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Please answer a couple of questions???? - 12/29/2012 10:02 PM   
splais


 

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Status: offline 		If I have an electric plane that has a 45a esc and the motor is pulling 44a on the ground at wot, will a 40c battery run batter and last long than a 30 or even 20c battery? In other words, for must performance do you need to match the "c" rating of a battery to the amps a motor is going to pull?

Second question. As long as your esc amp rating is higher than the max amps the motor is pulling you should be ok? As a follow on, can you have an esc with to high a rating; I.e., say, an 80a esc on a motor only pulling 40 amps?

Thanks so much.

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RE: Please answer a couple of questions???? - 12/29/2012 10:11 PM   
guver


 

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Status: offline 		It depends on whether you keep the TPS constant or the current/power constant.

If both set-ups were to run at WOT the 40C battery would run out before the 20C simply because it made more power therefore pulled a higher current.
If both set-ups kept the power equal then the 40C battery would outlast the 20C battery since it demanded you throttle back due to slightly higher voltage.
If both set-ups pulled the same current then they would run out at exactly the same time. The 40C would've made more power for the duration though.

When selecting a battery or esc , it is good to select one that is higher or equal to your MAX and you can't really size either one "too big" So it is ok to use a 40C pack on a set-up that pulls only 8C and it is ok to use an 80 amp esc on a set-up that pulls 40 amps.

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RE: Please answer a couple of questions???? - 12/29/2012 10:18 PM   
splais


 

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Status: offline 		Thanks very much, understood it all

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RE: Please answer a couple of questions???? - 12/30/2012 2:00 AM   
splais


 

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Status: offline 		I'm learning slowly. A follow-on question. Is the following a true statement. The power of a plane (thrust, speed) comes from the motor; the esc just controls and manages the power required by the motor and flight systems.

And finally it is the motors amp draw that determines how many amps your esc requires - true?

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RE: Please answer a couple of questions???? - 12/30/2012 2:14 AM   
guver


 

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Status: offline 		I would say it is the (kv of the motor and the) propeller. The motor has to be big enough to spin the prop of course. You've got it correct, the esc simply delivers what's required by the prop.

If we had two motors that were both 1200kv and one was twice as big a it had to be and the other was 5 times as big as it had to be then theoretically they would still both pull the same current with the same props. Beyond that if one esc was just barely big enough and another was twice as big they would still do the same job.

I hope I've explained it well , I'm still rather new to planes,props,ect. I'm sure there are minute differences here that we are skipping too. It's kinda similar with the two batts at 20C and 40C. If we were only pulling 5C then the difference between the 40c and the 20c battery would be minute.

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RE: Please answer a couple of questions???? - 12/30/2012 2:37 AM   
splais


 

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Status: offline 		I want to expand on your last paragraph, and believe me I'm getting close to my knowledge limit

I understand about battery C and how you arrive at amps (c x mah capacity / 1000). And I think I understand that if you have a 20c /2200mah battery it's max flow would be 44amps. So if your motor is drawing 44amps, that battery is probably not going to last long. And, conversely, if the battery was only rated at say 10c, your plane would be underpowered.

Does it follow, in the above scenario that using a 40c/2200 battery that equates to 88amps is a waste; being both heavier and more expensive than you need. In the above scenario a 30c/2200 battery would probably be perfect.

Am I basically correct?

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RE: Please answer a couple of questions???? - 12/30/2012 3:04 AM   
guver


 

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Status: offline 		Yep, absolutely in theory correct. Unfortunately the ratings on batteries are not very accurate and many have labeled them very optimistically. Many times we have to double the "c" rating to get a realistic figure. As it turns out most times the 40C overkill isn't a waste at all.

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RE: Please answer a couple of questions???? - 12/30/2012 3:11 AM   
guver


 

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Status: offline 		A side note about sizing components since you used an example of a 10C battery trying to do a 20C job. The battery will overheat and be way down on power because the voltage will fall on it's face under that 20C load of 44 amps.

ESC and battery can be oversized/overspecc'ed with no pros or cons other than a very slight power increase and a slight weight gain. Motor can also be oversized , but the weight gain is very real especially on the nose of the plane. It is the rpm and the prop that is key to power/current. I prefer to do it from front to back.

Select prop,rpm,motor,esc then battery. I am in the habit of carrying around a larger capacity battery than normal because I can get away with much cheaper lower rated packs and I like longer flight times. Hot rod flights of course can't do this. Lots of my flights are 15 minutes (which is 4C)

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RE: Please answer a couple of questions???? - 12/30/2012 3:14 AM   
splais


 

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Status: offline 		Yes I have learned that also. Thanks for all the good info.

I have an F27Q Stryker flying on a stock motor and a 45amp esc out of a Squall HP. It pulls 44amps at wot on the ground. I have 20c, 30c and 40c Turnigy 2200mah batteries. It does not seem to fly as well on the 20c batteries. Can't really tell any difference between the 30c and 40c.

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