Help Choosing a battery

[Jussi]

So I read Bartman's Understanding LiPo batteries post which was very informative but I have a couple of questions.

1. He mentions that picking the cell count is the first step but (unless I totally missed it) not how to do it. If you've decided on a motor and prop size, how can you determine the proper cell count battery.

2. I was reading someone's build on another site and they mentioned that they were using 8" props and using a 4S battery but if they wanted to upgrade to 10" they should use the a 3S battery. I'm assuming this has to do with the esc they chose. Is there a way to determine the current draw by a given battery, motor, prop combination to let you know what esc capacity is needed?

3. Is flight time of a copter only determined by the capacity of a battery. If you know the the auw, is there a formula or chart to tell you the approximate flight time will be given the capacity of a battery?

 

[Hexacrafter]

Jussi,
Please provide specifications for your aircraft....
Frame/ configuration/ mfg.
Motors
ESC
FC
This will allow the forum to provide the correct advice....

 

[Motopreserve]

Hey Jussi, welcome!

Hexa is right, listing the stuff you have already will help us help you. If you are still shopping for components, maybe this will help:

1. Hopefully the motors you choose will provide a data sheet that should list a series of calculations. This should include thrust, amp draw, efficiency etc for various prop sizes, at different voltages (battery S rating). The first thing you need to to do is determine your AUW, and then match motor/prop combo to lift it (goal should be 50% throttle to hover). These motors will provide the needed lift while being supplied a specific voltage - and there's your battery choice.

2. Providing a motor a given voltage will produce a specific RPM (this is where Kv comes in), so with 8" prop the thrust would be x, but with 10" prop, it would be y. In his case, Y would probably be too much, so he would lower the voltage (S) applied.

3. You can use ecalc to estimate flight time, but it's often inaccurate (although I had some luck with it). The flight time is determined by battery capacity fighting the weight. Also the efficiency of the motor etc. so unfortunately you will get diminishing returns by slapping more and more Mah onto a craft - since the weight will increase with it.

Hope that makes sense.

 

[Jussi]

Yes I'm mostly still shopping for components. The only thing I have bought so far is the frame (tarot 650 sport) and the transmitter (DX8). Right now I'm leaning towards a Naza V2 and Tarot gimbal. No idea of motor or prop yet so if you have a suggestion on that I would appreciate it as well. But plugging in some very rough numbers (including guessing on motor and battery) the auw should be somewhere north of 2.4kg .

So suggestions for a battery, motor, and prop combo to maximize stability and flight time?

Hey Jussi, welcome!

Hexa is right, listing the stuff you have already will help us help you. If you are still shopping for components, maybe this will help:

1. Hopefully the motors you choose will provide a data sheet that should list a series of calculations. This should include thrust, amp draw, efficiency etc for various prop sizes, at different voltages (battery S rating). The first thing you need to to do is determine your AUW, and then match motor/prop combo to lift it (goal should be 50% throttle to hover). These motors will provide the needed lift while being supplied a specific voltage - and there's your battery choice.

2. Providing a motor a given voltage will produce a specific RPM (this is where Kv comes in), so with 8" prop the thrust would be x, but with 10" prop, it would be y. In his case, Y would probably be too much, so he would lower the voltage (S) applied.

3. You can use ecalc to estimate flight time, but it's often inaccurate (although I had some luck with it). The flight time is determined by battery capacity fighting the weight. Also the efficiency of the motor etc. so unfortunately you will get diminishing returns by slapping more and more Mah onto a craft - since the weight will increase with it.

Hope that makes sense.

 

[Motopreserve]

You don't mention what price range you're looking for. As I'm sure you've found, there are many options. The lower cost motors I typically suggest (and use) are sunnysky, the higher end would be KDE or T-Motors. And there are many in between.

What's the budget?

 

[Jussi]

Will stick to lower to mid price motor. Is there a best bang for your buck option? And suggestions for battery? Thanks

You don't mention what price range you're looking for. As I'm sure you've found, there are many options. The lower cost motors I typically suggest (and use) are sunnysky, the higher end would be KDE or T-Motors. And there are many in between.

What's the budget?

 

[Motopreserve]

I'd say for the cheaper motors, best bang for the buck is sunnysky. Little more $ than the dirt cheap ones, but definitely higher quality, and inspires more confidence.

Batteries are like motor oil. Everyone has their favorites . I'd say on the cheaper side, the Nano-tech from hobby king have held up quite well.

 

[Jussi]

I meant want size motor and battery?

I'd say for the cheaper motors, best bang for the buck is sunnysky. Little more $ than the dirt cheap ones, but definitely higher quality, and inspires more confidence.

Batteries are like motor oil. Everyone has their favorites . I'd say on the cheaper side, the Nano-tech from hobby king have held up quite well.

 

[Motopreserve]

That's really going to depend on the exact weight of the craft. At this decision point, you do not ant an estimate. For argument
s sake; If you are estimating at 2.4Kg AUW - the sunnysky V3508-20 580kv motor will lift what you need (650g thrust/motor), with a 4S battery and a 15" prop - the max size prop for this frame (although some have mentioned 15" props hitting the canopy if you're using it).

What I suggest is for you to make a spread sheet with ALL of the components you plan to use, with their weight listed. That's everything....frame, gimbal, camera, etc. After you see what total weight will be (AUW), you can research some motors and see what their thrust capacity is for a given prop/battery combo. Factor in the weight of the motor at this point, and plan for extra weight of the battery, and you should settle on a system that hits full weight lift at 50% throttle.

 

[Jussi]

Thanks for the info. One thing that is a little confusing is sometimes they don't put the throttle percentage. In that case how do u determine what the hovering capacity is? For example here is the chart they give for the sunnysky x4110s 400kv. Also is there a site that u can see all these figures? Seems like the sunnysky page doesn't have them. I had to go to different store pages

http://www.buddyrc.com/sunnysky-x4110s-400kv-multirotor-brushless-motor.html

That's really going to depend on the exact weight of the craft. At this decision point, you do not ant an estimate. For argument
s sake; If you are estimating at 2.4Kg AUW - the sunnysky V3508-20 580kv motor will lift what you need (650g thrust/motor), with a 4S battery and a 15" prop - the max size prop for this frame (although some have mentioned 15" props hitting the canopy if you're using it).

What I suggest is for you to make a spread sheet with ALL of the components you plan to use, with their weight listed. That's everything....frame, gimbal, camera, etc. After you see what total weight will be (AUW), you can research some motors and see what their thrust capacity is for a given prop/battery combo. Factor in the weight of the motor at this point, and plan for extra weight of the battery, and you should settle on a system that hits full weight lift at 50% throttle.

 

[Jussi]

Also if I figure out what motor I need what values in the chart help you to decide on the battery. Besides the the voltage that is. What items in the chart will tell me what mAH and C value to choose?

 

[Motopreserve]

It's really frustrating. Especially when a company doesn't provide consistent data for all the models in the same line.

I'm sure there's some way to figure it out correctly from those charts, but I typically look at the thrust around the middle of the list, and guesstimate from there. Not very confidence inspiring . I haven't actually purchased a motor that didn't provide data in quite a while.

It looks like that motor might be overkill for your projected AUW.

I would do some searching around for people who have done builds with that specific frame, see what's worked and what hasn't, and that should get you closer to a good choice, if lack of data is stumping you.

Ecalc has a bunch of motors listed, but not all. And it can be a bit hit or miss.

As far as the C rating (discharge capacity), that will also depend on the total package. If you can find data, you can figure out the max amp draw the motor will require (given voltage and prop size) and shoot fkr something safely over that total.

For mah, the higher the heavier. So see what you can afford to put on the craft without limiting the balance of the system. It really is all related. Get your weights input, calculate the thrust needed, see how much room you have with a given system to put more battery weight on. Eventually you should be able to back into a well thought out power system.

 

[Jussi]

Ok so I think I've pretty much settled on the SS 3508 kv380. It looks like will do the job and is still affordable so that if I decide to go a higher end motor in the future I won't feel too bad about buying these first.

http://www.buddyrc.com/sunnysky-v3508-29-380kv.html


So if I look at 15x5 propeller table it looks like it draws 12.8A at 100% throttle. So do I need to find a battery such that

capacity *max current >= 4*12.8


Besides adding weight, is there a negative to going with a larger than necessary battery? Any risk of overloading the motor?


Also are all battery brands the same? If not, which are better?

It's really frustrating. Especially when a company doesn't provide consistent data for all the models in the same line.

I'm sure there's some way to figure it out correctly from those charts, but I typically look at the thrust around the middle of the list, and guesstimate from there. Not very confidence inspiring . I haven't actually purchased a motor that didn't provide data in quite a while.

It looks like that motor might be overkill for your projected AUW.

I would do some searching around for people who have done builds with that specific frame, see what's worked and what hasn't, and that should get you closer to a good choice, if lack of data is stumping you.

Ecalc has a bunch of motors listed, but not all. And it can be a bit hit or miss.

As far as the C rating (discharge capacity), that will also depend on the total package. If you can find data, you can figure out the max amp draw the motor will require (given voltage and prop size) and shoot fkr something safely over that total.

For mah, the higher the heavier. So see what you can afford to put on the craft without limiting the balance of the system. It really is all related. Get your weights input, calculate the thrust needed, see how much room you have with a given system to put more battery weight on. Eventually you should be able to back into a well thought out power system.

 

[Motopreserve]

Your calculation is correct. So you would need a battery that can handle 51.2 Amps at burst. This is the safe number - we will hope you never need to use it

To determine the battery needed - you multiply the battery's amps (mah/1000) by the discharge rate (C).

Example: a 10000mah battery with a 30C rating would produce the potential for 300A. Keep in mind these C ratings aren't always accurate, so erring on the side of caution is best.

If by size you mean weight, mah and discharge rate, there is no risk in overloading the motor - as long as the voltage of the battery is within the specs for that motor. The motor will pull what it needs.

I have had great luck with nano-tech, readymaderc Orange packs and Glacier. Not all batteries are the same, but the cheaper nano-techs have held up well. Proper care is what seems to be the biggest factor in battery life. Make sure you have a good charger and you charge/discharge correctly.

If you haven't already - read this thread:

http://multirotorforums.com/threads/understanding-lithium-polymer-batteries.12072/

 

[nikunj sharma]

hi ,
i m new to this..
i want to make my quad.
i have four 930 kv motors and 30 A ESC: props of 10* 4.7
my quad will weight about 1500 g (including evrything)
so pls tell me what battry should i use to have flight time of about 10-15 min.

Jussi,
Please provide specifications for your aircraft....
Frame/ configuration/ mfg.
Motors
ESC
FC
This will allow the forum to provide the correct advice....

 

[Motopreserve]

Do you have a link to the exact motors? Hopefully a page with some data for the motor???

 

[Jussi]

Is there any benefit in using 2 batteries either in series to get the necessary voltage or parallel to gain flying time?

 

[Motopreserve]

If you can afford the extra weight - dual batteries may help in case one goes bad - and has the additional mah as well.

 

[econfly]

I would add that the correct battery voltage for any rig carrying a gimbal and camera -- even just a gopro -- is almost always 6S. Maybe consider 4S for a 450 sized craft, but try coming up with a 6S build first and then see why 4S would be better (hint: it probably won't be). Forget 3S altogether.

Find a motor with thrust specs broken out by prop size (Tiger is excellent in this regard. KDE makes great motors, but just discount all of their claimed thrust values by 25%). Estimate total all-up weight with battery (endogenous problem) and find a motor/prop with thrust equalling all-up weight (for all motors combined) at 50% throttle.

Pick approriate ESCs for the chosen motors with well more than enough amp rating at max thrust (which you will never use, but you want over-spec on the ESC amp rating). Ideally voltage rating should exceed the build voltage (i.e., 8S max for a 6S build is a good choice; this helps in managing the current pulsing / capacitor stress issue).

Finally, if you find yourself on the edge of a spec sheet, reconsider or have a good reason for this. Aim for the middle. If a motor only works out at its max prop size or voltage get a bigger motor -- same goes if it only makes sense at the smallest prop size or voltage (get a smaller motor). Manufacturers like to offer a large range of claimed performance, and this means the edges are less than ideal.

Back to the battery: take total amps of the motors at thrust = all-up-weight. Divide by 3.5. That's about the amp-hours of battery you want (again, with the battery weight included in all-up-weight) for around 15 minutes of typical AP flight time. For example, say you are building a quad that has thrust = all-up-weight with motors pulling 8 amps. Four motors times 8 amps is 32 amps --- divided by 3.5 and you get a little over 9 amps. So, you should look for a battery that is about 9000 mAh (e.g., a pair of 4500 or 5000 mAh would be a good choice).

 

[Motopreserve]

Nice Rob! Never heard the 3.5 factor in that equation.

I learn something from you almost daily!