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Alkaline Rx batteries - 10/15/2012 5:56 AM   
Just_a_noob


 

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Status: offline 		 I was wondering if it would be taboo or just a bad idea to use four AA alkaline batteries to power the Rx in my plane? I want a 6volt power system and was just wondering if this would work. My plane has 7 servos so the current draw would significantly limit the number of flights per battery pack. I think each Duracell AA has approximatelty a 2700mAh rating. That would also be adversely affected as the voltage dropped across the batteries as they are depleted. I figured I could probably get an hour and a half to two hours out of four brand new batteries if I fly somewhat conservatively. Any thoughts?

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RE: Alkaline Rx batteries - 10/15/2012 7:31 AM   
Hossfly



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Status: offline 		YES, big time thoughts. Fer-Git it! You did not say what kind of servos, however with 7 servos I suspect you may be using some Digital. Digital servos take considerably more power than analog. Let's say, If you are operating landing gear, while using ailerons, elevator and throttle, your current drop with alkalines could well send you down to Ol' Moma Earth, even when relatively new.

Some years ago I saw a large nice scale model leave the flying field, go behind the facility, across the highway and plow into the ground about 10 feet from where a new garage was being built. We did a lot of talking and being nice to the new home owners. They forgave and we are all still a big happy family. Doesn't always work that way.

One can get away with a 3 channel light airplane and 3-4 analog servos of 42 OZ torque, but I will not try even that. It isn't worth it. Go to the garden shop ant a home-style store where the outside pool/driveway lights are at. They have a variety of batteries with up to 2400 or more mAh. Make a 5 cell pack and you're good for a number of flights.

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RE: Alkaline Rx batteries - 10/15/2012 7:35 AM   
Just_a_noob


 

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Status: offline 		Thank you for the reply. I will forget the alkaline battery idea lol. However, how do you know if the servos are digital? I have all standard servos, futaba s3003 I think is the number on them? 

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RE: Alkaline Rx batteries - 10/15/2012 8:12 AM   
mike109



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Status: offline 		G'day

As far as I know, Futaba 3003 servos are not digital. They are a standard analogue servo.

As for using Duracell or similar, I would not. Sanyo Eneloop cells can be bought for about twice the price of an alkaline cell and once hold their charge extremely well in storage. I use them in all my planes. You may be able to buy receiver packs made with them. Here in Australia they are readily available for about $30.

What sort of radio are you using? How big is the engine? What is the plane? And how big? How are you planning to fly it? All these will make it easier to suggest what you actually need. I am using 5 cell Eneloop packs in planes which I fly with a Spektrum DX7 and also with a Hitec Aurora 9 with engines up to 150 four stroke size.

Cheers

Mike in Oz

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RE: Alkaline Rx batteries - 10/15/2012 1:46 PM   
JohnBuckner



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Status: online 		Alkalines are an exceedingly bad idea for another reason and that is in most cases the fellows will use them in those cheezy four cell spring boxes. These have two failure modes and the are just waitning to fail First on many the plastic corners where the springs are will weaken with repeaded heat cycles (hot days and cold nights) and eventually the spring will push the corner away and the spring contact aginst the battery becomes intermittant and at that point you loose your airplane.

The other failure probability is simple those spring contacts will eventually corrode (it does not take much) and once agine the contact becomes intermittant.

Another possibility is some folks will solder up a pack (which can be done safely if you have the experiance) but this most often results in cobbled up packs which look like this one someone did and did result in the loss of an expensive glow ship.

John

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RE: Alkaline Rx batteries - 10/15/2012 1:47 PM   
KW_Counter


 

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Status: offline 		The biggest problem I see is the connectivity.
In a battery pack the cells are welded/soldered to each other.
Their chance of electrically coming apart is reduced considerably.
If you were to put the 4 alkalines in a plastic battery holder the chance
of an electrical separation is higher, the results catastrophic.

Good Luck,
KW_Counter

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RE: Alkaline Rx batteries - 10/15/2012 2:21 PM   
da Rock



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Status: offline 		OK, time for an opposing opinion....

It's not really an opposing one. Just different.

The idea about connectivity is spot on. It can be countered and countered successfully, but it's still a pain to deal with. but moving right along................

Once upon a time, long ago about the time NiCd RC batteries really were the new convenience and not long after lots of people were using dry cells............ in a land far to the Nawth of heeah....... I was flying gliders and starting to tick off the LSF levels one by one. I'd sorta run into the slope requirement in one of them. I think it was 8 hours. Well, my best slope glider was a 2 meter and I'd designed it with no room for a larger pack and the ballast compartment wasn't large enough for a 2nd pack. The problem that worried me most was whether or not my TX pack would last. I often had had to stop using the TX for that plane late in a long outing. so........

I noticed that dry cells had lots of capacity compared to the NiCd packs in question. So I got a couple of those Radio Shack snap in packs to try. I noticed right off the springs in one weren't what you'd call robust. Another brand was good. I still didn't like the airborne pack, so I soldered straps and such and shrinked it all tight. I flew it thermal for a day and it tested out great before, during and after. Made another couple of fresh ones, threw away the test one, and headed for the slopes. BTW, a bunch of the RTR cars today, and cheap RTF planes use snap in packs. Of course, those things aren't common in decent model airplanes.

With some care, lots of things work better one way than another. The dry cells were carbon back then. But they worked like gangbusters for what I wanted to do. Also, I knew the problems and limitations and paid attention.

I'm tickled to death our batteries today make all that obsolete. But if there was any reason to use them again today, I'd do it. But then not everything should be done by everyone.

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RE: Alkaline Rx batteries - 10/15/2012 2:52 PM   
jester_s1


 

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Status: offline 		It's great that that worked back then, but like you said the commercially available welded packs make it obsolete.

To the OP, I assume you're asking this question because you want to try and save money. One word of advice: DON'T.

Not that this particular plan of action would actually save you any money, but in general don't try to make the hobby cheaper by cobbling things together or coming up with some novel approach to home make something instead of buying a factory built one. You're traveling a road that is very well traveled, so the standard ways of doing things are the result of every way being tried and these work the best. You can save money by buying used airframes or by learning the scratch build, and when you get to know engines well you can save money by buying them used. But with electronics, new radio gear, servos, and batteries will serve you well. Buy from reputable brands (not Hobby King junk) and take care of it. That's the absolute cheapest way to do this hobby.

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RE: Alkaline Rx batteries - 10/15/2012 4:23 PM   
Rodney



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Status: offline 		I would be much more worried about the internal impedance of the cells you use. In general, the cells you reference have a very high internal impedance (will not allow high current draw without significant voltage drop within the cells themselves). A servo can draw in excess of 2 amperes when commanded to give full torque so if you command full control on two or more at once, you can draw some pretty high instantaneous currents that this type of cell can not provide. If you are powering a 2.4GHz system, this could easily create a "Brown-out" where you lose control completely until the system reboots itself.

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RE: Alkaline Rx batteries - 10/16/2012 12:53 AM   
Just_a_noob


 

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Status: offline 		Yes, originally my idea was to save money but I also didn't want the hassel of ordering new batteries and the chargers to accomodate them and then wait a week for them to arrive. But I would rather wait the extra time and use the correct battery as opposed to losing my model. Thanks guys!  

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RE: Alkaline Rx batteries - 10/16/2012 4:37 AM   
JohnBuckner



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Status: online 		A wise choice you will not regrete

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RE: Alkaline Rx batteries - 10/16/2012 2:58 PM   
jetmech05


 

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Status: offline 		No one is looking at the way the battery will discharge under load......if you were to graph it, the alkalines compaired to a Nicad or nickle metal are just about oposite of each other....with the alkalines dropping off at first then leveling out...just some random numbers here..... start at 5.0V drops to 4.4V quickly then discharges slower to unuseable....while the nickle metals or NiCads will drop a little at first say 5V to 4.8 then hold that maybe down to 4.6 for a while before dropping down to quickly to unuseable.
I know you said you had dumped the idea....this is for someone else that maybe thinking of using alkalines.......

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RE: Alkaline Rx batteries - 10/16/2012 10:12 PM   
Just_a_noob


 

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quote:

ORIGINAL: jetmech05

No one is looking at the way the battery will discharge under load......if you were to graph it, the alkalines compaired to a Nicad or nickle metal are just about oposite of each other....with the alkalines dropping off at first then leveling out...just some random numbers here..... start at 5.0V drops to 4.4V quickly then discharges slower to unuseable....while the nickle metals or NiCads will drop a little at first say 5V to 4.8 then hold that maybe down to 4.6 for a while before dropping down to quickly to unuseable.
I know you said you had dumped the idea....this is for someone else that maybe thinking of using alkalines.......


excellent point as well.



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