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Old Oct 11, 2006, 01:44 PM
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nunz15's Avatar
Flower Mound, TX
Joined Apr 2004
311 Posts
Question
NiMH Amperage

Hi all
I just started flying the new Poly Pulsar conventional tail model. Really nice flyer. I'm using a Hacker B20-12L geared 4:1. I'm using 7 cell NiMH batteries, IB1400s, which is perfect for the weight and balance. I've been conducting static tests with different props and have a question. With a 11x7 prop the amp draw is a pretty steady 25 amps. I run full power for 30 seconds and then cool for a few minutes. Each successive run draws similar amps. With a 10x6 prop the amperage is just over 17 on the first run with a cool pack, then goes up to a fairly steady 19 amps on each succesive run. Is this typical of the NiMH packs? They seem to perform better after they're warmed up. Also judging from the high heat and slight odor after running the static tests at 25 amps I'm thinking that it might be too much amperage for the B20 and the battery. Some of you have a lot of experience with NiMh, any inputs? Thanks.
Mike
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Old Oct 11, 2006, 02:20 PM
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Lenny970's Avatar
Greeley, Colorado, USA
Joined Feb 2000
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Mike,
25 amps may be pushing that motor a little hard but should be OK for short runs with some cooling before the next run. That battery will handle 25 amps quite well. Yes, the pack will perform much better when it's warm. Probably doesn't matter for fun flying, but for competition you want to take the pack off the charger nice and warm and fly immediately.

Good luck,
Lenny
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Last edited by Lenny970; Oct 11, 2006 at 02:26 PM.
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Old Oct 11, 2006, 04:47 PM
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Terre Haute, Indiana, USA
Joined Aug 2004
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I agree that 25 amps is not much of a problem for an IB1400 pack. I have an application in which I run a 7-cell IB1400 pack at around 35-38 amps for 15 - 20 second bursts, and it does fine (although its overall life will be shorter as a result).

FF
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Old Oct 11, 2006, 05:36 PM
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United Kingdom, England, Stratford-upon-Avon
Joined Feb 2001
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I wouldn't bench test the B20 for more than a few seconds at 25A. The outside may cool after a few minutes, but the inside will still be hot and repeated runs will probably cause damage.

Yes NiMH cells perform much better when hot. Most fliers like to charge them fast (to get them hot), and launch immediately.

Neil.
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Old Oct 11, 2006, 07:24 PM
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nunz15's Avatar
Flower Mound, TX
Joined Apr 2004
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Thanks to everyone for the response. Neil, are you saying that that bench testing will be different than flying because of airflow? I have no cooling slots in the Pulsar at all, and I wasn't going to cut any. It gets up to 700-800 feet in 30 seconds so it never runs full blast for very long. Lenny and FF, I agree that the IB1400 works well. Voltage stays right up there until about 1200mah and then drops off. The meter shows just under 1300mah usable with ESC cutoff set at 5 volts. After motor cut out the voltage recovers to around 8.
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Old Oct 12, 2006, 05:30 AM
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United Kingdom, England, Stratford-upon-Avon
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Quote:
Originally Posted by nunz15
Neil, are you saying that that bench testing will be different than flying because of airflow?
Not because of forced airflow, because:

a) Of boredom. When bench testing one wants to complete the test so is tempted to repeat the run as soon as the motor seems cool. But when flying on a good day you could not need the motor for 15min after getting to 700 ft.

b) Lower current. The prop will probably unload in flight.

c) Leaks. Although you may not have arranged explicit cooling air there will be leaks at the wing/fuse junction, the model is moving, and the air will be cooler at altitude. All these will mean that there will be improved cooling when flying.

d) Improved conduction. If you use a metal spinner heat will be sucked out of the motor through the shaft. This will occur more when the spinner is being cooled by the airflow.

Regards,

Neil.
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Old Oct 12, 2006, 05:41 AM
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nunz15's Avatar
Flower Mound, TX
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Thanks Neil. And actually, my next question was going to be is there any way to estimate the inflight amp draw based on the static draw? Seems logical that there'd be some percentage of drop as airspeed and prop speed get closer. I may just compromise and buy a 10x8 prop, I just wasn't sure if 10x8 is appropriate for the Pulsars climb speed.
Mike
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Old Oct 12, 2006, 07:27 AM
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Hervey Bay, Sunny Qld, Australia
Joined Jun 2001
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NiMH .... HOTC (hot of the charger) works best. THOUGH I have been using GP2000's HOTC and as they get hotter from the amps drawn and I end up with a pack that is boiling after a flight. So hot that the shrink wrap melted . Have re-wrapped the pack and melted and finally covered the cells in a shrink tubing so air could get to them (I even made the air intakes+ outakes, larger too). I am TOO tight with money to wrap the pack in Kapton tape, however I am thinking of getting IB4200's or GP3300's instead and wrapping the pack in Kapton tape then (the GP2000 are too far gone / ABUSED ).

I found a product at my local auto store today called "Xtreme tape". Made in USA so I guess somebody has to have heard of it on this forum . Apparently can take up to 260 degrees C. I thought I'd give it a try.
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Old Oct 12, 2006, 03:44 PM
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estonia
Joined Jun 2004
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I have 16 cell pack made of IB1400 and i`ve taken out about 74A for 15 seconds, so they are the best weight/amperage ratio i`ve seen before.
Usual load for them in my pack is about 40-50 A, in next season i`m gonna buy at least 2 packs more and see how they will work in competitions.
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Old Oct 12, 2006, 04:00 PM
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United Kingdom, England, Stratford-upon-Avon
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Quote:
Originally Posted by nunz15
And actually, my next question was going to be is there any way to estimate the inflight amp draw based on the static draw?
You can't easily estimate the current change. It will probably go down by ~ 15% in flight, but the reduction (if any) depends very much upon the climb angle. The big problem is that some high pitch props are stalled when static, and actually pull more current when moving. Because stall = turbulence = chaotic flow the numbers aren't readily computable, so you have to play safe or measure with a Micropower elogger or similar.

Neil.
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