Battery Pack capacity and C ratings and so on

[A.A. Barry]

Timbo, what you are saying is that to achieve 4000mah, in each nacelle, is that i would need to have both 7.4v bats rated at 4000 each.. getting it slowly B

[Tim Mackey]

Barry. I assume you are after fitting 2 seperate batteries for a twin ?

TBH its better to use one battery to feed both motors simultaneously - that way you dont incur the risk of one battery running low before the other which could of course cause the LVC on one ESC to cut the motr before the other - undesirable of course

Now, having said that,  you can still use two seperate batteries - one in each nacelle - but cabled up to become one pack IYSWIM.

 

Doubling up batteries /cells in PARALLEL doubles the capacity but NOT the volts.

Connecting in series has the opposite effect - EG: Twice the volts but same capacity.

All this stuff is in the "batteries de-mystified" article I did in the last two magazines.

[A.A. Barry]

Oh well I'll stick to I.C engines it's easier , put fuel in the tank and away you go, to much cofigeration reqired for E.P. I understand the power reqired in I.C model but trying to "nut out" E.P????????    TA...    B

Edited By Barry Wetherell on 24/03/2009 08:43:31

[A.A. Barry]

Timbo, ...rethinking(me).. if I added 2x 11.1v  x 2400 3s. bats per nacelle, would that give me a duration of 12 mins?????  THat's 22.2v and not using as much prop speed????    Barry

Edited By Barry Wetherell on 25/03/2009 03:28:54

[Tim Mackey]

Thought you had gone back to the oily hand brigade

22V means you are putting 2 x 3s in series, so your revs will be high and capacity remains at 2400mahr. Power increase dramatically due to the high voltage, so duration will drop.

Barry. IF you are serious about rying to put together a suitab;le setup for a particualr project, then I am happy to help.... but these things take time to calculate, research and write about, and I simply dont have that time ( or indeed inclination ) if the person then adapts the ...."oh I cant be bothered anymore but thanks anyway " attitude.

This is NOT aimed at you spoecifically - honestly - and as I say, if you feel you are really likely to go ahead, please post the full details of the model, what your objectives are, and we will gladly help.

[shaun bellringer 2]

 

hi timbo

 

how do you go about choseing a motor say for a simple trainer 40"  at moment .46 2 stroke .what size motor would do the job. how would one calculate to get the right size.then how to calculate what speed controller.then calculate battery size say for 10min flight ....ie how do you put all these numbers together to work correctly.

 

great thread by the way .allready learnt alot thanks shaun.

[Tim Mackey]

Well Shaun, one of the things about electric flight ( which if you approach it with a positive attitude ) is that there is more flexibility in choosing an electric powertrain for a model than with IC. Different props and batteries on the same motor can turn a model from a slow thrusty model, into a fast speeder - providing of course the airframe - and pilot - can handle it!

For a simple trainer of 46 IC size I would probablya aim for around 80 Watts per pound of AUW as a minimum so if the model weighs ( all up ) say 5lb, then aim for 400 - 500watts.

You dont want a high revving screaming motor ( another benefit of electric flight ) you turn a larger than normal prop slower = same power, but more efficient and quieter. 

Consider anything above about 1500 Kv as quite a high revving mnotor, so lets go for about 1000 Kv or so ( these are quite flexible TBH ) and this should be pretty happy swinging a decent size prop of say 9 or 10 " diameter...about right for your average trainer.

To keep the current manageable at the desired 500 watts, I would use a minimum of 3 cells, which under normal load will give around 10.5 Volts or so. 

a little bit of maths shows us that 500 (watts ) divided by 10.5 ( volts ) = 47 ( amps )

So we now know that the rig will pull around 47Amps at  WOT ( wide open throttle )

This figure will reduce a bit in the air as the prop "unloads"  - by maybe 25% so current will be more like 35 - 40 A. This needs a speed controller of 40A - safer will be 50 Amp

As for flight times, this is mainly a function of  cell capacity but is also afffected by weather, and flying style etc. However roughly speaking...

If the rig pulls say 40A and the battery is 4000mahr capacity this is 4A an hour.

We want 40 Amps out of it ...so its 10 times more.

Divide the 60 minutes in a hour by 10 and you get 6 minutes flight time.

Conclusion?

500 watt 1000Kv ourunner motor, 4000mahr 3S battery, and a 50ESC.

 

Of course a lot of the flight on a trainer is at low throttle, so in good conditions, you could maybe get double that, I would say expect an average of  8 - 10 minute flights.

However in a fast EDF jet - here the throttle is at full most of the flight, so 6 minutes would be closer to reality!

HTH

Edited By Timbo - Moderator on 28/04/2009 21:37:24

[shaun bellringer 2]

 

thank you timbo

 

going to print off this and other pages .read and get my head around it ?.

 

i like most of your readers very much appreciate your time and dedication you give us beginners.

 

thank you all shaun

[Tim Mackey]

Hope you find it helpful Shaun - and TY for the comments, it is always good to get feedback.

[Dave Doughty]

Hi

 

Feel a bit silly asking this but......I went out and got myself a proper charger as that was the best advice, i have a Ultramat 16. The thing is it shows you how to connect Lipos with the balancers supplied but how  do you connect the NiMHs do i use the balancers supplied ? Sorry i'm just confused!

[Tim Mackey]

No balancer when using NiMhs...just connect the main charger leads to the pack.

[Dave Doughty]

ok another dumb qus, i'm to plug the 4 cell NiMH into the front of the charger where the balancer connector would go. Theres a  ground pin and 6 other pins, the battery has a red and black wire i guess just like a car the black is the ground wire?

sorry i know it sound a bit darft.

[Tim Mackey]

Im confused.

The chargers main output is red and black ( pos and neg) banana plug style sockets. Connect the battery to these via the ( presumably ) supplied flying leads. You dont use the balance board or anything else.....just connect the battery to the output terminals.

 

 

[Dave Doughty]

Think i've worked it out, i do not have the extra leads to use with my type of battery, guess i should of asked if they came with it, silly me.

Thanks for the help though.

[mark guerin]

anyone know if i can get a battery for a flycam one2 ?

thanks.

[Andrew Smith 5]

Hi please excuse if this has been answered as I have recenly joined this very helpfull forum,

I understand how to get approx flight time worked out on the first page, my query is, is it worth getting the new overlander extreme batteries which have a higher c rating than the sport which has constant 25c, im using A30 16m hacker with pulso 33a esc 12x6e prop as recomended by LHS in eflite sukhoi, by using higher c rating pack will it give more power to motor, or does the motor only draw what it needs and the lower rated pack be ok.

 

Im due to pick up a wattmeter very soon so can get figures accurate

[Andrew Smith 5]

Seem to have found what I'm after, digesting it slowly

[Andrew Smith 5]

If my setup is pulling 28 amps static WOT out of 2200 3s constant 25c burst 40c pack How do I work given ampage out into c rating for the battery, I ask as I'm looking at getting a higher c rated battery but don't want to waste money if I don't need to. I've learnt how to work out flight duration from this thread but can't get c rating yet. Thanks Andy

Edited By Andrew Smith 5 on 14/01/2011 18:07:58

[Tony Smith 7]

One "C" is the nominal current that the battery would deliver over one hour.

 

So for a 2200mAh battery then 1C = 2200mA (or 2.2A).   If your battery's rated at 25C should be able to deliver 25 x 2.2A = 55A.

[Andrew Smith 5]

Thanks for the reply tony very clear thanks, So there probably is no point me upgrading to the higher c rated battery of 60c(Overlander extreme currently using sport) or can you see an advantage. thanks Andy

[Tony Smith 7]

I would have said no advantage, but I don't have the experience of many here.  On the face of it you're drawing only about half the rating of your battery so I can't see that it would be improved much by upgrading.

 

I think that a higher "C" battery would have a lower internal resistance, so would be more efficient, but I can't imagine that would amount to much.

[ian ludwell]

Hello guys,a question that others may have come across,i have had 2 batteries,both 3s 2200 mAh 11.1 Volts,one was aTornado and the other an Overlander.

 

The Tornado issue was cell 2 was losing capacity in comparison to cells 1&3,and also starting to swell so i disposed of this.( lasted about 12 months)

 

Now the Overlander has the same issue but without the swelling.After balance charging at 0.3 Amp the voltage is about4.1 v/cell,but the capacity for cells 1&3 show 99% whereas cell 2 shows 89%.

 

After a few hours the capacity for cell 2 goes down to 70% and the next day the capacities are ,cell 1 - 4.218 V 99% ,cell 2 -3.803 V, 35%, cell 3 - 4.238 V 99%..

This looks like the end,can this be made into a 2s 7.4 battery or should it be disposed of.(lasted about 7 months)

 

I have used these on a Mutiplex Easy star with 20A motor,5.5x5.5 prop,i use full power to get it airborne and up to decent altitude,they are 25C sports rated,could this be the fault

Many thanks..ian

[Tim Mackey]

25C rating should be fine.

Why are you charging at 300m/a - normal charge rate is 1c ( 2.2A)

 

It can be made into a 2s , but frankly, unless you are very careful, and skilled with soldering ( maybe even aluminium paste ) then I would forget it.

LiPos are so cheap these days I wouldnt bother - and try another brand

[ian ludwell]

Hi Tim,thanks for your reply,so are you saying dispose of this battery?

The reason i was charging at 0.3 Amp was that i thought a slow charge would be benefical to the battery life.

If as you say i charge at 1C will this be ok for the battery life charging as you say a 3s 2200mAh at 2.2 A....cheers ian.

[Tim Mackey]

Yes.....and yes.