Batteries

[jfro]

My big octo is relatively efficient in the configs I fly. I'm using Mikrokopter electronics, so I have exhaustive data logging.

My copter is an X8 with 500mm booms on a Cinestar-like hub. I have the full MK stack including the "Double Quadro 2XL Cool" power board, KDE 4012 motors and T-motor 16" CF props, and a MōVI M5.

What I see on my machine is a current draw at hover of about 58A with the GH4 (AUW about 22 lbs.) and a draw of 67A at hover with the 5D Mark III (23 lbs). I'm using dual Tattu or Pulse 10000mAh LiPos. The highest current draw I've seen in a full-out climb (not that I generally do that) or an arrested descent is about 110A. In a normal flight for filming purposes, the copter consumes almost exactly 1000mAh/minute, giving me a 15 minute flight time with 25% reserve (although I usually fly 8-10 minutes).

I have a 850mm x8 w/ 3520 Avroto's. I fly 18-20lbs w/ 2x8000 mah 6s 25c batteries. Also a 650 quad with KDE 3520's.

You mention you generally fly about 8-10 minutes but can get around 15 minutes. I have similar situation and am curious as to what you think the overall effect on battery life is when you fly @ 8-10 min vs 15 min. Last year I pushed the crap out of my 5000mah 4s batteries and some have held up better than others.

Some of the experienced fliers on this forum have mentioned that by not going below the 3.65 or 3.7v range while in the air will greatly increase your battery life. I thought the 80% rule (80% of 4.2v) would be 3.36 so last year I set my alarm to 3.5.

I built a new quad and x8 with 3520 kde and the avroto 3515's this year planing on being in the 15 - 20 minute for the camera's each would carry. This was based on running close to the 80% rule. Following advice of others, I've been experimenting with landing in the 10 minute range. So far my batteries look great and my charging times are reduced, but I'm not 100% that I'm not being too cautious for the sake of my batteries. This year my 6s 25c batteries land so much cooler than my 5s did last year, that I'm guessing this has an impact to.

Do you notice much of a temperature difference between your 10-15 minute flights and if so are you concerned about the heat?. Do you feel the 15 minute flights are shortening the life of your batteries as opposed to what you'd get flying mostly 8-10 minutes?

Be interesting to see total flying time on wearing out a battery that wasn't abused. Say 100 - 10 minute flights for a total of 1000 minutes verses 66.6 - 15 minute flights for a total of 1000 minutes.

Enjoying this battery conversation. Still interested in peeps opinions on how much overhead is justified and is that brand specific.

 

[R_Lefebvre]

Me? I'd use a helicopter.

So for example, to achieve the 4.6 kg/m^2 disk loading my quad has, with with a 10kg AUW, you'd need 2.17m^2 of disk area. On a quad or X8, that would require 33" props. On an octo, 23" props. I don't have to tell you how big of a machine that would be. But that's what you'd need to do to be able to load it with enough batteries to fly for 30 minutes.

By comparison, a simple 700 heli, has 2.06 m^2 of area, and an 800 is 2.54m^2.

I know it may seem odd to focus so much on the disk area, but this is the key place that hovering efficiency is determined. It's a natural law. The disk loading gives you the basic hovering power. Then your drive system efficiency makes things less efficient from there. There's less to be gained in the drive system, than there is in the disk area.

This talk is too brief, but gives you some introduction to the topic:

 

[R_Lefebvre]

IMO, landing at 3.7V is too soon. I land at 3.5V, and that seems to be the comfortable 80% capacity range.

The heat of the batteries doesn't have a lot to do with how far you drain them. It's more determined by the internal resistance and the amperage load.

 

[stevemaller]

I have a 850mm x8 w/ 3520 Avroto's. I fly 18-20lbs w/ 2x8000 mah 6s 25c batteries. Also a 650 quad with KDE 3520's.

You mention you generally fly about 8-10 minutes but can get around 15 minutes. I have similar situation and am curious as to what you think the overall effect on battery life is when you fly @ 8-10 min vs 15 min. Last year I pushed the crap out of my 5000mah 4s batteries and some have held up better than others.

Some of the experienced fliers on this forum have mentioned that by not going below the 3.65 or 3.7v range while in the air will greatly increase your battery life. I thought the 80% rule (80% of 4.2v) would be 3.36 so last year I set my alarm to 3.5.

I built a new quad and x8 with 3520 kde and the avroto 3515's this year planing on being in the 15 - 20 minute for the camera's each would carry. This was based on running close to the 80% rule. Following advice of others, I've been experimenting with landing in the 10 minute range. So far my batteries look great and my charging times are reduced, but I'm not 100% that I'm not being too cautious for the sake of my batteries. This year my 6s 25c batteries land so much cooler than my 5s did last year, that I'm guessing this has an impact to.

Do you notice much of a temperature difference between your 10-15 minute flights and if so are you concerned about the heat?. Do you feel the 15 minute flights are shortening the life of your batteries as opposed to what you'd get flying mostly 8-10 minutes?

Be interesting to see total flying time on wearing out a battery that wasn't abused. Say 100 - 10 minute flights for a total of 1000 minutes verses 66.6 - 15 minute flights for a total of 1000 minutes.

Enjoying this battery conversation. Still interested in peeps opinions on how much overhead is justified and is that brand specific.

Jfro,
I'm no electrical engineer, but I have a little experience with these things, so let me comment. First of all, I think we all have to understand that there is a connection between voltage and current/amps with batteries, but they're very different animals. Without going off road here and exposing my lack of actual knowledge, let me just say that the 80% rule is not only quite arbitrary, it's not very specific. Are these folks saying you should take your voltage to 80% or use up 80% of the current in the battery? Either one is possible but neither is very useful. The reason for that is that you want your batteries to operate within a range of safe voltage (4.2V to 3.something) and be able to utilize as much current as possible (for example, 4500mAh of your 5000mAh LiPo). Knowing your batteries is critical, as pushing a battery too far can be catastrophic both to the battery and your copter.

By way of example, I bought a bunch of Zippy 8000 6S batteries when I built my copter. I had good luck with the 4S models, and given the cost, was planning to use them on my big rig. But what I found is that these were completely useless. I routinely saw the voltage on these sag to the 3.2-3.3V range after only using up 6000mAh of the theoretical capacity of 8000mAh (confirmed by my charger afterwards). I then did some research and bit the bullet and bought some of the new Tattu 10000 6S batteries, and it's a world of difference. I can get 9000mAh out of these and still be at 3.5V in the air under load. I fly dual batteries, but even so, like I said, I rarely push these. It is good to know, though, that they can handle it if I get a "hey, can we get one more angle on that shot?"

So, like I said, take this with a grain of salt, as I'm certainly no [MENTION=3314]R_Lefebvre[/MENTION] when it comes to real engineering, but I do have a lot of hours logged with these systems.

 

[R_Lefebvre]

The 80% rule is based on mAH used, not voltage. But there's also a pretty hard limit at 3.0V per cell. If you really load a battery, you could see the voltage sag to lower than 3.0V/cell, while it still has half it's capacity left. I believe this is bad for the battery too.

What kind of amp load were you using with those Zippys where you had the problem?

 

[gtranquilla]

One thing I cam across only a couple of days ago while working on an industrial electrical project involving larger HP motors was this very interesting fact....
The motor torque is directly proportional to the square of the supply voltage.
Larger rigs use larger diameter props which may turn slower but still require more torque.
Both the cost of the motor and its efficiency are very dependent on the design voltage as the torque requirements increase.

Jfro,
I'm no electrical engineer, but I have a little experience with these things, so let me comment. First of all, I think we all have to understand that there is a connection between voltage and current/amps with batteries, but they're very different animals. Without going off road here and exposing my lack of actual knowledge, let me just say that the 80% rule is not only quite arbitrary, it's not very specific. Are these folks saying you should take your voltage to 80% or use up 80% of the current in the battery? Either one is possible but neither is very useful. The reason for that is that you want your batteries to operate within a range of safe voltage (4.2V to 3.something) and be able to utilize as much current as possible (for example, 4500mAh of your 5000mAh LiPo). Knowing your batteries is critical, as pushing a battery too far can be catastrophic both to the battery and your copter.

By way of example, I bought a bunch of Zippy 8000 6S batteries when I built my copter. I had good luck with the 4S models, and given the cost, was planning to use them on my big rig. But what I found is that these were completely useless. I routinely saw the voltage on these sag to the 3.2-3.3V range after only using up 6000mAh of the theoretical capacity of 8000mAh (confirmed by my charger afterwards). I then did some research and bit the bullet and bought some of the new Tattu 10000 6S batteries, and it's a world of difference. I can get 9000mAh out of these and still be at 3.5V in the air under load. I fly dual batteries, but even so, like I said, I rarely push these. It is good to know, though, that they can handle it if I get a "hey, can we get one more angle on that shot?"

So, like I said, take this with a grain of salt, as I'm certainly no [MENTION=3314]R_Lefebvre[/MENTION] when it comes to real engineering, but I do have a lot of hours logged with these systems.

 

[Motopreserve]

I'm definitely no wizard when it comes to these power systems, so I typically turn to those that are

in addition to many knowledgable people here on this forum, I have learned a ton about batteries, chargers, power supplies etc HERE. When the 80% rule was discussed on that page, I didn't take it as "law," but felt it was probably a good corroborating resource to all the folks here who refer to the 80% rule and how to determine that with the specific rig/power system you run.

I adhere to the rule as a guideline, often landing earlier than that rule would dictate. I figure the batteries are expensive enough, offer enough variables from brand to brand, and could cause enough grief to ply it on the safe side of these guidelines.

I have a telemetry sensor which shows consumption and voltage, and I reference that data against the mah put back in by the charger. I keep an excel sheet of this data for later reference, which includes: start voltage, end voltage, flight time, consumption, mah replaced (and calculation formula relative to 80% rule), charge rate, IR etc. so far it seems to keep me honest, and I don't feel I've damaged or over discharged any batteries.

 

[Quinton]

Me? I'd use a helicopter.

So for example, to achieve the 4.6 kg/m^2 disk loading my quad has, with with a 10kg AUW, you'd need 2.17m^2 of disk area. On a quad or X8, that would require 33" props. On an octo, 23" props.

Don't you just love this place, and learning about stuff like this.. (another 23:06min of my life gone watching videos
Thanks for sharing.

 

[stevemaller]

What voltage are you using? 4S or 6S?
I'm assuming 6S, in which case, you're using 132 Watts/kg to hover. You're not efficient unless you're under 100. My quad does 90 W/kg.
The issue is you're lifting 10kg on about 0.51 m^2 of disk area. Or 19.6 kg/m^2
Compare that to my quad, which is 2.099kg on 0.456m^2. That gives 4.6 kg/m^2.
This means that to fly, your props must accelerate the air to a much higher speed than in my case. That is less efficient. There's no way around this rule unfortunately.

Robert, I did a few back of the envelope calculations with my thinking cap and my slide rule, and I don't quite see the same numbers as you do. Can you clarify? Thanks!

1. What do you mean by prop disk area? Do you mean the area transcribed by a single prop as it spins?

2. How did you get to a disk area of 0.51 m^2? A 16" prop has a radius of 0.2032 m, therefore it's disk area is 0.129717 m^2. An octocopter will have total prop disk area of 1.6256 m.

3. How did you calculate the "efficiency values"?
Given a 6S battery at 25.2 volts, the two data points for the GH4 and the 5D are:
GH4: Weight = 9.97903 Kg, 58A, 1461.6 watts. So the "efficiency" is 146.4 watts/kilogram.
5D: Weight = 10.4326 Kg, 67A, 1688.4 watts. So the "efficiency" is 161.8 watts/kilogram.

4. Why should the efficiency be less than 100?

 

[R_Lefebvre]

1. In the case of flying machines, it's the total area of the disks, when viewed from the top. And X8 only has 4 disks unfortunately.

2. 0.129717 * 4 = 0.519. 0.129717 * 8 = 1.037736

3. I don't use 4.2V/cell. I use a nominal voltage of 3.8V/cell, as this is typically the average cell voltage throughout a flight. This is just an estimate, without actually have more data. I didn't do your numbers with the 5D, it's interesting to see how the efficiency really tanks just from the extra half kilogram. This is the problem with hovering efficiency.

4. Just an empirical number, based on experience. 100 is a good target to shoot for.

 

[Ronan]

With lipo shipping being such a pain (and slow), where do people order their high quality 6s lipo's in CONUS?

With a new multicharger i'm changing all my batteries, probably to PULSE or GENSACE, and it would be nice to receive them in a timely manner... lol

 

[stevemaller]

With lipo shipping being such a pain (and slow), where do people order their high quality 6s lipo's in CONUS?

With a new multicharger i'm changing all my batteries, probably to PULSE or GENSACE, and it would be nice to receive them in a timely manner... lol

Check HeliDirect for Pulse LiPos.
Tattu are super hard to find. Ask Mrs. Google.

 

[tombrown1]

I guess I don't understand these expensive lipos. I've been running NanoTech 8,000s for a year and a half with hardly any problems. I think I've had a cell go bad on two of them, but it's easy to spot. One cell will start to be very difficult to balance, and then you replace it. Shipping comes quickly from Washington.

I can buy two NanoTech 8,000s for $100 cheaper than 1 16,000 Pulse. Or one NT 8,000 for $20 cheaper than a 6,000 pulse.

And they work great!

As far as reliability - none has failed me in flight - but if one did, then I'm running two in parallel with alarms on each - if one goes bad then the alarm goes off and I land quickly under power from the other lipo.

 

[Old Man]

I haven't run numbers for energy density but I use Max Amps batteries because they have worked better than anything else to date. The fact their prices are considerable lower than what one pays for Thunderpower in the U.S. doesn't hurt, nor that they were found superior to anything else tried for a 40lb electric motor experiment to replace a gas engine on a UAV. But a story about shipping might be in order. Due to a mix up between some people in placing some battery orders it was discovered some critical batteries that were needed for a high profile flight demo never got ordered. With three days left before the demo Max Amps finally got the call and promised they could get them to us in time. Max Amps is in Washington and they had to ship to Texas. Two days later there were (6) 5s 6000ma batteries sitting on the door step.

 

[jfro]

I guess I don't understand these expensive lipos. I've been running NanoTech 8,000s for a year and a half with hardly any problems. I think I've had a cell go bad on two of them, but it's easy to spot. One cell will start to be very difficult to balance, and then you replace it. Shipping comes quickly from Washington.

I can buy two NanoTech 8,000s for $100 cheaper than 1 16,000 Pulse. Or one NT 8,000 for $20 cheaper than a 6,000 pulse.

And they work great!

As far as reliability - none has failed me in flight - but if one did, then I'm running two in parallel with alarms on each - if one goes bad then the alarm goes off and I land quickly under power from the other lipo.

I have 6 of the 8000 6s nano techs. So far, all are good as I've not pushed my flight times so no real stress. Curious, how do you replace a cell on those in case I ever run into a bad cell.

I have that on 2 of 5000 4s batteries, 1 hard case, 1 soft, so maybe I can learn something today.

 

[jfro]

Old Man, service is worth something. When I buy, I weigh price and service and can go either way. So far on batteries, I've opted for price.

I've never used either Maxamps or Thunderpower, but just googling them, they seem to be pretty much the same price on the 5000 mah 4s, but max amp a little pricier on the 8000 6s ($15). Compared to the nano 8000's I'm using, they are $315 vs $112 . That's a significant difference. I have about $700 into 6 batteries and that keeps me flying all day. So far, I've had no issues, but can't compare flight times to anything else as these are my first 6s batteries. I do get a little better times than ecalc says. I tried some Turnigy's and won't buy them again as I was much happier with the GensAce 4s's. Not sure I will ever buy the real expensive lipo's unless I have a catastrophic failure on something I own.

I haven't run numbers for energy density but I use Max Amps batteries because they have worked better than anything else to date. The fact their prices are considerable lower than what one pays for Thunderpower in the U.S. doesn't hurt, nor that they were found superior to anything else tried for a 40lb electric motor experiment to replace a gas engine on a UAV. But a story about shipping might be in order. Due to a mix up between some people in placing some battery orders it was discovered some critical batteries that were needed for a high profile flight demo never got ordered. With three days left before the demo Max Amps finally got the call and promised they could get them to us in time. Max Amps is in Washington and they had to ship to Texas. Two days later there were (6) 5s 6000ma batteries sitting on the door step.

 

[tombrown1]

Don't replace the cell. Discard and buy a new one. There may be a way to do it, but I'm not smart enough to figure it out. Not worth the trouble.

 

[Ronan]

No one talks about it... what about weight?

Are all batteries equal in that regard? Mine are pretty heavy, over 800g for a 6000mah, x2, that's 1600gram in battery weight, killer!

 

[fltundra]

No one talks about it... what about weight?

Are all batteries equal in that regard? Mine are pretty heavy, over 800g for a 6000mah, x2, that's 1600gram in battery weight, killer!

Multistar 10AH4S 10C only weighs 801g.
10AH6S 10C at 1189g

 

[Quinton]

Multistar 10AH4S 10C only weighs 801g.

Time for a new thread/spreadsheet battery power/weight ratio..

Desire Power 6S 35C continuous

5200 mAh 736g
6200 mAh 883g
8300 mAh 1115g
10400 mAh 1591g
20000 mAh 2492g

..without discharge connector

..or put another way..

7.06 mAh/gram
7.02
7.44
6.53
8.02