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Old Oct 02, 2012, 04:27 AM
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You didn't mention if it was just one battery for the two EDF's. But it seemed likely it was that.

At 116Amps that is 2.3mins flying time at WOT!!!
Mine (2x 3000mAH) gives 3.4mins at WOT. (both cases theoretical - my timer is 3.0mins on the 3000's)
You could back off and not use WOT.... I only use 50% throttle most of the flight anyway (but the 50% throttle point seems like 80% power really).
But anyway, it still adds up to 2.3mins at $70 versus 3.4mins at $40 (2 batteries), except you can go a BIT faster - only a bit, as the drag will eat a large amount of the higher thrust it has anyway (it is a squared law and thus EATS power).
Would need to see speed results to know the exact difference.... (I guess 10kph???)

Holding voltage is somewhat irrelevant to flying. It isn't really that important and only really matters to FAST flying. Mine is well over scale flight speed anyway. All my scale aircraft are over scale speed, with 'cheap' appropriate batteries used.

A battery doesn't care if it drops volts, that is just what C/IR causes and doesn't matter to the battery itself to any degree of importance. It is 'normal' to the battery and if you are not running them hot and puffing them (excessively) then all is fine anyway. My 3000's get 'hot', but just to normal expected temps. (minor puffing)
You only need low IR, to hold volts, if you want to the top ends of power. In most planes this is excessive speed - just for 'fun', not for scale. Fine if that is what you want it to do, but I have some fast planes and some average/slow ones... and no one watching cares either way. The majority of spectators are "That looks great" etc, not "That is fast." ("too fast for scale"). But some planes it is nice to have a fast one - just one or two will do..... LOL

If I go to 3300's (pair), or 3700's (pair), that is 6600mAH and 7400mAH... WAY more than 4400mAH.... and then $68 the pair (for both types - still $2 under the Nano cost). Power is up then (not needed anyway), and flight time still up also. 3.5mins and 4.0mins WOT. So a Nano still does not win that comparison EXCEPT for the 10kph better it got, and FAR less flight time in comparison then.
It just doesn't add up..... Not to AIM for the Nano (or any high C battery), or buy it intentionally... So I would not ADVISE anyone to buy expensive batteries. EXCEPT, as mentioned, if you want a high performance plane - and then it had best be some slick F-16 type jet, so you can get the use of the power with low drag. And even then.... for that higher power case you really want better motors than these $13 L2855's if you want it to last too! You don't even want more than 1100W max used on these! If not 1000W preferably.
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Old Oct 02, 2012, 04:31 AM
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"+1 on this combo. It's the cheapest effective for airframes up to 1.2kv."
That is plainly wrong. It costs $10 MORE than the 5S path. For LESS power.
"Cheapest"???
ANY 4S combo is a waste of time/money/result. (against that 5S combo)
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Old Oct 02, 2012, 04:53 AM
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Well, the ME262 is a twin. A bit hard to write too much on an android phone.

Peter, I am with you on the 2300kv/5s but the guy asked for 4s. Works really well in my vampire and F-86.
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Old Oct 02, 2012, 06:10 AM
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If a battery sags out its voltage excessively then its not coping with the load, simple as that, there is no magic here, dropping voltage means inability to cope with load.
This hurts batteries over time, they overheat easier due to the higher internal resistance and if you discharge them too far when stressed you will get plating out of the lithium, the byproduct being oxygen which is what causes swelling. Age will also cause them to swell up much easier when loaded up. Once this process starts the cell loses capacity and will eventually fail.
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Old Oct 02, 2012, 06:36 AM
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Quote:
Originally Posted by PeterVRC View Post
Just for info to Burdo and others:

You mention 50Amps from a 4S 2650mAH 35C, they cost $30
It weighs 291g
50A x 14.0V (??) = 700W
You will need a 60A ESC or higher.
It would last 3.1minutes at WOT.

If you used a L2855-2300kv on a 5S Zippy 3000mAH 20C battery, which is $20 ($19.80), and weighs 379g, you get 1100W at 55A and would last 3.3mins at WOT. Same ESC needed.
But you also have the option that you could fly at 70% throttle, for the same thrust as the 4S combo, and get over 4mins easily.

So the total combo was $10 less for 5S and you got 45secs more flight time.
The 80g heavier weight is a nothing really, plus foamies are too light anyway, and the huge amount more power will exceed that difference by miles too.
It would be about 900g thrust for the 4S and 1.30Kg for the 5S (on that battery).
For that $10 less......

Nothing compares to that 5S combo, it just slaughters all others for the cost and with 'good' output (equal or better than 4S). Nano Schmano, LOL.... all that matters is the end result - and what it cost
You can't even justify 4S on "I dont need as much power".... as it is CHEAPER to have MORE. Or, throttle limit it to get more flight time anyway. You can even restrict airflow ON PURPOSE, to get a better jet sound, whilst using the less power amount that will cause, and have the lower thrust you only 'needed' if you really wanted to.

There is just no other combo of use at all, for this 1100W and under situation.
Then above it (1100W upwards) you do start to have a FEW options/choices you could make. But even then, that next level is almost only HET (with a narrow window for cyclone power), unless you can get a cheap Turnigy XK (which none are around anymore). So really, the useful effective/cost choices in the CS10 are quite small.
Peter the guy asked for a 4S set up which may be due to already owning a number of 4S lipo's and not owning ANY 5S lipo's, which would then make your setup cost more expensive....

The setup I quoted pushes my airframe around with relative ease (ef-16). I only fly at 1/2 to 3/4 throttle for most of the flight so I get great flight times without the additional weight......
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Old Oct 02, 2012, 12:28 PM
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Quote:
Originally Posted by Burdo View Post
Peter the guy asked for a 4S set up which may be due to already owning a number of 4S lipo's and not owning ANY 5S lipo's, which would then make your setup cost more expensive....

The setup I quoted pushes my airframe around with relative ease (ef-16). I only fly at 1/2 to 3/4 throttle for most of the flight so I get great flight times without the additional weight......
Both of you have answered my question. Thanks.
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Old Oct 02, 2012, 06:32 PM
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Voltage sag is NOT an issue... it is a natural by-product of any battery.
They have Internal Resistance and that, combined with the load's resistance, form a voltage divider. Thus SOME voltage MUST apply over the battery.... that is what the SAG is.
"Sag" is actually a misleading term.... the word should be DROP. Sag makes it sound like a bad thing, when it is not a problem at all.

It is not actually bad for the battery at all, it is just a cost of the IR and totally expected and normal (known to designers of the battery). All that matters is EXCESSIVE HEAT.... not just 'normal expected heat'.... and as long as you aren't over the designed operational heat levels, it is fine and of no detriment.

So as long as your application does not puff the battery too much, all is fine and your application was technically fine to do. And if you use a 20C battery in a certain combo that is suited for it, then it will do the job fine.
eg this L2855-2300kv case.... 20C drives it totally fine, for good power. 65C will drive it X amount better (not 3x better though!!), but at far more cost than its ratio gave you!

Lower IR means the resistance of the battery becomes less than the load's (motor's) resistance, compared to another higher IR battery. Thus volts drop less, because there are less 'used' across the battery, and thus you get more power because the load had a bit more volts across it.... BUT that lower IR battery always costs a LOT more. In general (always?) a cheaper battery, which is because it is a higher IR battery, will give X amount less power.... but will be cheaper than just X amount less dollars.
eg Low IR power of "100" for $100, higher IR power of "80" for $60
You pay quite a premium for lower IR,so unless you NEED it for some reason, it is a waste. Somewhat like owning a Ferrari in a city where it can never go over 80kph in..... you pay a LOT more, for potential you may never need, or use.

(Complete battery/electronics explanation needs a LOT more words than just this even! But this covers most of the key points)
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Old Oct 02, 2012, 06:46 PM
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I can see that people still aren't going to understand this battery stuff.....
It just so happens that this L2855-2300kv combo is a perfect case for demonstration and explanation!

The load the CS10 puts on a 2300kv motor that is TRYING to do its unloaded RPM (until a load is on it) means it would use "Current X", IF it can get that from the power source - battery. When current flows, the battery/load voltage divider causes some to be lost over the battery - according to its IR - so the motor gets "Voltage X", which then cause "Current X".... it is sort of a LOOP that will reach an equilibrium very quickly.... of what current the motor ends up running at.
Two batteries..... a 5S 3000Mah 20C and a 5S 3000mAH Nano 45C.....
60A sustainable, and 135A sustainable.
The Nano has less IR so the equilibrum has the system settle at a higher current level - because the voltage drop across the Nano battery is less, thus more volts are over the motor = more power.

Real results of this combo show: 20C = 53Amps typical (in a plane with low airflow resistance), 45C Nano 58Amps.
20C power = 1050W, 45C power = 1200W (not my number).
53A is within the 20C's specs.... but getting to close to max. It will run 'hot' - not puffed excessively, but as the battery depletes this will head towards being more of an issue.
58A is well within the 45C's specs. It is actually way under-utilised at this load (like the Ferrari analogy), so it is just cruising through the task, probably just warm at worst.

In this case you got a little bit more power from the 65c, as it was overkill, and the 20C coped fine with its job - though you have it running near its designed maximum, and better not push it hard right down to depletion.
Ideally you would use 25C or 30C. BUT they cost 50% more than the 20C. So you could throw way a 20C battery reasonably often, over time of course, versus paying 50% more.
20C is also lighter than 30C or higher. (of course)
Then the Nano 65C.... over triple the price! And it was never a great use anyway.
It has a lower voltage drop.... but that never helped you much anyway.
If you WANTED or NEEDED the 150W more it gave, then you might decide the triple cost was worth it.....

So the bottom line is use the BATTERY for the JOB required. No less..... and most likely no more.....
There is some leeway to head upwards (higher C) viably.... accepting the higher weight and cost.... if you have valid reason for that. But really, the bottom line of 'need more' is 'I want more speed". And the amount more you are going to get is going to cost you a LOT, for every bit higher you head for.
In most cases the lower C solution is more than adequate speed for the given plane. eg still higher than scale speed.
But if you want 'rocket speed', then you are in the region of 'needing' higher C..... but also a better motor!!!
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Old Oct 02, 2012, 08:31 PM
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Sigh, seriously do you read what you type or just bang away...I used the term sag because its commonly used in US forums and understood by most.

If the voltage "drops" excessively then the battery is not coping with the load. Thats what I said and its a simple concept. I have been working with a batt engineer and his chemist brother for 6 years now, I discuss Lithium technology with him regularly. Voltage sag IS an issue if excessive as it is an indicator that the battery is overloaded for the application, and overloading the cells will cause premature wear and reduced cycle life.
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Old Oct 02, 2012, 11:50 PM
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You obviously aren't talking about the right things with him then.... (your statements are not actually wrong, but they don't actually tell/explain anything of great use either - very vague generalisations, and easily LEAD to misunderstanding of how it all truly works).
Plain and simple IR is a factor of a battery. For all the various loads you could ever choose to use on it, it will reach an equilibrium of current based on the load resistance and the battery resistance (IR - which is somewhat dynamic even).
The only difference (so far) between low IR and higher IR is what that equilibrium will be.
Low IR, less voltage drop (over the battery), more current/power for that systems load. Higher IR, more voltage drop (over the battery), thus less current/power for that system load.
Both batteries will run totally fine and within their specs. The higher IR battery fed less power, less current, so over its own IR (P=I2 x R) the power 'wastage' was no more than the low IR battery. The IR limited the current of the system anyway.
The battery, and its specs, all cover and were designed to do this totally fine.

The AMOUNT of volts 'dropped' over the battery, increases in all cases of load current increase - low IR or not - it is irrelevant. It is normal.... a cost/side-effect of any battery... and only reaches a point to matter at all if the battery is driven to try to feed higher than its C rating. And TEMPERATURE, not voltage drop, is the best indicator of that limit.

But using a lower C battery, due to its higher IR, is even a way to limit current of a set load. As long as you do the maths/tests and it doesn't overheat the battery, there is zero issue doing that.
The amount of voltage drop itself, over a battery, is totally irrelevant to it functioning fine or not - it only reaches a point to matter when the POWER of the resultant I2xR=W overheats the battery.

eg
24v battery.....
drops 0.5V due to the load/current created by the equilibrium of impedances... stays cool = no issue.
drops 1.0V due to the load/current created by the equilibrium of impedances... ... gets a bit warm = no issue.
drops 1.5V due to the load/current created by the equilibrium of impedances... ... gets a bit hot = no issue.
drops 2.0V due to the load/current created by the equilibrium of impedances... ... gets very hot = An ISSUE at this point.

Dropping anything up until 2.0V was zero issue to that battery.

"If the voltage "drops" excessively then the battery is not coping with the load."
That is a fairly useless sentence as it has no definitions..... "excessively" - what does that mean?
Like my 4 examples above, where did 'excessively' start? 1.0v, 1.5v? The amount of voltage drop was IRRELEVANT, until HEAT showed what amount of voltage drop in that exact battery/load system began to matter. 2.0V in that case.
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Old Oct 02, 2012, 11:52 PM
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"Sag" is a useless and misleading term because 90% of people around here are "electronic lay-persons", so they instantly see sag as a 'bad' term. It is not a bad term at all, just a fact of life with a battery. So whoever came up with that dumb word for it didn't do much of a favour for all modellers!
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Old Oct 02, 2012, 11:53 PM
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you forget one of the main benefits of high C lipos...

faster Charge rates
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Old Oct 02, 2012, 11:57 PM
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Probably true.....
Some of your cost is returned in that faster turn-around. Something to consider.... if that is a useful thing for your case.
But you then also need more money for 'big' chargers to feed them that higher charge current!!
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Old Oct 03, 2012, 12:19 AM
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Quote:
Originally Posted by anlucas View Post
Well, the ME262 is a twin. A bit hard to write too much on an android phone.

Peter, I am with you on the 2300kv/5s but the guy asked for 4s. Works really well in my vampire and F-86.
Which F-86 are you running on 2800kV at 4s?
thx Clemens
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Old Oct 03, 2012, 01:34 AM
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Originally Posted by mopetista View Post
Which F-86 are you running on 2800kV at 4s?
thx Clemens
The Starmax or Venom as it is referred to in the relevant rcgroups thread.
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