Batteries

[Ken Culbertson]

This probably belongs in the Electric forum but it reall is just a quick question.  I an using a 2200mAh 4s 30c battery right now on my Cobra 2820/12 motor (Castle 50 ESC) running at 9800 rpm and landing at 80% utilization.  Can that battery be replaced by a  "3S 20C-30C 5A 5000mAh 11.1v" and product similar results.  I know what all of these numbers mean now but I am still not clear on the relationships of all these numbers.  The 5A - what is that? or is it simply restating the 5000mAh?

I suspect that it will work but the motor may run a bit hotter. 



Ken

[Tim Wescott]

For the same energy storage, cell count (or voltage) times mAh should be the same.  2200mAh times 14.8V is roughly 32 watt-hours (I did that in my head -- go right ahead and check my math).  5000mAh times 11.1V is around 55 watt-hours.  So you're getting way more energy.  Are you sure the battery pack isn't that much heavier?  Or are you looking for nose weight?

BUT:  The motor needs voltage to spin.  I'd have to look up the numbers, but with a launch voltage of 11.1V and 9800 RPM, you'd need a kV of around 1200 RPM/V to be sure you wouldn't be sagging at the end of the flight (kV needed is RPM / (0.75 * launch voltage)).  I dunno what that motor will do, and I'm too lazy to check!  The input current to the ESC would go up by 33%, but if it's not burning up now it probably won't, because the lower voltage will actually be easier on the transistors (I'd still check).

The "5A" part probably means the rated charging current.

[Joe Ed Pederson]

Ken,

STRIKEOUTS ADDED NOV 30TH BY THE AUTHOR

The number of cells has no bearing on how long the battery will run a given motor!  The only thing that determines how long the battery will run a given motor! The only thing that determines how long the battery will run the motor is the milliamp rating.  The first two sentences are not correct.  See Tim Wescott's post three posts down.  The C rating also doesn't have anything to do with how long the motor run will be. 

The purpose of the C rating is to tell you how many amps the battery will comfortably deliver.  This matters because every motor is rated for a maximum amperage draw.   The battery must be able to deliver at least the maximum amp rating of the motor.  To determine if the battery has the capacity to deliver the amount of amperage the motor will draw you multiply the milliamp rating times the C rating.   So, a 2,200 mah battery (which is the same as 2.2 amps) with a 30 C rating will deliver 2.2amps x 30C = 66 amps.  So, any battery with a 2,200 mah 30C rating can support a motor with a peak amperage draw of 66 amps.   The formula for how many amps the battery will deliver at any given moment stays the same regardless of how many cells the battery has (amps x C rating = amps the battery can deliver).  So, a 5,000 mah (5 amps) battery rated for 20C will deliver: 5 amps x 20C = 100 amps.  Also, a 5,000 mah battery rated at 30C will deliver: 5 amps x 30C = 150 amps. So, a 5,000 mah battery at 20C or 30C is way beyond what you need.  As I said, the number of cells (voltage) does not determine how much juice a battery can store or deliver. 

To repeat, how long the battery will run the motor is not dependent on how many cells the battery has. Flight times are only dependent oon the milliamp rating if the battery.  Also, how big a battery is needed to power your motor is not dependent on the number of cells, but rather the milliamp rating and the C rating of the battery.  Now, the manufacturer will tell you how many cells the motor was designed to run on.   The manufacturer will tell you the minimum and maximum number of cells the motor was designed to operate with.  If you want to burn up or shorten the life of a motor that is rated for a maximum of 4 cells, try running it on 6 cells.  It won't last long, if at all.

So, if you want the same run time and want to go from a 4S battery to a 3S battery, the milliamp rating should stay the same and the C rating should stay the same. In other words a 2,200mah 4S 30C battery will give the same run time as a 2,200mah 3S 30C battery   [As Tim's post below demonstrates, they won't be exactly equal.  In the example he gives there is an 8% difference going from a 5S to 6S battery.] And both a 3S and 4S battery rated at the same 2,200 mah and 30C have the same capacity to deliver the same 66 amps to the motor.  Interesting enough, most 2,200 4S batteries will be the same length as a 3S 2,200.  The only difference will be the 4S will be one cell thicker.

However, voltage determines thrust.  You will never get anywhere near as much thrust using a 3S as you were getting from the 4S no matter how big a milliamp rated battery you use no matter how big a milliamp rated battery you use [See Tim's post below.] or how high a C rating you use.


Now the only way to know how many amps the motor is actually drawing is to buy a Wattmeter and check it for each different prop you use (every different prop diameter, pitch and manufacturer will draw a different amount of amps for a given motor/battery combination).  In fact, if you attach a scale to the tailwheel to measure the thrust, the watt meter will be able to tell you which prop gives the most thrust for the least amperage draw (longest flight time.)  I checked the You Tube reviews of different wattmeters and the one that reads accurately at different voltages is the "Watts-Up." I just checked and it is available on Amazon for $29.95 but wasn't at Tower Hobbies, Horizon Hobbies, or Hobbyking).  This is the one I bought since I was building a lot of Flite Test foam board RC models and was using non-standardized motor, prop combinations.

If you don't want to get bogged down in the weeds with a watt meter, use combinations of motor, prop, ESC, and battery one of the suppliers recommends.  And if you  just want to risk it, run the motor for a while and check to see if the motor, ESC, or battery gets hot.  It's not unusual for these items to get a little warm, but they shouldn't burn you to touch them.  Also, if your battery swells, or the shrink wrap on your ESC bubbles, you were drawing too much current.

Each electric motor has a Kv [See Tim's correction 3 posts down] rating which stands for kilovolts which means how many rpms in a perfect world the motor will spin a reasonable prop for every volt the battery can deliver.  So, a 1,000 Kv motor on a 3S battery will deliver 1,000 Kv x 11.1 volts (a 3S battery) = 11,100 rpm.  The reason a 4S battery will give you a lot more power is that for every volt increase the rpms will increase whatever the Kv rating is.  Thus, the same 1,000 Kv motor on a 4S battery increases to 14,800 rpm (1,000 Kv x 14.8 volts = 14,800 rpm).

 Now what may seem odd is that you must run a smaller diameter pitch prop on a 4S battery than on a 3S battery or the motor will draw a lot more amps. In other words, if you change from a 3S battery to a 4S battery and don't decrease the diameter of the prop, you will burn things up.  But since you are thinking of decreasing the voltage by going to a 3S from a 4S you will want to increase the diameter or the pitch of the prop from what you were using on the 4S battery to properly load the motor. [These statements are not true.  See Tim's post below.] Again, a wattmeter is the only way to know if the motor is loaded properly (not too few and not to many amps).


Incidentally, since I have a watt meter, I have found that the range of props the motor manufacturer says you can use may overload the motor (and thus the ESC as well) under certain prop/battery combinations.  So, even if you use a prop recommended by the motor manufacturer there is a possibility you may be overloading the motor/ESC.  The only way to tell for sure is with a watt meter.

Hope this helps,
Joe Ed Pederson

[Ken Culbertson]

Hope this helps,
Joe Ed Pederson

Thank you gentlemen.  You have convinced me that the $10 a battery is not worth taking a perfectly happy 4s setup and re trimming for a 3s only to have to add an oz of nose weight as well.  I will stick with 4.  The combination I have now gets me a full pattern, about 10 extra laps and I am on the ground under 6 min with 20% still remaining on the battery.  Why mess with something that works.  I think the Castle 50 ESC keeps flight data.  I need to download it and see what I am actually doing.  I am using a 12 x 6P cut down to 11.5 and re-balanced (tailwind on takeoff).

If I read whet you are telling me correctly then replacing a 2200mAh battery that only uses 80% with a 2800mAh battery is not making any difference. And that explains why I don't see any difference when I fly a 2200 one flight and a 2800 the next.

Thanks Again - Question Answered!

Ken

[Joe Ed Pederson]

If I read whet you are telling me correctly then replacing a 2200mAh battery that only uses 80% with a 2800mAh battery is not making any difference.

Thanks Again - Question Answered!

Ken

If you're only using 80% and still have plenty of laps left with a 2,200 mah battery, then, yes, all you will be adding with a 2,800 mah battery is dead weight.  In fact, having a battery that uses about 80% of your battery each flight has the additional benefit of giving you close to the maximum life span on the battery as well.  With your motor/prop/battery combo and the length of motor run, that 4S 2,200 mah battery is the sweet spot.  That's as close to perfect a match between motor/prop/battery as you could possibly get.

Also, manufacturers of Lipo batteries have done research that shows that Lipo batteries will last longer if you store them at 3.85 volts per cell.  Most battery chargers have an automatic function that brings the battery back to 3.85 volts per cell whether the cells are above or below 3.85 volts per cell.  So, it's best to put your batteries back at storage voltage (3.85 volts per cell) if they are fully charged and you aren't going to use them the next day.  And it is best to bring them back up to storage voltage if you brought them home after using them for a flight.


Lipos will also last longer if the time spent in 90 F plus environments is limited as much as is practical.

Joe Ed

[Tim Wescott]

Ken,

The number of cells has no bearing on how long the battery will run a given motor!  The only thing that determines how long the battery will run the motor is the milliamp rating.

Upon what do you base this claim?  Have you done the experiment with a proper speed-regulating stunt setup?  It runs counter to the experience of some pretty competent guys I've seen post here, to my own experience designing motor drives in industry, to the fact that if you go up in cell count and fly at the same speed that nothing burns up.  In fact, here's a quote from some recent testing:

- I also ran a test of 5S vs. 6S setup using the Cobra 2826-12 motor (everything else same - prop, ESC, etc).  Strangely, the 5S setup consumed 21.46 Watt*hr for the 3 min run, while the 6S setup consumed 23.3 Watt*hr.

So Mike made a 20% change in voltage, and saw an 8% increase in the energy used -- if your claim were true, then a 20% increase in voltage would cause a 20% increase in energy used.  Assuming the same motor speed (which is what happens if you're running a proper setup) that energy would have to go toward heating up the ESC, the motor, or both.

However, voltage determines thrust.   You will never get anywhere near as much thrust using a 3S as you were getting from the 4S no matter how big a milliamp rated battery you use on or how high a C rating you use.

Upon what do you base this claim?  Have you done the experiment with a proper speed-regulating stunt setup?  As long as the battery voltage is higher than the voltage required to spin the motor at the speed determined by the timer and ESC, then your claim runs counter to basic physics.  If the prop is spinning at a given speed it doesn't matter how that prop is being spun, the prop will deliver the same thrust.

Each electric motor has a Kv rating which stands for kilovolts

I'm not even going to ask what you're basing this claim on.  It's just wrong.  In math an engineering, 'k' is often used as a handy constant, the way that 'x' is used to denote a variable.  The "Kv rating" is the voltage proportionality constant.  Regardless of how much it may look like "kV", which does, in some contexts, mean kilovolts, it does not mean kilovolts in this context.  That's probably why it's carefully capitalized to read "Kv" instead of "kV".  The world of science and engineering is a big and complicated place, and there's only 26 letters in the alphabet.  Even using Greek letters, there's still not enough letters that everything gets its own single-character constant.  That's just how life is in the nerd lane.

Now what may seem odd is that you must run a smaller diameter pitch prop on a 4S battery than on a 3S battery or the motor will draw a lot more amps.

Upon what are you basing this claim?  Have you done the experiment with a proper speed-regulating stunt setup?  It is only true for an unregulated motor.  If you're using a timer/ESC system that regulates the motor speed properly and you have sufficient voltage headroom, then this is not the case.  With a proper stunt setup the motor will spin at a constant speed (or at a speed determined by how the flight's going, but not the battery voltage) and will deliver the same amount of energy to the prop.  Ignoring efficiency, that will consume the same energy from the battery, which means less current for higher voltages.  The inefficiency of the setup will eat into that somewhat, but unless you've thrown the ESC/motor combination into a really inefficient mode of operation, the current drawn from the battery will be less -- see the quote of Mike Alimov's post, above, for the 5S vs. 6S case.  This is going to vary by motor and ESC.

In other words, if you change from a 3S battery to a 4S battery and don't decrease the diameter of the prop, you will burn things up.

Upon what are you basing this claim?  Have you done the experiment with a proper speed-regulating stunt setup?  It runs contrary to -- well -- everything that's been pointed out before.  Unless your combination was on the hairy edge of burning up on 3S, or was improperly set up (i.e., not enough headroom to regulate speed properly) then the combination will be less efficient, but most likely not bad enough to burn things up.

[Fred Underwood]

Tim, Thanks for your answers and explanations above. 

If you're only using 80% and still have plenty of laps left with a 2,200 mah battery, then, yes, all you will be adding with a 2,800 mah battery is dead weight.  In fact, having a battery that uses about 80% of your battery each flight has the additional benefit of giving you close to the maximum life span on the battery as well.  With your motor/prop/battery combo and the length of motor run, that 4S 2,200 mah battery is the sweet spot.  That's as close to perfect a match between motor/prop/battery as you could possibly get.

Also, manufacturers of Lipo batteries have done research that shows that Lipo batteries will last longer if you store them at 3.85 volts per cell.  Most battery chargers have an automatic function that brings the battery back to 3.85 volts per cell whether the cells are above or below 3.85 volts per cell.  So, it's best to put your batteries back at storage voltage (3.85 volts per cell) if they are fully charged and you aren't going to use them the next day.  And it is best to bring them back up to storage voltage if you brought them home after using them for a flight.


Lipos will also last longer if the time spent in 90 F plus environments is limited as much as is practical.

Joe Ed

Agree that heat can shorten the life or decrease the number of cycles for lipos.  A little different take on the 80% rule however.  If you use 80% of a 2200 mah battery, that is 1760 ma.  If you use the same 1760 ma from a 2800 ma battery, you use about 63%.  80% is considered safe, and we use that for weight savings.  If you need the weight for balance, then the 2800 is a good deal as you can charge to about 85% full and will then discharge to about 22%.  That may give many more cycles.  It only works well if the weight is not a penalty.  OTOH, it is good to keep in mind that charging to less than 4.2v is good if you don't need a full charge.  4.2v/cell then sitting in a box at the field may shorten the life/cycles.  Some of the smart phones suggest that undercharging to 85 - 90% may add cycles.

For reference, some information here
https://batteryuniversity.com/index.php/learn/article/how_to_prolong_lithium_based_batteries

Please don't take this as trying to pile on to Tim's explanations.

[Ken Culbertson]

Tim, Thanks for your answers and explanations above. 

Agree that heat can shorten the life or decrease the number of cycles for lipos.  A little different take on the 80% rule however.  If you use 80% of a 2200 mah battery, that is 1760 ma.  If you use the same 1760 ma from a 2800 ma battery, you use about 63%.  80% is considered safe, and we use that for weight savings.  If you need the weight for balance, then the 2800 is a good deal as you can charge to about 85% full and will then discharge to about 22%.  That may give many more cycles.  It only works well if the weight is not a penalty.  OTOH, it is good to keep in mind that charging to less than 4.2v is good if you don't need a full charge.  4.2v/cell then sitting in a box at the field may shorten the life/cycles.  Some of the smart phones suggest that undercharging to 85 - 90% may add cycles.

For reference, some information here
https://batteryuniversity.com/index.php/learn/article/how_to_prolong_lithium_based_batteries

Please don't take this as trying to pile on to Tim's explanations.

So much to learn, so little time.  I have to get this discharge/storage thing under control.  Just out of curiosity, how many cycles is average?

Ken

[Curare]

So much to learn, so little time.  I have to get this discharge/storage thing under control.  Just out of curiosity, how many cycles is average?

Ken

Ken, how long is a peice of string? I've been using using lipos for 10 years, and I can tell you that I have had packs with hundreds and hundreds of cycles, and some that have lasted less that twenty. In the olden days (2008 ha!) the big issue we were struggling against was pack discharge capacity, (this as you probably know now is your C rating, i.e. 30C is 30 times the capacity of the pack as your max amperage), and for high output operations -like ducted fans- they'd fall away quickly. Those same packs in a prop driven plane did countless flights, (5 airplanes, flying daily for nearly 4 years?) in the end they just couldn't deliver the amps, and were retired with full honours. My first F3A packs had 300 flights and were still good enough to do practice flights with when I sold my competition ship with everything (including batteries).

Fast forward to where we are today, and current draw is not puch of an issue, every pack is 40C or over unless you're buying trash. The big issue I seem to face these days is that of diminishing capacity. You buy a 2800mah pack, and use 2000mAh in a flight, and everything seems ok, but somehwere there's an issue, it's as though you've taken 2000 out, but can seemingly only put 1999 back in.

Over time you'll notice that the battery still has the same "punch" or current draw but now won't complete flight. it's as though your 2800 pack has now shrunk to an 1800!

In situations where weight is of primary importance (stunt especially), it's hard to get more capacity 'headroom' in a pack without incurring a weight penalty, but I can say this phenomenon seems to happen more with the cheaper packs.

I also suspect that those with higher cell counts (6s for instance) see slightly less amperage and may get less issues reduction in capacity, but can't say for certain.

[Ken Culbertson]

Ken, how long is a peice of string? I've been using using lipos for 10 years, and I can tell you that I have had packs with hundreds and hundreds of cycles, and some that have lasted less that twenty. In the olden days (2008 ha!) the big issue we were struggling against was pack discharge capacity, (this as you probably know now is your C rating, i.e. 30C is 30 times the capacity of the pack as your max amperage), and for high output operations -like ducted fans- they'd fall away quickly. Those same packs in a prop driven plane did countless flights, (5 airplanes, flying daily for nearly 4 years?) in the end they just couldn't deliver the amps, and were retired with full honours. My first F3A packs had 300 flights and were still good enough to do practice flights with when I sold my competition ship with everything (including batteries).

Fast forward to where we are today, and current draw is not puch of an issue, every pack is 40C or over unless you're buying trash. The big issue I seem to face these days is that of diminishing capacity. You buy a 2800mah pack, and use 2000mAh in a flight, and everything seems ok, but somehwere there's an issue, it's as though you've taken 2000 out, but can seemingly only put 1999 back in.

Over time you'll notice that the battery still has the same "punch" or current draw but now won't complete flight. it's as though your 2800 pack has now shrunk to an 1800!

In situations where weight is of primary importance (stunt especially), it's hard to get more capacity 'headroom' in a pack without incurring a weight penalty, but I can say this phenomenon seems to happen more with the cheaper packs.

I also suspect that those with higher cell counts (6s for instance) see slightly less amperage and may get less issues reduction in capacity, but can't say for certain.

Thanks, this helps.  I flew a lot of RC before the Lipo.  Is this anything like the nicad memory issue?  Should I run the batteries down to say 3% before I recharge them and how damaging is on the batteries to leave them charged for say a week?  Also, if I read you correctly, having a higher "C" value does not damage the ESC.

Ken

[Tim Wescott]

... in the end they just couldn't deliver the amps, and were retired with full honours. ...

An RC clubmember I knew kept all of those, connected together in series-parallel (to 14.8V, I think).  He'd charge them slowly at home, then use them to charge his other batteries at the field -- the overall capacity was there, but the discharge rate wasn't.

[Fred Underwood]

Many of us fly with 25c batteries with 2700 - 2800 mah cells, so they can put out about 67.5 amps continuous.  In general higher C batteries are heavier with larger diameter leads.  The standard TP batteries are 25c batteries as are many Zippy Compact.

I try to undercharge when I can still have enough capacity, and not leave my batteries at full voltage setting and still get around 100 cycles/pack of flying.  I have 3 packs, 6s, that I use to run my charger at the field and can recharge a pack or two, or use to top off packs that were purposely undercharged for travel or sitting at the field.

More ideas here
https://stunthanger.com/smf/gettin-all-amp'ed-up!/battery-discharger/

They old batteries flew about 100 cycles each, and probably have another 100 cycles but those 3 batteries field charging one pack at 5 amps or about 1.8C are only discharging at about 0.6C each.  The capacity of those pack are also low.


Lipo capacity degrades with cycles, not a memory problem.  So, no, don't discharge them lower that 20%.


C value relates to the battery pack ability to release of energy (non-engineer speak).  That will be controlled by the motor needs and the ESC and for our CL needs you will get similar results from 25C or higher.  All have the ability to feed the system at a higher rate than needed.

[Joe Ed Pederson]

My apologies to Ken, Tim, and everyone for the errors in my lengthy post above.  My experience with electrics is in the RC realm.  I was in error in assuming that electric CL setups run at near full throttle setting.

As Tim's quote of Mike Alimov demonstrates, my statement that the number of cells has no bearing on how the battery will run a given motor is wrong.  There is some effect, as Tim points out from Mike's post, the difference in Mike's case was an 8% increase in watt hrs motor draw when Mike went from a 5S to a 6S battery.  But as Mike's post demonstrates, his surprise at the results indicates that Mike didn't go to 6S to get a longer motor run or shorter motor run.  What I wanted to convey is that when buying a battery, of the three ratings on a battery (milliamp, number of cells, and C rating) the milliamp rating is what gives you the general idea of how long the battery will run a motor at a given speed setting. 

In RC, where full throttle is used frequently, a 5S battery at full throttle won't give you nearly as much thrust as a 6S battery at full throttle.  But this is apperently irrelevant to CL Stunt, so I wish I hadn't written it.   It's also because RC applications frequently use full throttle, that if you go up in cell count you usually have to go down in prop diameter to keep from burning up the motor.  But this, too seems irrelevant to CL Stunt, so I wish I hadn't written it.

Again, my apologies for the errors I wrote and the things that don't transfer from RC to CL Stunt applications.  I should have waited for someone else to answer Ken's questions.

Joe Ed Pederson

 

[Tim Wescott]

In RC, where full throttle is used frequently, a 5S battery at full throttle won't give you nearly as much thrust as a 6S battery at full throttle.  But this is apparently irrelevant to CL Stunt, ...

It is, unfortunately, not completely irrelevant.  Because battery pack voltages sag under high current load and at the end of a flight.  If you don't have enough overhead, then the system hits a point where the throttle is at 100% and the motor speed is determined by its load and the battery voltage.  Since those times are when you most want power from the motor, you want to avoid them.

That's the reason that people running active timers also tend to run extra cells -- the active timers ask for more from the system than the fixed-speed timers do, and that drives you to needing more battery for an otherwise identical system.

Another difference from RC practice is that it's quite common to have a motor/prop/battery combination that would burst into flames if you held it at full throttle.  And again, it's more common to have that with the active timer systems.  We can get away with that because the way the system behaves is to never hold it at full throttle, and never even approach full demand on the combination except briefly, and at the end of the flight when the battery is sagging.

All of this, in turn, makes our electric power systems a bit of a chore to wrap your brain around.  But at least you don't have to worry about getting a battery with the wrong proportion of castor and synthetic.

[Curare]

Thanks, this helps.  I flew a lot of RC before the Lipo.  Is this anything like the nicad memory issue?  Should I run the batteries down to say 3% before I recharge them and how damaging is on the batteries to leave them charged for say a week?  Also, if I read you correctly, having a higher "C" value does not damage the ESC.

Ken

Fred's touched on it for the most part but i will add my 2c, and say that you can forget everything you know about nicads, when talking lipo. The'yre completely different animals. A deep discharge of a lipo cripples it (having done it a few times by accident), where as you know, a few cycles of a nicad and you're almost back to where you were.

Trickle charging doesn't work with lipos as they're VERY VERY intolerant of overcharging. Nicads sort of cascade voltage when they are overcharged, 'filling' the lower voltage cells, lipo's just catch fire if you try that, which is why it's essential you use the balance leads when charging. The charger will monitor the cells, and pull voltage from the higher cells to balance them.

A good charger is key to long battery life, and you really want one that can charge and discharge to a decent storage level as well. Don't store them fully charged, or fully depleted as that degrades capacity.  To an extent everything you do degrades capacity (including not using them) you just have to manage and mitigate degradation as best you can.

[Tim Wescott]

Trickle charging doesn't work with lipos as they're VERY VERY intolerant of overcharging. Nicads sort of cascade voltage when they are overcharged, 'filling' the lower voltage cells, lipo's just catch fire if you try that, which is why it's essential you use the balance leads when charging. The charger will monitor the cells, and pull voltage from the higher cells to balance them.

Chemically, NiCd (and lead-acid, and a lot of other older rechargeable technologies) generate hydrogen that's recombined back to water if the charge rate is low enough.  LiPo's precipitate metallic lithium, which grows little whiskers to poke through the polymer from anode to cathode -- which is why they catch fire.

[Curare]

Chemically, NiCd (and lead-acid, and a lot of other older rechargeable technologies) generate hydrogen that's recombined back to water if the charge rate is low enough.  LiPo's precipitate metallic lithium, which grows little whiskers to poke through the polymer from anode to cathode -- which is why they catch fire.

I knew it had something to do with lithium dendrites, but wasn't entirely sure on the details, and didn't want to spread misinformation. Thanks for the memory jog, and schooling me on Nicads too!