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Electric flight Batteries...for beginners - please keep it simple

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Tim Mackey27/08/2009 19:31:49
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OK here we go with the very basics you need to know - I am only discussing Lipo here, cos far and away this is definately the most popular form of electric fuel today.
LiPo ( Lithium Polymer ) batteries come in all sizes, capacities, and voltages.
The "S" in the pack description refers to the number of cells in Series, and the "P" is the number of cells in Parallel. If a battery does not contain cells in parallel this letter is usually dropped from the "name".
2s 2p = 2 cells in series, and they are in parallel with another 2.
3s = 3 single cells in series.
Easy right!
Putting cells in parallel keeps the voltage the same but doubles the capacity.....
( the length of time the battery will give power for, or the amount available as a maximum. )
1 X 2000m/a cell = 3.7V 2000m/a
2 X 2000m/a cells in parallel still = 3.7V but now its 4000m/a capacity,  and so on.
 
Putting cells in Series increases the voltage each time, each cell is 3.7V ( nominal ) so...
2s battery = 7.4 and a 3s = 11.1V and so on. Capacity remains the same throughout.
 
Motors require power to turn, and the more they are "loaded" with bigger and bigger props, the more current they will take from your battery....lots of it !
If you are using a high power motor and want lots of speed, big props, heavy large models, or long flights and so on then you will need a battery with lots of CAPACITY .
 
The Voltage of the battery only drops a little with this loading, and the voltage will determine how fast the motor turns ( RPM  = Revs Per Minute ).
Increasing the voltage by using more cells will not only make the motor spin faster, but will cause more current to flow also, and the power goes up as well. Of course, sometimes you dont actually want a high revving model, so beware about using high voltage batteries in this case.
As you will see in the MOTORS thread, the motor choice is also very important.
 
Batteries must be properly charged, and "balanced" to ensure each cell in the pack stays at the same level as its neighbour.
You CANNOT use a charger that is not specifically designed for Lipos to charge the batteries.
You can read a lot more about batteries and chargers from these threads and articles
 
 
 

Edited By Timbo - Moderator on 30/08/2009 23:47:10

Tim Yates28/08/2009 09:20:22
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Hi Timbo.
Hope you don't mind me replying on  this thread. Can i give an example of battery and a battery supplier and ask for some assistance? The plane manufacturer description is (li po ofcourse) "3S 11.1v 1300"
 
So, 3 cells in series = 11.1volt and 1300mah
 
It is for a hobbyzone supercub LP. My local store has told me spare batteries are 36.99 which to me seems like it should be for a pair (when i look around) so i am looking on line at giantcod.co.uk (for example). They have several '1300  3S 1P' but they also give a figure of 20C - 30C or 25C - 35C or 30C - 40C
 
Which should i choose and what does the 'C' figure refer to?
 
Kind Regards
Tim
Tim Mackey28/08/2009 11:35:56
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Of course I dont mind Tim - this sort of question, and forthcoming answer is just what beginners need to see.
Prices of LiPos are very competitive at the moment - and LMS generally will be higher than online "shops" - giant cod are keenly priced.
The C rating of a battery refers to its Capacity, and its ability to actually deliver multiples of that capacity.
 
For instance, if your 1300 ma pack was rated at 1C that means it can only deliver its capacity of 1300 ma and no higher discharge rate is allowed.
This theoretically means you could "pull" 1.3 Amps out of it, for 1 hour.
If it was a 2C pack then it could supply its1300 X 2 m/a ( twice as fast ) or, put another way 2600 m/a.
If indeed the motor did consume 2600 m/a then this will only last for half an hour.
 
So...in the case of the quoted packs....
20C packs will be able  if required  to pass currents of 1300m/a X 20.
1.3A X 20 = 26A maximum current allowed.
30C packs would be 1.3A X 30 = 39 Amps ........and so on.
 
Of course, as shown in the 2C example above, the time that the battery will be able to deliver is reduced with higher C rates.
The 30C pack would be only good for 2 minutes at 30C .
An easy way to work this out is simply divide 60 ( minutes in an hour ) by the "C" figure ( 30 in this case ) 60 /30 = 2.
However,remember that just because a pack is capable of such high discharges, it does not mean it has to be used at those rates.  Indeed, "pushing" packs to their maximum rated "C" means not only very short flight times, but considerably reduced overall lifespan of the pack.
Higher C rated packs generally weigh more than lower C rates.
As a rule, I generally try to use my packs at only 50% of their claimed maximum "C" rate.
So, if your rig is pulling 20A, then get a battery which is capable of 40A maximum.
In the case of the 1300mahr pack, this would be 30C. ( 40 / 1.3 = 30 )

Edited By Timbo - Moderator on 28/08/2009 11:43:10

Tim Yates28/08/2009 14:46:27
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Thanks. I think i have that understood in a simple way in my simple mind. C rating is the ability of the battery do deliver the power.
 
The battery with the super cub is 15C continuous and 20C burst (guess that makes it 15C - 20C).
For me to buy extra batteries i will use 20 to 30C - higher C rating than the origional standard battery but the smallest on said web shop.
 
Thanks so much!
 
I now continue to learn more about leccy flight - which ESC, which motor and how to choose which goes with which and which goes with which plane.
 
 
A mod i want to do to the Cub is fit a brushless motor and appropriate speed controller. Going to try and work out what i need myself but may be a forthcoming post on that. 
Tim Mackey28/08/2009 15:39:00
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Always nice when someone actually tries to work things out for themselves...given of courese, the correct guidance.
Dont forget to check the two sub sections on motors and ESCs then .
Tim Mackey30/08/2009 23:42:50
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             A bit more...pinched from another thread in which I posted this....
 
Any battery will supply ( theoretically ) its stated CAPACITY for 1 hour, in other words you are discharging at 1C.
Now,if you want to double that RATE to 2C discharge rate ( irrespective of the capacity ) then it will only supply its juice for half an hour...and so on.
 
Therefore the quick and easy way to gauge your duration at WOT is to simply divide the 60 ( minutes in an hour ) by the C rate,  assuming the battery is being used at that C rate.
Take for example a battery of 1300mahr CAPACITY  and its potential maximum C Rate was 20C.
Its maximum current flow therefore is 20 X 1.3Amps = 26Amps.
Therefore if you actually discharged it at its full rate of 20C ( 26A ) the calculation is....
60 / 20 =3  ( minutes )
Got it ?? 
Yorkman31/08/2009 01:09:41
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Sooooooooooo.......if the setup pulled 13amps at WOT duration would be 6 minutes? 10C?
Sorry if that seems an obvious question/answer-just trying to get clear in my head!

Edited By Timbo - Moderator on 31/08/2009 11:21:56

Tim Mackey31/08/2009 11:23:17
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Yes! Because at 13 Amps the RATE is 10 ( 13 / 1.3 ), so the maths is again....
60 / 10 =6
Well done sir - no homework for a week
Yorkman31/08/2009 13:57:38
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Thanks for your patience Timbo-got it now!
Tim Yates02/09/2009 12:40:37
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Timbo
Firstly many thanks for some great threads to help us get our heads around leccy flight set ups.
 
I 'think' you have kind of answered this but maybe i am just asking in a different way?
Can you touch on the MAH rating of batteries? Of course first we need to know how many amps our motor will draw; example a 450 watt capable, 11.1 volt motor will draw 40.5 amps (watts divided by amps).
Need an ESC of 40.5 amps or greater (as you already mentioned give a safe margin with ESC's).
 
Battery: Here is where i am a little stuck.
It needs to be 11.1 volt for the given example which means '3S' (3 cells in series each of 3.7v). The question now is regarding MAH of the battery, in short the more MAH the longer the battery will run for? Is that correct?
 
Next::
Battery 11.1v (3S), 1300MAH (1.3 amp), C rating 20
The battery could discharge in 3 minutes at it's peak (60 minutes / C20 rating)
Approx run time of battery with mentioned motor 32.07 minutes (MAH 1300 battery / 40.5amps max draw of motor).
 
Now if all of that is correct i'm a really happy guy!
 
 
Tim Mackey02/09/2009 13:17:51
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Hi Tim...just a small point of order - I am sure its just a typo but  to quote you...
 
"Of course first we need to know how many amps our motor will draw; example a 450 watt capable, 11.1 volt motor will draw 40.5 amps (watts divided by amps)"
It is Watts divided by VOLTS
Yes, the more mahr ( capacity )  the battery has the longer it will supply the current.
 
if you want to be a little more accurate when estimating setups , then I alays allow 3.5V per cell under load for the expected battery voiltage.
So.... a 3s battery will be
3.5 X 3 = 10.5V.
If the Watts are 450,  then 450 / 10.5 =  43A. ( 42.8 to be precise )
Dont think of the motor in terms of voltage.... motors will run on all sorts of voltages, including much higher figures than the "norm"... what you need to do then is "prop down" in order to kep the CURRENT down...its current that kills motors.
 
 "Approx run time of battery with mentioned motor 32.07 minutes (MAH 1300 battery / 40.5amps max draw of motor)".
Sorry mate....  not quite right
The run time of your 1.3A battery  @ 1.3A is an hour.
At  2.6 A its going to be half an hour
At  5.12A its going to be 15 minutes and so on.
 
As I say, forget about the motor maximums etc, you just need to know the current being drawn by the motor with the particualr prop... and this is where the wattmeter comes in.
 
 

Tim Yates02/09/2009 14:06:10
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Thanks. I 'm getting there.....
Yes it was a 'typo'.
 
I appreciate i'll need to put a watt meter in the circuit and see what the motor / prop draws. I'm just trying to work out kinda ball park figures and equations with an aim to popping it all into excel with wonderful formulas and it'll then basically tell me what each system needs. Maybe that exercise has alreayd been done?
My aim is to work out how many minutes the battery would run for with said motor, thsu you can choose the correct battery....
 
With the best intentions and little leccy knowledge
Tim
 
 
 
 
Tim Mackey02/09/2009 15:31:53
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Hi Tim.... sometimes of course, with all the best will in the world, our plans are limited by reasons of space, weight etc. There are several factors involved in the decision as to what battery to use, and I have given a fairly comprehensive response to your post, but decided it was going outside the remit of this sub-section so its over HERE.
Hope this helps....
Tim Yates02/09/2009 20:24:38
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Thanks Timbo. Despite best efforts i'm really struggling to get my head round this but ma sure when i do i'll never forget it!
 
I'm still puzzling on
The benefits of 3,4,5 cell. Why would one plane use
MAH, ok i get that now, the bigger the MAH the more energy it stores, the bigger the better (longer flight time). Likewise i can work out which Amp ESC i would need for a given motor as watts / volts.
Working out which C rating battery i need for a given motor is somewhat puzzling.
If a kit says you need a 450W motor how many KV, volts, is this as motors aren't sold in watts (looking from a well known big fish website).
 
I'm sure i'll get there but until i understand it i'm loathed to order a kit or plans and the bits to complete it. I could post on here and i'm sure all the top folk would tell me what i need but i'd like to understand what i need.
 
Appreciating ALL the help already given.
A puzzled Tim
 
Tim Mackey02/09/2009 21:55:47
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Choose the cell count to help keep the amps lower, yet the watts high enough for your needs. Generally speaking lots of volts and not many amps is better than viccky verrky.
If you need very hign revs, and lots of watts, ( large EDF perhaps ) then you need high volts. Slow flying slow revving models.... low volts.
 
You dont generally need to work out which C rating you need ( unless you need very high performing batteries in EDFs etc ) - the capacity itself is the important thing.... just ensure that the battery C rating will at least allow the amount of current you are pulling to actually flow. C rates can be a bit academic TBH - and often wildly optimistic - alsoo, remeber if the battery is being used at 30C then you have a 2 minute flight!
 
If you need 450 Watts, then on a 3s Lipo thats 42A so just about do-able.
Therefore get a motor thats happy at 42A or more, and choose the KV to suit the prop size you will be using.
Small prop =high Kv .....Big prop = lower Kv.
 
You will get there, well done so far
 
Tim Mackey05/09/2009 22:18:34
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From another thread started by someone else cam a question about C rates and do we need these very large C rated batteries that some places sell. Heres an example....
 
The rig pulls 40 A, and you have a 4000m/a battery 20C rating.
At 40A thats 10 X its capacity =  therefore its being used at 10C....half its capability
At  WOT that will last 6 minutes. ( 60 / 10 = 6 )
 
The same exact rig ...you have a  4000/ma battery at 30C rating
At 40A thats 10 X its capacity =  therefore its being used at 10C....1/3rd its capability
At  WOT that will still last 6 minutes. ( 60 / 10 = 6 )
 
Huge C rates ( some of which are untrue anyway ) are only really necessary where very high current is required, but perhaps the model / budget cannot cope with a huge capacity battery. Example.....
 
EDF jet - pulls 60A - battery 3000m/a 20C ( 60 A maximum discharge )
60A is 20 times its capacity, and its absolute maximum capability. It will not have a long life, and voltage will be likely to sag under a 60A load.  flight time....3 minutes
 
Same 3000 m/a battery but this battery has very high C rate of 40C ( 120A maximum discharge )
60A is again 20 times its capacity, but its only  working at half its capability.
Pack will have a longer life, and maintain a good high voltage under the 60A load
Flight time remains the same of course....3 minutes.
Get it ?
Tim Yates06/09/2009 08:18:55
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Thanks for your patience Timbo!
I have read all your posts on this about 20 times and also looked on other sites. End result is it's all come together. I think just reading the same stuff but explained by different folk in different ways was helpful. I also found a good thread on watt flyer site with lots of easy good info' for the beginner, i could PM it to you if you like for reference. In essence it's the same as everything you have listed and explained just written differently.
I am greatful for your time and patience on this.
Tim
 
 
 
Tim Mackey06/09/2009 09:17:43
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Glad to here your getting there Tim.
No links to Wattflyer or anywhere else thank you - I am well aware of all the other forums and have gravitated to just "ours" almost exclusively these days

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