are lots of other things to consider when buying lipo batteries but am still trying to get my head round a lot of it myelf, hopefully someone who speeks plain english will pick up this thread and we can both learn something,,
phil

[Alistair Taylor]

Hmmmm - not many replies to your question Johnnie.

Similarly, I'm not expert, but the following may help.

As well as putting cells in series (e.g. 1s, 2s, 3s) so that their voltages add up, you can also connect them in parallel, so that the voltage remains the same, but the capacity available adds up (e.g 1s1p, 1s2p, 1s3p).

In the first case (series) you connect the -ve terminal of the first cell to the +ve terminal of the second (and so on) the same as if you had a row of batteries end to end e.g. in a torch. The speed controller would then be connected to the -ve and +ve connections at each end.

In the second (parallel) you would link all the -ve terminals together to one common terminal, and all the +ve terminals together to a separate common terminal, a bit like sticking four straws into four separate cans of fizz, and drinking from them simultaneously (this is a crap analogy but I can't think of anything better). The speed controller is connected to the common +ve and -ve terminals shared by all the cells.

e.g. with 3.7V 1000MaH lipo cells

1s1p = 3.7v 1000mah
2s1p = 7.4v 1000mah
1s2p = 3.7v 2000mah
3s1p = 11.1v 1000mah
1s3p = 3.7v 3000mah
3s2p = 7.4v 2000Mah
2s3p = 11.1v 3000Mah
3s3p = 11.1v 3000mah

The other major consideration is how many amps the lipo cells are capable of safely supplying - the much touted "C rating".

Bear in mind that milliamps (Ma) are 1/1000 amps

The C rating purports to show how many amps a lipo cell can supply, at its limit.

e.g. 3.7v 1000Mah lipo cell, with 10C rating

10 x 1000Mah = 10,000Mah, or 10 amps (10,000/1000)

Take the C-rating as an optimistic maximum never to be exceeded, and you shouldn't go far wrong. Treat it as a target to aim for, and there will be trouble ahead. The harder you push any battery, the shorter its useful lifetime.


AlistairT

[Johnnie]

Thanks for the answer Alistair.
Still a bit confused about the C rating though.
I thought that if I was wanting to charge a 1000 Mah battery say with a 100 ma charger then I would have to charge it for 10 hours. So simplistically in reverse then I could draw 100 ma for 10 hours or 1 amp for 1 hour etc.
So in your example above for a 1000 Mah battery I would expect to draw 10 amps for 6 minutes.
So where does the 10,000 Mah come in.

Johnnie

[Aslan]

This might also help.Your reverse theory sounds good but things arn't what they seem Johnnie. Different lipos of the same capacity (e.g. 1000Ma) have different C rates.One might only be able to supply 5C(5000Mah or 5 Amps)and another might be able to supply 10C(10,000Mah or 10 amps)Then you get into amps a motor draws
making sure your motor does not draw more than the lipos can provide.......and it goes on and on and on....Also Nicad and Nimh batteries are a different kettle of fish altogether.A different type of charger is needed for Lipos. DO NOT use a standard charger for Lipos.(Not assuming you will,but just in case Johnnie) Lipos are generally only used in electric flight.Good luck

[Johnnie]

Just found another defintion of C on the internet.
Current is rated in C's. C is how long it takes to discharge the battery in fractions of an hour. For instance 1 C discharges the battery in 1/1 hours or 1 hour. 2 C discharges the battery in ½ or half an hour.
So I guess that is the same logic as I used above i.e. a 10C battery will discharge in 6 minutes.
Based on this definition if the battery above was a 2000 Mah capacity and it was 10C the max discharge current would be 20A.
Now I am really confused

[Aslan]

As Aistair said the C rate is how many amps a Lipo can supply.....Forget about the fractions of an hour one....The important figure is the Amps a battery can supply,the length of time the battery lasts will depend on the amps being drawn.

[Mike Rolls]

Johhnie
What you are being told is good information. Just one aspect I don't thnk has been clearly covered;
As has been said you will find different Li-pos quoting different 'C' ratings and these are the maximum safe dischage limit. They are NOT the safe charging limit. The vast majority of Li-pos must not be charged at greater than 1C regardless of their maximum discharge rate. There are now some cells coming on the market capapble of being charged at higher rates (2C-3C) but they are very much the exception.

It is vital not to charge Li-pos at an excessive rate. At the very least they will be ruined; at worst they will will burst into flames.

Mike

[Aslan]

Good point Mike.It is very important to do your homework on lipos Johnnie,donm't treat them in the same way as other batteries at all.Would strongly recommend using a cell balancer as well.

[Alistair Taylor]

You wait three days, and suddenly three answers all arrive at the same time :o)

Coming at this from another angle...

To work out what lipos you need, first work out how many amps your motor setup is drawing (or alternatively how much power your plane needs to fly).

e.g. A parkflyer (small-ish but not an indoor plane) is going to need 150-200watts for reasonably exciting performance.

Watts = Volts x Amps

So a 3s lipo (11.1V) will need to supply between 13 and 18 amps to achieve this.

11.1V x 13A = 144W
11.1V x 18A = 200W

So for this sort of setup you would need a 10C 3S1P pack made of cells with at least 1300 MaH capacity(13amps/10), but preferably closer to or greater than 2000MaH capacity to allow a safe margin

A 20C 3S1P pack would need to be made of cells with at least 750MaH capacity (13amps/20) but preferably closer to or greater than 1000MaH (bearing in mind that it will not fly for that long with only 750MaH available)

A 2s lipo (7.4V) will need to supply between 20 and 27 amps to achieve this.

7.4V x 20A = 144W
7.4V x 27A = 200W

So a 10C 2S1P pack must be made of at least 2000MaH cells, preferably 2700MaH or more.

Again - this is coming at things from the other angle - first work out how much power you need, then work out what pack you need as a minimum.

Now revisit the advice on pairing packs - if you decide to use a 3S2P pack (six cells in total in two banks of three - 11.1V but twice the MaH or capacity to deliver amps) you can effectively halve the amps drawn on each cell.

So a 3S2P pack of 10C cells could be (ooh my brain, it's too early) made of cells of (minimum) 750 MaH capacity (13 amps/[10x2]), because pairing them would effectively double this to.....1300MaH. Although 1000MaH cells would allow a greater safety margin (and longer flights)

A 3S2P pack of 20C cells could use 375MaH cells (but preferably greater capacity for safety).

A 2S2P pack of 10C cells could be made of cells between 1000MaH and 1400MaH capacity for the same reason (1000 x 2 = 2000Mah, 2000MaH x 10 = 20amps).

Important rule - when you start pairing up packs, you're going to need some way of making sure that they are all in the same state of charge - i.e. a balancer. If the cells are not matched up, things will get hot, smokey and go bang in short order.

Hopefully this helps. Working out what lipo you need does not require quantum physics, but there's a lot of algebra in it.

If you wonder why I keep talking about safe margins, google "lipo fire" and have a look at the you tube videos of lipo fireballs. It's quite salutary. Treat them with the sort of respect reserved for chainsaws and driving in london and you'll be OK.


AlistairT

[Johnnie]

Mike
Not sure what is the good information you mean. i.e. the forum or the internet as they are at odds with each other.
If you Google "Lipo safe discharge current" there is overwhelming agreement about the meaning of the C rating of a battery. It is not the maximum current that the battery can supply. However, the maximum current can be derived from the C value using the following.

Max current = Mah capacity of battery X C rating / 1000.

From one particular manufacturer you can find the following information on their batteries (and I am presuming that they know best)

the figures are in order Capacity(Mah), C Rating, Max Discharge current.

450, 25, 11.3
600, 25, 15
850, 25, 21.3
1000, 25, 25

etc
etc

You will be able to see that this agrees with the above formulae.

As Aslan says above the important figure is how much the battery can supply and from that the length of time the battery will last can be calculated from the capacity. However, the trick is to know what that amperage figure is.

Johhnie

[Johnnie]

Alistair
Took so long to write my reply and trying to do my day job that our messages crossed. Need some time to work it out but I think you are agreeing with the definition of C.

John

[Alistair Taylor]

John

you're right that the C-rating is not itself the maximum amperage figure, but it indicates this, so most people just say that the C-rating is how many amps a lipo can supply.

I find it easier just to convert most MaH figures into amps, then you're dealing with one or two digits, not six!

So a 1300MaH battery is....a 1.3 AH battery...i.e. it will deliver 1.3 amps for an hour, or 13 amps for a tenth of an hour (six minutes).

If it's a lipo, you'd charge it at 1.3amps maximum. If it's a 10C lipo, you'd discharge it at 13 amps maximum (and expect it to last six minutes).

Hope this helps

Alistair

[Johnnie]

Thanks Alistair
Clear to me now.

If anybody from the magazine is reading this, this would make a great subject for an article.

Johnnie

[Aslan]

Sorry Johnnie you've completely lost me here. The figures you gave(from the internet) exactly match the formulas given on this thread.The reason most people would use the C rating as the max amps a lipo can supply is because it is important to know when choosing a motor etc. although the max discharge rate would obviously be the same,when discharging for battery maintenance purposes,you would not discharge at the max rate.This might sound confusing but... .......it is:):)

[Alistair Taylor]

Aslan

I think the confusion is in the terminology.

A 20C rating doesn't mean that a lipo battery can supply 20 amps, which, if you were new to lipos, you could be forgiven for assuming if someone said to you the C-rating is the max amps a lipo can supply. The C-rating is really just a component of the formula outlined above that will enable you to calculate the max amps a lipo will supply.

In a similar vein, a 150Watt power system consumes 150 watts of power from the battery, but the airframe itself does not experience 150Watts of thrust (Ok I know you measure thrust in Oz or g but stay with me), due to inefficiency losses in the electronics, motor (gearbox?) and prop.

AliT

[Johnnie]

Aslan
You have lost me or we are talking at cross purposes.

For example.
A 25C lipo battery does not necessarily have a max current discharge rating of 25amps.

If the lipo is 1000 Mah then the max current would be 25 amps.
If the lipo is 500 Mah then the max current would be 12.5 amps.
If the lipo is 2000 Mah then the max current would be 50 amps.

In other words the C rating of the battery only coincides with the max amps if the battery is a 1000 Mah battery.

Rereading the thread again and your responses I can see you have some confusion between Mah and Amps. The two units are completely different. Amps is a measure of current flow and Mah is a measure of capacity. On your post of the 30th a battery cannot have a capacity of 1000 Ma - the correct term would be a capacity of 1000 Mah. This means that the battery could theoretically supply a current of 1000 Ma for one hour or alternative 100 Ma for Ten hours and so forth.

In the your same post a 5C , 5000 Mah battery would have a max current of 25Amps not 5 amps and a 10C, 10,000 Mah battery would have a max current of 100 Amps and not 10 amps.

The reason why we need to know the max amps is crystal clear.

Johnnie

[Alistair Taylor]

My brain hasn't ached this much since trying to sort out the mortgage.

I ought to own up at this point that I only own three lipos - two in X-twin planes, one in a PiccoZ.

They are, or course, locked away in a flame proof steel cabinet, sat in a bath of salty water, wrapped in flame retardant blankets, in an isolated asbestos shed in the middle of a lake.

AliT

[Tim Mackey]

Not sure if this has been mentioned...confess to NOT having read ALL the responses in detail :( but please rememeber if you use any of these formulas to decide a suitable pack, the REAL voltage of a 3s LiPo when in use, is more likely to be around 10V NOT 11.1V. Secondly, and more importantly, if you use a battery, or calculate your require battery based on it actually being used at its maximum potential C rating,2 things will result.
1) A dissapointingly short pack life overall.
2) An equally poor flight time !!
IE if a pack is rated at 20C, and you use it at 20C, this equates to 3 mins duration!! If you use the same pack at 10C, flight time is doubled, and batterylife will be much longer. This marketing hype that is currently fashionable to claim vast C ratings is pretty stupid really. I now use F/Power packs in all my electric powered planes, but never use them at more than 50% of their claimed C rate. Even F/P claim huge C rates, with even bigger "burst" C rate figures, but as far as I am concerned, it is NOT the way to use your LiPos.

[Johnnie]

Alistair
Know what you mean. However, I am convinced that the paracetamol has something to do with the fact that in 5 days i have advanced from knowing nothing about lipos to thinking that I am an expert!!
Timbo
You make an execellent point. It would not the way to go to run these batteries at max current ratings. However, finding out what that is makes it easy to run them at 50%.

Now back to the day job

Johnnie

[Mike Rolls]

Johnnie
It's looks as though you have got there.

BTW - when I said 'good advice' I was referring to the answers you had received here uyp to then (and since). Don't know what you had read on the net at that point
Regards
Mike

[Johnnie]

Mike
The fact is what I read on the internet was correct - i.e was the good advice when it came to the C rating. There is no doubt that some of the replies received were suggesting that the C rating was the max current e.g. if it was a 25C battery then the maximum current was 25 amps. This is absolute nonsense. So that advice was also absolute nonsense. There was lots of good replies and information thought on the designation of batteries etc.

Johnnie

[Aslan]

O.K..... here we go....if the C rating is 25 on a 1000Mah battery the max that battery can supply is 25 x 1000 = 25,000miliamps =25amps. If the C rating is 25 on a 500Mah battery the max that battery can supply is 25 x 500 = 12,500miliamps = 12.5amps.The C rating is relevant to the mili amps per hour....I,m getting very angry here,there is no nonsense been spoken here.The advice been given here by all is good advice.Not all batteries of the same capacity have the same £%*($ng C rating. You really need to think about it before you make comments like that.Again I stress that what you read on the internet is exactly what you read here.The 25C battery with a max of 25amps is based on a 1000Mah(or 1amp)battery...DAW
Oh,and Timbo is right,the max is not the way to use them.

[Alistair Taylor]

Johnnie

Aslan, Miek and myself have taken the time to explain lipos for you as best we can, voluntarily, unpaid, and at the risk of a rollicking if we're at work while doing it. Regardless of whether you take this advice or not (based on your understanding of whether it is right or not), calling it absolute nonsense could..dare I say it.. be considered a tad ungrateful? Aslan made the vital point that you don't use the C-rating as a guide to charging. Ignoring this could lead to you destroying your packs before you've even had a flight out of them.

Aslan

I read back to the first examples we gave - and we both used easy to deal with numbers - i.e. 10C and 1000Ma

So 10C was 10 amps and 5C 5 amps.

I think this is what confused matters.

Do you fly with lipos? How do deal with charging/avoiding getting closely aquainted with the fire brigade? I fly with NiMH and NiCads at the moment, but I kep toying with the idea of Lipos.

AlistairT

[Johnnie]

Alistair
I am loosing the will to live.
Suggest you go back in your box. I had no problem with your information. As I said there was a lot of good replies - yours was included in that.
That is until you came to the C rating.
Quote
The C rating purports to show how many amps a lipo cell can supply, at its limit.
Unquote
This is not correct. As Aslan thought at the begining people take this to mean that 10C or 25C mean 10 amps and 25 amps.

I do agree though that your further posts clarified that
However, somewhat pompous that you think you are giving advice voluntariy and unpaid. Wake up smell the coffee. This is a forum!

Now to Aslan
Get angry all you like. Suggest you go back and read your posts.
1. quote
Your reverse theory sounds good but things arn't what they seem
unquote
The reverse theory is absolutely correct
2.Quote
5000Mah or 5 Amps
Unquote
This is an absolute nonsense statement. Mah stands for milliamp hours, amps off course is amps. One term is capacity the other is current.
3. Quote
C rate is how many amps a Lipo can supply
Unquote
This is is nonsense. Agree you have retracted now by trying to show you understand it. I am sure this new found knowledge has come about because of this debate we are having.
4. quote
Forget about the fractions of an hour one
unquote
Again nonsense.
Simple algebra will show that in fact the C rating does give the fractions of an hour the battery will theoretically last at maximum rated current.
Maximum current in milliamps = C rating X Capacity
Time battery will last at max current = capacity / max current
Answer = 1/C rating in unit of hours
QED the C rating shows what fraction of an hour the battery will last at max current.

On your last post you are just repeating what I said in my last post. No doubt trying to show your new found knowledge.

Johnnie