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battery c rating

how critical?

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Yorkman08/01/2020 17:04:03
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Having read the article in the latest RCME mag about how the L39 Albatros wouldn't get off the ground with less than a 50c rated Lipo, I wondered what everyone else's experience is?

I've just acquired a 10 year old Lander Grumman Panther with a 68mm 5 blade fan for a 4s battery. My model shop has 30c rated Lipos to fit, but I'm wondering if l need to go higher?

The Panther is 36" span with a flying weight about 42oz.

Andy4808/01/2020 17:31:19
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Generally speaking you need to take the C rating with a huge pinch of salt. Perhaps C stands for crude.

However, I do find that somewhat higher C rating batteries do last a lot longer, especially if not used anywhere near their upper C rating limit.

Colin Carpenter08/01/2020 17:49:38
645 forum posts
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I purchased an FMS Avanti EDF for 4s. Try as I might with 4s lipos that gave excellent performance on aerobatic prop planes it wouldn't get off the ground ! 2 new higher C rating lipos later and bingo !!! Success !!Try what you have in stock but dont yank it off if the runway end is approaching ! Keep spending !😨😨😨😨😨😨 Colin

Former Member08/01/2020 20:49:17
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PatMc08/01/2020 21:23:09
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Posted by Tom Sharp 2 on 08/01/2020 20:49:17:

Too high a C rating and you risk burning out your motor seeing that it will have been made for a bygone specification.

Nonsense.

High C rating means that the battery's IRS (internal resistance) is lower. This means that the battery terminal voltage (output) won't drop as much as a similar battery with a lower C rating. In other words the higher rated battery loses less energy in the form of heat than a low rated one.

However, as a marketing tool the C rating number is invariably exaggerated but it does seem that the exaggeration is applied across the range with most makes. Meaning that the higher rated is still better than a lower rated from the same producer.

Keith Miles 208/01/2020 22:43:19
392 forum posts
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Posted by Tom Sharp 2 on 08/01/2020 20:49:17:

Too high a C rating and you risk burning out your motor seeing that it will have been made for a bygone specification.

The “C” rating will not burn out a motor. Only excess current in the motor will do that.

The battery, obviously, dictates the maximum applied voltage. The motor, and the load placed upon it, then determines the current for a given applied voltage and speed command from the ESC.

The “C” rating, in practice, will determine the level of voltage drop under high load/high current demand.

The higher the “C” rating the lesser the voltage drop will be under a given load.

The lower the “C”” rating the higher the voltage drop will be for a given load.

Ever seen a light bulb dim slightly when initially switching on an electric kettle? That’s voltage drop caused by a brief initial rise in current drawn by the kettle before the current stabilises and voltage rises back to the previous level.

The lower the current drawn by the motor, the lower the “C” rating can be to minimise volt drop.

The higher the current drawn by the motor, the higher the “C” rating may need to be to minimise volt drop.

A motor that exhibits poor acceleration, for example, but produces maximum speed might benefit from a higher “C” rating to cope with the initial starting current of the motor and the related initial voltage drop.

This is why a wattmeter is useful because it can give separate readings for voltage, current and power. Any voltage drop can be easily detected.

And power readings alone (watts) are not definitive of the most efficient motor performance because you could be running at low current high voltage or high current low voltage with high current creating heat i.e. wasted energy.

What I would suggest you should be aiming for, initially, is maximum power at maximum available voltage but minimum current. This should give the best performance/endurance combination I would think.

So, the only possible disadvantages of choosing a battery of too high a “C” rating would be increased size, weight and cost!

In the cases quoted, sounds like a voltage drop issue to me, now resolved!

Edited By Keith Miles 2 on 08/01/2020 23:16:19

Steve J08/01/2020 23:29:21
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Posted by Keith Miles 2 on 08/01/2020 22:43:19:

The “C” rating will not burn out a motor. Only excess current in the motor will do that.

For a given ESC/motor/prop combination, a battery with a higher C rating will lead to the ESC and motor seeing a higher voltage and current at full throttle than they would with a lower C rated battery. If the motor is close to the limit with the lower rated battery, the higher rated one could kill it.

Keith Miles 209/01/2020 00:57:20
392 forum posts
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5Posted by Steve J on 08/01/2020 23:29:21:

Posted by Keith Miles 2 on 08/01/2020 22:43:19:

The “C” rating will not burn out a motor. Only excess current in the motor will do that.

For a given ESC/motor/prop combination, a battery with a higher C rating will lead to the ESC and motor seeing a higher voltage and current at full throttle than they would with a lower C rated battery. If the motor is close to the limit with the lower rated battery, the higher rated one could kill it.

Sorry, no!

Read again what PatMc and I have said and then do some online research about batteries and basic theory. There is plenty of it. And stay away from forums for obtaining such knowledge until you have the necessary incite to enable you to sort the wheat from the chaff.

When you have done that, come back and point out where at least two of us have misunderstood the theory, at least one of whom is a retired electrician.

I, for one, am always happy to be re-educated or to learn something new.

Edited By Keith Miles 2 on 09/01/2020 01:00:03

Don Fry09/01/2020 07:19:12
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Keith, bit puzzled, but not enough to go and find out.

But I seem to remember, Pd = i times r.

In the the case of a lower C rated battery, its internal resistance is greater. So working Ohms Law above, substute a higher C rated battery the current goes up, less resistance, same Pd, more current

Now, Watts law says W = Pd times i. So in the case of fitting a higher C rated battery, transferring the current from our findings from Ohms Law, same pd, that gives us more watts. A few more revs on the prop therefore. A bit more stress.

So if the combo was marginal on the lower C rated battery, it might burn out with a few more watts being extracted.

What is wrong with the above analysis.

Going back to the original post, I've got a small ducted fan jobby. Fitted with low C batteries, performance and duration is pants, and it eats batteries, destroying them in a dozen flights. Fit a high C battery, it goes well, flies longer, battery is not stressed

Steve J09/01/2020 08:51:36
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Posted by Keith Miles 2 on 09/01/2020 00:57:20:
Posted by Steve J on 08/01/2020 23:29:21:

For a given ESC/motor/prop combination, a battery with a higher C rating will lead to the ESC and motor seeing a higher voltage and current at full throttle than they would with a lower C rated battery. If the motor is close to the limit with the lower rated battery, the higher rated one could kill it.

I, for one, am always happy to be re-educated or to learn something new.

Take two batteries that differ only in C rating. Connect to the same prop/motor/ESC setup. Open the throttle. More current will be pulled out of the higher C rated battery than out of the lower.

gangster09/01/2020 09:07:54
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The moral of the story is to measure the current if you change anyway especially the prop. Also we must realise that so much of this kit is cheap. Do not run it to the limit. Allow yourself 10% headroom when buying the kit including the battery’s. ie if you anticipate 40 amp current draw fit a 50 amp esc. The C rating of the battery’s are also the figment of the manufacturer’s imagination in some cases, we at least according to folklore

Nigel R09/01/2020 10:14:15
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Posted by Keith Miles 2 on 09/01/2020 00:57:20:

point out where at least two of us have misunderstood the theory, at least one of whom is a retired electrician.

If the motor is right on the button with a low C battery, a high C battery can supply slightly more current and slightly higher voltage, perhaps enough to push the motor over the edge.

It's a contrived case, granted, but correct.

Dickw09/01/2020 10:29:15
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734 forum posts
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Lowering the IR can make a considerable difference in performance. Cold days kill performance so apart from getting a battery with a higher C rating, you can also consider lowering the IR by keeping the battery warm before flying.

I actually I keep mine at 35 degrees C in a heated box when performance is critical - particularly in cold winter weather.

Dick

Keith Miles 209/01/2020 12:37:21
392 forum posts
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A battery has a given maximum voltage.

The current it delivers depends on the load and what the load DEMANDS.

The “C” rating, amongst other things, will determine its susceptibility to voltage drop on load i.e. when current is DRAWN from the battery.

The load, in this case, is the prop/ motor combination which should only be allowed to draw the current it requires to deliver a given level of power within the motor’s specified current limits.

If the “C”;rating is too low for the application, the voltage will drop, current will tend to rise to maintain a given power figure BUT if the load is too high (e.g. wrong motor/prop combination) the battery will be unable to deliver the current REQUIRED therefore LIMITING the maximum power available.

If a high “C” rating battery of the SAME voltage is applied to the SAME load, the battery voltage will remain HIGH and the battery will be MORE able to deliver the current DEMANDED by the load.

As one contributor appears to have found, a model that would not perform on a low “C” rated battery is now fine on the higher “C” rated one, probably because the battery could not deliver the necessary current without a drop in voltage.

In a sense, you could say that a low “C” battery might, in fact, protect a motor from burning out as it will be incapable of delivering the power demands (voltage and current) of an incorrect motor/prop combination.

Conversely, if you apply a high “C” rated battery to the SAME incorrect motor/prop combination the battery will be perfectly capable of meeting the DEMAND from the load and will happily supply current beyond the limits of the motor with obvious potential consequences!

No battery, of correct voltage, will damage anything connected to it but, if anything, quite the reverse!

A motor, however, will certainly burn out if too great a load is placed on it. It is the prop that places load on the motor and raises the current, not the battery.

Also, any electrical engineer will tell you that when you design any electrical system you START with the expected LOAD and work BACK from there in selecting the ancillaries that will meet that load plus a bit of “headroom”.

So, if you find that a motor burns out on the higher “C” rated battery but seemed fine on the lower “C” rated one, I would suggest that the motor/prop combination was probably wrong to begin with and whilst the power reading on your wattmeter might have seemed correct, a good chunk of those “watts” might have been producing more heat than prop speed!

Ask yourself this question.

Would the “C” rating matter if the prop was removed?

P.S. Profuse apologies to those who already have the technical knowledge. Merely trying to help those who don’t!

Edited By Keith Miles 2 on 09/01/2020 12:43:48

Keith Miles 209/01/2020 13:10:10
392 forum posts
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Posted by Steve J on 09/01/2020 08:51:36:
Posted by Keith Miles 2 on 09/01/2020 00:57:20:
Posted by Steve J on 08/01/2020 23:29:21:

For a given ESC/motor/prop combination, a battery with a higher C rating will lead to the ESC and motor seeing a higher voltage and current at full throttle than they would with a lower C rated battery. If the motor is close to the limit with the lower rated battery, the higher rated one could kill it.

I, for one, am always happy to be re-educated or to learn something new.

Take two batteries that differ only in C rating. Connect to the same prop/motor/ESC setup. Open the throttle. More current will be pulled out of the higher C rated battery than out of the lower.

Further to my recent post which should have covered your question but just in case you are still puzzled after reading it.

In your example , did you take a reading of the on-load, full throttle battery output voltage in each case?

And I do say voltage NOT watts.

I suspect not.

Go try it!

No cheating. Same motor and prop. Two same voltage but different “C” rated batteries.

Then come back to us with your results!

 

 

Edited By Keith Miles 2 on 09/01/2020 13:31:20

Keith Miles 209/01/2020 17:55:55
392 forum posts
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Posted by Steve J on 09/01/2020 08:51:36:
Posted by Keith Miles 2 on 09/01/2020 00:57:20:
Posted by Steve J on 08/01/2020 23:29:21:

For a given ESC/motor/prop combination, a battery with a higher C rating will lead to the ESC and motor seeing a higher voltage and current at full throttle than they would with a lower C rated battery. If the motor is close to the limit with the lower rated battery, the higher rated one could kill it.

I, for one, am always happy to be re-educated or to learn something new.

Take two batteries that differ only in C rating. Connect to the same prop/motor/ESC setup. Open the throttle. More current will be pulled out of the higher C rated battery than out of the lower.

Sorry, I meant to say that you are absolutely correct on that last point!

I, and certain others, have said nothing different.

And you will note that you said "pulled".

Also correct!

So, if your prop/motor combination i.e. the thing that is "pulling" the current, "pulls" current that exceeds the maximum rated current of the motor (which it will be more than happy to do if you have the wrong motor and prop combination!).........?

Eventually the penny might drop!

smiley

 

Edited By Keith Miles 2 on 09/01/2020 18:20:28

PatMc09/01/2020 20:31:58
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Posted by Nigel R on 09/01/2020 10:14:15:
Posted by Keith Miles 2 on 09/01/2020 00:57:20:

point out where at least two of us have misunderstood the theory, at least one of whom is a retired electrician.

If the motor is right on the button with a low C battery, a high C battery can supply slightly more current and slightly higher voltage, perhaps enough to push the motor over the edge.

It's a contrived case, granted, but correct.

It's so contrived that the same case could be made for using a battery that's previously been OK during mild temperature days but on a hot day would supply slightly more current ... etc.
Or if a similarly C rated battery just happens to be from a better batch & is connected...
Or if ...

Absolute nonsense

It's just not worth considering

Yorkman09/01/2020 20:42:38
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I seem to have poked a hornet's nest! I think from all the above my conclusion must be that basically a higher c rating simply means that the battery will maintain a higher voltage under load, therefore boosting take-off power....? (as reported in the RCME article) and of course, the only way to determine if a higher c rated battery is required is to see if the thing will take off or not....

Don Fry09/01/2020 20:47:00
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PatMc, you are correct. But, we are discussing boundary conditions. And they bite your backside when crossed.

I cite the charred fusalage of a glider on this forum today, or indeed when you see a F1 car fail spectacularly. A bit of headroom helps.

Peter Beeney09/01/2020 20:51:12
1593 forum posts
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@ PatMc

Exactly so. Quite agree.


Keith,

Again with the greatest respect,but there’s just one little point of interest relating to your post, I think the whole chunk of “watts” that you read on your watt meter is actually measuring the produced heat. The mechanical turning action of the motor is the result of a deflecting action between two magnetic fields. All the heat is generated by the current flowing though the resistance of the conductor, no heat is created by the action between the electrically created magnetic field and the permanent magnet.

PB

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