Battery numbers breakdown. Need help.

[Chadd]

Where can i find out about what all the numbers mean in respect to the Li-Poly batteries.

ex.

2200Mah
10 - 20C

and so on.

What going to give me the longest flight times with my Trex?

What does the Amp rating on the ESC's do?

If anyone has a good link to this informnation please let me know, thanks.

[mudbogger2]

Here is a good place to start. http://www.batteryuniversity.com/

It has a ton of info and will take a while to read but for someone not familiar with all the battery stuff it is priceless reading.


The amp ratings on ESC's are the ammount of amps that it can continuosly provide to the motor with out damaging or shutting down.

[jrohland]

Battery packs have three basic specs: volts, amps and capacity.
I will discuss only LiPo packs here.

Volts comes from the number of cells in the pack. Each cell should be able to deliver 3.7 volts under load when the pack is freshly charged. For that reason, the pack voltage is always shown as 3.7xcell count. So a 3 cell pack is 3.7x3=11.1 volts

MAH (milliamp hours) is a measure of capacity. That is, how many amps can this pack deliver for one hour. In the case of the 2200 MAH, the pack can deliver 2.2 amps (2200/100) for one hour.

C is the maximum amps the pack can delivery without being damaged. To determine the actual number of amps the pack can deliver, take the MAH and multiply it by the C. So, your 2200 MAH, 20C pack can deliver a maximum of 44 amps.

Now, if you are pulling 44 amps from the pack, it will discharge in 1/20 of an hour or, 3 minutes. Remember the pack can deliver 1C (2.2 amps) for 1 hour. If you pull 20C you will discharge it 20 times faster than at 1C so, the pack will discharge in 1/20 on an hour or 3 minutes.

The other side is how fast can you recharge the pack. This is also rated in C and is usually 1C. So, at 1C you are charging the pack at 2.2 amps and, an empty pack will take about 1 hour to charge. (It is more complicated but, that is for a different post.)

So:
MAH is how much power the pack holds.
C is how fast the power can be taken out of and, put back in the pack.

Higher MAH will give longer flight time to a point. Remember, bigger MAH packs weigh more. At a point, the battery pack weighs so much, the heli does not fly well--or at all. Over time most T-Rex owners have found that 2100 to 2200 MAH packs are the best balance between weight and flight time.

jrohland

[Chadd]

great information thanks.

[Shuttle RG]

Great info!

Now for some novice questions, can you explain what the different battery designations mean, ie. 3s2p, 4s1p, 6s3p, etc.? Which is better, 1p, 2p, etc? What is the difference between Serial and Parallel and which is more important?

Also, how do you add two packs? Say 2 x 3s2p = 6s2p? You don't add the second numbers together, just the first, correct? Is it better to have one 6s pack or two 3s packs?

Am I correct in saying that the first task in setting up an e heli, is to decide upon the lipo packs size and MAH before the correct motor / ESC combo can be determined? If so, how does one determine the best size/MAH lipo for a given heli? By its weight or other factors?

Example: I have a Hirobo Shuttle that I want to convert. The Shuttle will weighs about 7-8 lbs (AUW). It has a 77T main gear and an 8T pinion for a 9.625:1 ratio. This ratio cannot be changed. Right now I am trying to determine what lipos to use; 4s, 6s, 8s, or 10s? I would ideally like to get ~15min. flight times (sport flying, no 3d). How does one solve this problem?

Thanks in advance for any info.

Clark

[Shuttle RG]

Anyone?

[jrohland]

Now for some novice questions, can you explain what the different battery designations mean, ie. 3s2p, 4s1p, 6s3p, etc.? Which is better, 1p, 2p, etc? What is the difference between Serial and Parallel and which is more important?
None is better than another. Let me explain. First it is important to understand what it means to wire a pack in series and/or parallel. Packs are made up of some number of cells. Each cell is wired to the other cells in series, parallel or some combination.

When cells are wired in series, their voltage adds up. So if you have 3 cells and, each cell is 3.7 volts and, they are wired in series, i.e. 3s, the pack will have a voltage of 11.1 volts.

When cells are wired in parallel, their milliamp hours (MAH) add up. So, if you have a pack of 6 cells wired as a 3s2p that means the pack will have 3 times the voltage and 2 times the MAH of each cell. Let's make up a battery pack:

We have 6 cells. Each cell is 3.7 volts and 2100 MAH. We wire one group of 3 cells in series. We wire the other 3 cells in series and we wire those 2 groups of cells in parallel. See the diagram.
http://www.helifreak.com/album_mod/upload/cfcc23bf3688c1f9b559e11ab4b9a575.jpg
From the diagram we have a 3s2p pack with 11.1 volts and 4200 MAH.
Also, how do you add two packs? Say 2 x 3s2p = 6s2p? You don't add the second numbers together, just the first, correct?
From the explanation above you can see that you can't just add one set of numbers or the other. You need to know what you want from the batteries.

If you have 6 cells and you need 22.2 volts, you would need to wire them in series, i.e. 6s. If you want 11.1 volts and 4200 MAH, you would wire the 6 cells as 3s2p.

If you have two 3s1p battery packs of say 11.1 volts. Now lets say you need 22.2 volts. You would wire those 2 packs together in series. That would give you a 6s1p group.
Is it better to have one 6s pack or two 3s packs?
If you have a charger and/or balancer that can do a maximum of 5s packs, it might be better to have two 3s packs. Most folks would rather not deal with the hassle and extra weight of 2 separate packs. However, when you get into say 10s sizes, it is more common to see people using two 5s packs because they charge faster with two separate chargers.
Am I correct in saying that the first task in setting up an e heli, is to decide upon the lipo packs size and MAH before the correct motor / ESC combo can be determined? If so, how does one determine the best size/MAH lipo for a given heli? By its weight or other factors?
In my opinion you should decide what you want your rotor head speed to be in RPMs. From that you can decide what Kv rating your motor needs and, from that you get the voltage you need from your batteries. As to the power rating you need from your motor and ESC, you should see what the manufacturer recommends. Also, it is an excellent idea to ask other pilots on the appropriate forum what motor; ESC; cell count; MAH and C ratings they recommend for the model you are looking at.
Example: I have a Hirobo Shuttle that I want to convert. The Shuttle will weighs about 7-8 lbs (AUW). It has a 77T main gear and an 8T pinion for a 9.625:1 ratio. This ratio cannot be changed. Right now I am trying to determine what lipos to use; 4s, 6s, 8s, or 10s? I would ideally like to get ~15min. flight times (sport flying, no 3d). How does one solve this problem?
This problem should be presented to others with a similar setup. However, to me it sounds pretty close to a TRex 600. You could use that as a starting point to determine your ideal setup.

Again, I would start with a target head speed and, using the fixed ratio, determine the motor speed you need. From that determine the appropriate Kv and what pack voltage would work best.

The ideal MAH is the most difficult because you need to settle everything else first. Once you know your cell count (voltage), AUW and head speed, contact a few battery sellers and ask what MAH they recommend.

jrohland

[spork]

jrohland,

That's a great summary. But I have two extremely small nits:

In the case of the 2200 MAH, the pack can deliver 2.2 amps (2200/100) for one hour.

I think you mean 2200/1000. Probably just a typo.

MAH is how much power the pack holds.

Technically the capacity (times voltage) describes how much energy the pack holds. Power is the rate of energy delivery. But you probably weren't trying to be entirely technical on that point.

[jrohland]

...I think you mean 2200/1000. Probably just a typo.
...
Technically the capacity (times voltage) describes how much energy the pack holds. Power is the rate of energy delivery. But you probably weren't trying to be entirely technical on that point.
Spork,

You are exactly correct. Thank you for correcting those points. I should get you to edit my posts before submitting them. :glasses:

jrohland

[Shuttle RG]

Very Nice! Thank you for taking the time to explain it all. Of course, I now have more questions.

I would start with a target head speed and, using the fixed ratio, determine the motor speed you need. From that determine the appropriate Kv and what pack voltage would work best.

So, when selecting the proper head speed, should I select the head speed at hover, or the highest head speed I expect to run? Right now, I am looking at 1600 to 1800 RPM.

A] 1600 - 9.625 X 1600 = 15,400 Motor RPM.
B] 1700 - 9.625 X 1700 = 16,363 Motor RPM.
C] 1800 - 9.625 X 1800 = 17,325 Motor RPM.

So, assuming my calculations are correct, which HS component do I use? It seems to me that I should use the highest I expect to run, correct? The 1800?

Now, given the HS, how to I determine the kv required and what pack voltage would work best?

Thanks again,
Clark

[WillJames]

You also need to consider the efficiency of the setup. Those numbers A,B & C look like the theoretical maximum. You will probably get about roughly (maybe very roughly) 85-95 or so % of those numbers.

So for instance example A:
1600 - 9.625 X 1600 = 15,400 X .95 = 14,630 real Motor RPM at 100% throttle. This is only an example, I have never seen a 95% efficient motor on any of my setups.

It should be
kv of motor x voltage x efficiency / gear ratio = Headspeed
I use the nominal voltage of the pack explained below on GOV setups.

Are you running a governed or non-governed ESC. At 100% on a non-governed setup, the headspeed will start out in example a at approx 14,630 and by the end of the flight will be a lot lower than that.

If you run a governed setup, gear for higher HS, at say 75% or so to = approx 1600 then the voltage drop and corresponding headspeed loss will happen much later in the flight and give you a lot more consistent run.

This is only one example and there are a lot of factors like motor choice, ESC choice, Battery pack choice, blade choice, weight of your heli, etc, but I posted this as an example to get you thinking about what exactly you want because you seem to be grasping the math pretty well above and interested.

[spork]

For what it's worth, you might want to find an electric as similar as possible to your shuttle in size and weight, and copy the common battery, motor, ESC, and gear ratios from that heli. Not as much fun, but you'll likely save a few bucks and some trial and error.

[Shuttle RG]

Excellent guys! Thanks for all the feedback.

My goal is to understand the process in selecting batteries, motors, esc's, etc. I already have several "setups" that are similar to my situation for reference. I am really trying to verify any of these setups on my own rather than just trusting that they'll work for my application. That is the reason for all the questions.

Back to my quest... :D

Ok, to help me grasp what is being said I have put the information gathered here into an excel spreadsheet.

You also need to consider the efficiency of the setup.

I have built a table (see pic) that takes the efficiency into account (assuming no GOV, yet). Now what? Sticking to my original 1800 head speed, is it as simple as selecting a motor with a kv that matches the pack size I would like to use? If there isn't a motor with a kv that matches the kv listed, do I select a motor that has a higher, or lower kv than what I am anticipating?

Enough for now, one step at a time.

Thanks again,
Clark

[spork]

My goal is to understand the process in selecting batteries, motors, esc's, etc. I already have several "setups" that are similar to my situation for reference.

Cool. That's half the fun of the hobby. I just thought I'd offer the suggestion, but I definitely understand the quest to learn.

Sticking to my original 1800 head speed, is it as simple as selecting a motor with a kv that matches the pack size I would like to use?

I'm not the expert here, but I'd say no. In my way of thinking the KV is kind of like the gear ratio. High KV = high-speed lower torque; while lower KV = lower speed and higher torque (assuming the same wattage).

So I think what you really need to look at for your application is the required wattage. Then you can always choose the pinion to get the desired headspeed. I think the important specs are going to be wattage and efficiency.

But I'm here to learn too. Hopefully someone will enlighten us both if I got this wrong.

[Shuttle RG]

So I think what you really need to look at for your application is the required wattage. Then you can always choose the pinion to get the desired headspeed. I think the important specs are going to be wattage and efficiency.

Thanks for the input Spork! Yeah, that's the rub with my particular application, I can't change the pinion. So, I'm stuck with the 9.625:1 ratio.

Clark

[spork]

[quote=spork]Yeah, that's the rub with my particular application, I can't change the pinion. So, I'm stuck with the 9.625:1 ratio.

I don't think that should be a major worry. I would think you should be able to find a motor of the appropriate wattage and KV. I know some guys wind their own motors. I've never tried that.

[WillJames]

At first glance, I think your table is fine. Good starting point.

The other thing to remember is that you can figure until the cows come home on paper, but some setups FLY better than others, no matter what it shows on paper.

Flying is the final test to see if a setup is happy, and not all setups no matter what it says on paper are 100% all the time.

Have fun, you are definitely learning as we all are. Dontcha LOVE the E-Helis!! I do!! :smokin:

[spork]

I just took a look back at your table. I think there's a slight mistake at the end.

You say: Motor RPM = head speed x gear ratio x efficiency factor.

But motor RPM and head speed are kinematically linked by the gear ratio. This is not where the efficiency factor is applied. I think it works like this

Motor RPM = head speed x gear ratio

and

Motor RPM = KV x voltage x efficiency factor

But I obviously couldn't take the 430L out of my T-Rex 450, put it in a 60 size bird, and get the same head-speed (even though the KV, voltage, and efficiency factor remain the same). This is because my T-Rex motor is simply not capable of putting out that much power. I think the T-Rex hovers somewhere in the range of 150 watts. The bigger heli will require much more power (somehting on the order of linear with weight). So twice the weight will need twice the watts. This spells a bigger battery (not necessarily more voltage), and a bigger motor (not necessarily more KV).