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What battery

What battery

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Robert Edney23/02/2018 16:50:32
44 forum posts
8 photos

Hi chaps, the last time I started a new thread while still on the same build I got a bit of a telling off, I did post this question on the other thread but got no response so at risk of offending anyone here goes, I have taken the term taking my time to another level but I am finishing the covering which is not as easy as it looks on you tube but I'm getting there, I have got the servo,s , radio and ordered a brushless motor kit with ESC and prop. Finally the question, it says to use a 3s battery but I'm confused with what one 1500 my etc there seems to be so many, I am quite pleased with my build so far and can't waste money on something that's not right, any help would be great.

Dave Hopkin23/02/2018 17:20:13
3650 forum posts
293 photos

There are many factors that affect "which battery"

There is the question of size and weight - the bigger the battery the bigger and heavier it gets - obviously in any model there is a physical limit to the dimensions of the battery, there is also the question of weight - the Lipo is one of the heaviest things in the model so it will have a dramatic effect on balance - in most models the battery needs to go as far in front of the desired CoG as possible to avoid having to add (dead) weight to balance it - most people if they need more weight up front use a bigger capacity battery rather than lead

Secondly there are the electrical questions? Duration of your flight and the ability of the battery to supply the current demanded by the motor/prop combo

To estimate duration we need to know the current drain - that can be established roughly by taking a reading of the amperage at half throttle - divide the battery capacity by the current drain and yo have a rough estimate of duration of flight - the bigger the capacity the long the flight

The current draw by the motor will vary with the prop you fit, a small one will use less current but provide less thrust, a larger one will give more thrust but use more current

So where do you start????

We have a 3S 1500 Battery - you have ordered the motor, esc and prop - so when those arrive the best thing is to set up a test bed - fix the motor to a lump of wood (not balsa) and shove it in a vice- connect up the radio and bing in the ESC and test the motor WITHOUT the prop, you will need a WATT METER to check the current draw is below the ESC Rating with the motor running- now fix the prop and again read the current - it will have jumped up a LOT.... make sure that at full throttle there is still at least 15% headroom between max current drawn by the motor and the ESC rating

As a rough (very) guide to power you will need (ish) at least

100 Watts per Lb of model for a typical training/sports model

If you motor/prop/battery combo gives at least that - then great, if not one of them has to be changed

Robert Edney23/02/2018 17:54:38
44 forum posts
8 photos

All I wanted was to build a little plane and see it fly not send men to the moon, now this is starting to get silly, I'm certainly not going to start buying watt meters etc, surely there must be some sort of scale like for an 800g plane a 3 s 1000 mh lipo but you're starting to scare me with all the scientific stuff

Dave Hopkin23/02/2018 18:00:13
3650 forum posts
293 photos

The intention was not to scare you, but without details of the plane its an impossible question to answer and rather than just ignore you I was trying to illustrate the variables

if the plane is 800g then you will be looking for around 150Watts

To get 150 Watts from a 3S lipo then you will be pulling around 12 Amps max - so say 6 Amps average

So your 1000Mah lipo will give you around 5 minutes flying time - give or take

So its a reasonable set up so far

What motor and prop have you ordered?

Robert Edney23/02/2018 18:08:32
44 forum posts
8 photos

I bought it from Amazon as a kit it says it is for models in the 700-1200g range, 800 being optimum, I don't know how to post links but the reviews all seemed positive so I bought it, if I had known I would have waited but I thought seeing as my plane is in it's optimum range it would be OK and then I only needed a battery, never mind

Pete B - Moderator23/02/2018 18:26:36
7376 forum posts
702 photos

All of Dave's advice above is sound, Robert, as the reality is that mismatching components such as motor, ESC and prop can easily result in having a model that has too little or too much power, or even prompt an unwanted electrical firework display!

In your model's case, however, it's a simple motorised glider, so the size of the battery will dictate the duration of the power run, as you'll find you'll just use the motor to climb to height then glide, repeating until you've used the available battery power.

With any glider, it's important to not add unnecessary weight which will degrade the gliding performance, so I'd go for a battery that is sized to a) fit the available space and b) provide sufficient weight at the nose to balance the glider at it's correct centre of gravity without needing extra weight, if possible.

If your model is flight-ready, apart from the battery, then balance the wings on the C of G indicated on the plan and see how much weight is required to achieve the balance point. Then choose a battery of about that weight, provided it will fit in the model.

For example, a 3S 1300mAh pack weighs about 100g, a 1600 mAh about 140g.Either of these should give you several climbs of a couple of minutes, sufficient to gain height. Once you've established how much weight is needed, then research the available battery packs at your LMS or one of the online retailers and buy accordingly.

The fact is, it's not a 'one size fits all' hobby - you have to do your research to get the best possible result - and that's what makes it such an absorbing hobby!smile


ps - to post a link, just copy the address of the page, click on the link symbol 5th from right on the bottom row of the reply box and paste the address where indicated and click 'OK'. That's it!

Edited By Pete B - Moderator on 23/02/2018 18:28:53

Robert Edney23/02/2018 18:35:43
44 forum posts
8 photos

Thanks Pete now that makes good sense to me and obviously I hadn't thought about it like that but that Doe's sound like a good idea, killing two Birds with one stone

Robert Edney23/02/2018 18:42:56
44 forum posts
8 photos
Robert Edney23/02/2018 18:43:54
44 forum posts
8 photos

I think I did it if I did that's what I bought

Robert Edney23/02/2018 19:16:48
44 forum posts
8 photos

Finally posted a link to what I bought and everyone went out, LOL

kc23/02/2018 19:42:48
5390 forum posts
160 photos

Well they probably didn't reply because there is not enough info on that motor to give any opinion! When you get the motor tell us the motor numbers and details of the ESC. Then people can offer more advice.

Dickw23/02/2018 20:34:13
279 forum posts
39 photos

Scroll down a bit on that link to see motor details as follows:-

Weight(including connectors): 70g
Size: 27.5x30x30mm
RPM/V: 1500KV
Constant current: 40A
Size: 45x24x9mm

As said before, pick a battery sized to fit the space and with a weight to get the CG right, but something around the 1200mAh mark should be a good starting point.


Edited By Dickw on 23/02/2018 20:37:32

Robert Edney04/03/2018 21:11:52
44 forum posts
8 photos

I was doing the covering with the iron from the kitchen and melted it once or twice so I have ordered one from hobby king but I've got to say for my first attempt I am quite pleased, I've put some photographs in my album, what do you think, I hope it fly's

Denis Watkins04/03/2018 21:30:38
2489 forum posts
126 photos

Judging from the size of the table Robert, then you have built a nice looking small glider.

I have such a model using 2s and 3s small lipos about 1000mah, and using a 20A ESC

As the model is just power assisted for gaining height, then essentially a glider.

Was a power assisted glider your intention? As no undercarriage is in the view

Or a fully rise off ground aeroplane, where power is used throughout the flight?

Edited By Denis Watkins on 04/03/2018 21:31:50

Robert Edney04/03/2018 21:44:35
44 forum posts
8 photos

There was no undercarriage on the plans which worry's me a little bit as I think the tail might get damaged on landing, but it was always going to be a power assisted glider, may be when I trust my gluing ability I might try to build a bungee launched plane but I have visions of it breaking up on launch, but I must admit that I think I got the bug and am deciding what one next and this one hasn't been out the house yet

Denis Watkins04/03/2018 22:04:24
2489 forum posts
126 photos

OK Robert, hang fire buying anything else.

Am guessing 40A is over the top

And doubt the airframe will carry a 1500mah pack

Could you type out the wingspan of the glider, and flying weight from the plan if it is there

Robert Edney05/03/2018 08:13:00
44 forum posts
8 photos

The glider is 1500mm and about 800 g but I don't know if that is with all the motor kit and stuff or just the plane

Denis Watkins05/03/2018 08:34:24
2489 forum posts
126 photos

Apologies Robert, I just read through, and you have good advice and predictions on the 40A set up you bought.

My concern is the model in the picture does not look to be 28oz/ 800grms

And as a powered glider, it does not need motor thrust all of the time.

Apologies Robert, but now weigh the build on that table, and come back with the result

Robert Edney05/03/2018 08:37:15
44 forum posts
8 photos

I just double checked the measurement and it should fit snugly with a little sponge around to stop it flopping around

Robert Edney05/03/2018 08:54:05
44 forum posts
8 photos

Unfortunately I have no scales but just looked at an e-mail from the supplier and he said the one he built was in the 750 760 range and said mine would be about that depending what wood and covering I use

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