NIMH receiver packs

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NIMH receiver packs

Postby Beej » Mon Jul 18, 2011 10:39 am

Hi All,

i have just traded up from 40 sized models and standard servos to a very nice Hangar 9 pulse XT 60 (Saito 100). On board I have 4 Futaba S3050 digital servos and a 3010 on the throttle (+ Aurora 9 radio). Because of weight/balance issues I went for a 6V eneloop 2000mah receiver pack rather than the Nicads I usually use. A member of my club advises strongly against this set-up, cautioning against large voltage drops with this many digital servos in a large-ish model, presumably due to the high internal resistance of the eneloops. A quick google etc sees lots of information out there cautioning against using NiMh receiver packs in high drain applications and it all makes sense to me. My question for you very learned people is whether "high drain" in this instance is referring to large models like 30% IMAC etc etc, or whether I should really have some concerns about my set-up, especially if I fly it "hard"?

I guess I can always go back to the Nicads, or invest in some A123s (+ new charger!). Also I assume that both the receiver and the futaba servos would be happy at the 6.6V of the A123s.

Advice appreciated!!!! (I have read Bruce's article on batteries)

Many thanks in advance!

best to you all

Beej
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Re: NIMH receiver packs

Postby ergocentric » Mon Jul 18, 2011 2:11 pm

6V will drain faster than 4.8V, Ohm's Law

NiMh have higher internal resistance than most other chemistries

Those are not the most powerful/power hungry servos available

It's a 60 size plane, not a 100cc gas 3D plane

So unless you look up all the stall current on the servos and test your receiver pack to a reasonable level you won't know. (I would suggest adding all current plus the stall current for two servos).

this is one of those questions, how high, how far, how fast, how much...
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Re: NIMH receiver packs

Postby RCModelReviews » Tue Jul 19, 2011 4:50 am

If you can afford to -- bite the bullet and go for the A123 packs and charger. They're not very expensive these days and are so much better than either NiMH or Nicad.

No risk of false-peaking causing you to fly with a partly-charged pack, no self-discharge to speak of, much lower internal resistance and the ability to be recharged to about 90% of their capacity from dead-flat in around 15 minutes.

You can build your own pack for around $30 and a cheap 4-button charger with LiFePO4/A123 capabilities can be had for not much more than $20 these days. Not too much of an investment when you compare it to how much other money you've got tied up in your plane.
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Re: NIMH receiver packs

Postby Beej » Tue Jul 19, 2011 5:21 am

Thanks Bruce - I'm not very good with a soldering iron so would be happy to buy some pre-built A123 packs. It's fair to say the Hobby Stores here in Oz have a fairly modest range of A123 options as far as I can see. I know you are careful not be seen to promote specific brands but any possibilities you could point me to for A123 receiver packs would be great either here or by PM. I can see them on a number of web sites such as Nobs, HK etc but I bet not all are created equal! Hyperion here in Oz seem to have some nifty chargers so I guess they are a reasonable option for that end of things.

cheers,

Beej
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Re: NIMH receiver packs

Postby ergocentric » Tue Jul 19, 2011 2:54 pm

Seems you were looking for a reason.

NiMh will work in most cases, A123 packs are available from HK (using actual A123 cells with the nanophosphate) if you feel the need for a pack that can deliver 50c and can be charged in less than 20 minutes.

Look for the A123 logo and brand name, look for thick wire and heavy connectors.
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Re: NIMH receiver packs

Postby ergocentric » Tue Jul 19, 2011 3:06 pm

Another thought:

Definitely buy a multimeter and the ECO6 or whatever the latest multifunction charger is so that you can discharge/test your batteries

i.e. you can measure current draw on your flight pack with one servo stalled, discharge at that rate to see what the battery can do
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Re: NIMH receiver packs

Postby ZigZag » Thu Nov 10, 2011 6:48 pm

Hi Beej

Better late than never :-). I anguised over the same issue with my 2 metre Great Planes Lancair, running 9 servos, 5 of which are digital. I was contemplating something very complex and moderately expensive with LiPO or LiFE power and a voltage reg. I spoke with the chairman of our local club who has flown serious models for years and, whilst nothing in my research contradicted Bruce's advice, the chairman suggested going with the latest crop of "instant" NiMH batteries. I ordered a 2100 mAh 4 AA cell pack from Vapextech in the UK (worth looking at - they may ship to Oz) and had the maiden flight a couple of weeks back which was the "proof in the pudding" for the RX pack. After fairly extensive ground running and much phaffing about we were airborn with a 10 minute first flight, which included lots 'n lots of stick waggling and flap extending to test the performance envelope. A check of RX battery voltage after landing indicated that my concerns were groundless. There was plenty of oomph left for another couple of flights at least. So, take a voltage checker to the field with you. Fully charge your RX battery and try a flight. One flight won't kill the battery and there is every chance that the residual voltage will give you enough for another couple of flights. Or, go the other way, spend the bucks and go for the LiFE option. Belt and braces never hurt :-)

Cheers
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