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Model Forum / Radio Controlled / Helicopters / October 2005Need help in picking type of battery
Hi All: I have an X-Cell 60 with the stock OS61 engine and a R138DP Receiver with channels 22 on it and on the Futaba 7CHP. I will be running 5 digital servo's with a Gyro. So now my question is what do you flyers think is the best type of receiver battery to run. NiCad or NiMH? I would like to run 6 volts instead of the 4.8 since I'm running digitals. I can really use any help you all have to give! Thanks Brian > Hi All: > [quoted text clipped - 4 lines] > would like to run 6 volts instead of the 4.8 since I'm running > digitals. I can really use any help you all have to give! First of all, be careful. An X-cell 60 with digital servo's needs 6 volts like I need a third armpit or an itchy arsehole. 6 volts from the same capacity battery means less flight time too. 6 volts speed the servo's up and give them somewhat more torque, but with the digitals, do you NEED that extra? I seriously doubt it, unless you're ragging the bollocks off the heli doing extreme flying, which, given the type of question you asked, I also doubt. As to NiCad or NiMh? Your choice, but NiMh don't tolerate vibration as well as NiCads, so I don't use them in the flying bits, although I do use them in the Tx. A 3000mAh Nicad will give you plenty of flying time and if you're smart, you can install it in such a way that it's quickly removable so you can slip a fresh battery in its place, should the need arise.  SignatureBeav Reply to "beavis dot original at ntlworld dot com" (with the obvious changes) > As to NiCad or NiMh? Your choice, but NiMh don't tolerate vibration as well > as NiCads, so I don't use them in the flying bits, although I do use them in > the Tx. A 3000mAh Nicad will give you plenty of flying time and if you're > smart, you can install it in such a way that it's quickly removable so you > can slip a fresh battery in its place, should the need arise. Yep, just use any "standard" 4 cell NiCD subC pack designed for the job. If you get an SCR rated pack it'll push out more than enough power and you can fast charge it too (subject to there being a second fast charge lead supplied, one with a Tamiya style plug on it). Capacity from 2100 upwards should suffice. www.overlander.co.uk is a good place to browse. You can charge SCR packs with 5 amps+ so topping up at the field is quick. | 6 volts from the same capacity battery means less flight time too. This is the standard wisdom, though I wonder just how true it is. After all, a pack with 5 cells will have more total energy than a pack with 4 cells. When asked to explain it, people tend to dig up ohms law and after a little math declare that a 4 cell pack will last 20% longer than a 5 cell pack. Which would be accurate if the load had a constance resistance, but this is _not_ the case with servos. (Though it probably is close to accurate with the receiver and maybe the gyro.) In any event, taking everything into account, I suspect a 5 cell pack probably does get used up faster than an otherwise identical 4 cell pack under normal use, though I think the difference is a good deal less than 20%. But I haven't tested it. (Ok, enough rambling.) In any event, it's hard to tell the difference between servos powered by 4 cells and servos powered by 5 cells for most. | 6 volts speed the servo's up and give them somewhat more torque, but | with the digitals, do you NEED that extra? | | I seriously doubt it, unless you're ragging the bollocks off the heli doing | extreme flying, which, given the type of question you asked, I also doubt. I concur. It's also extra weight which will reduce performance, use more fuel and reduce your flight times ... | As to NiCad or NiMh? Your choice, but NiMh don't tolerate vibration as well | as NiCads, so I don't use them in the flying bits, although I do use them in | the Tx. A 3000mAh Nicad will give you plenty of flying time Do 3000 mAh SubC NiCds exist? The highest capacity subC NiCds I've seen are 2400 mAh. (subC NiMH cells, on the other hand, I've seen go up to 3700 mAh.) | and if you're smart, you can install it in such a way that it's | quickly removable so you can slip a fresh battery in its place, | should the need arise. As another poster suggested, subC cells can be recharged quite quickly, especially the NiCds. The car guys sometimes charge them as fast as 4C ... and possibly faster. Though I'd not suggest that for your RX packs, but 2C or 3C (if you're in a hurry) would probably be just fine. (But having two packs is a better idea.) SignatureDoug McLaren, dougmc@frenzy.com Elwood: What kind of music do you get here ma'am? Barmaid: Why, we get both kinds of music, Country and Western. > | 6 volts from the same capacity battery means less flight time too. > > This is the standard wisdom, though I wonder just how true it is. > After all, a pack with 5 cells will have more total energy than a pack > with 4 cells. Even so, the time they take to flatten i less with 6 volts than it is with 4.8. That's one of the reasons I stopped using 6 volts aeons ago. > When asked to explain it, people tend to dig up ohms law and after a > little math declare that a 4 cell pack will last 20% longer than a 5 > cell pack. Which would be accurate if the load had a constance > resistance, but this is _not_ the case with servos. (Though it > probably is close to accurate with the receiver and maybe the gyro.) I wouldn't give it a figure (as a percentage) coz it'd be just a guess. > In any event, taking everything into account, I suspect a 5 cell pack > probably does get used up faster than an otherwise identical 4 cell [quoted text clipped - 5 lines] > In any event, it's hard to tell the difference between servos powered > by 4 cells and servos powered by 5 cells for most. Actually, the difference in speed (of the older servo's) is VERY noticable. > | 6 volts speed the servo's up and give them somewhat more torque, but > | with the digitals, do you NEED that extra? [quoted text clipped - 6 lines] > I concur. It's also extra weight which will reduce performance, use > more fuel and reduce your flight times ... Unless of course, the heli needs sme extra nose weight. Then a 6 volt pack can help. > | As to NiCad or NiMh? Your choice, but NiMh don't tolerate vibration as > well [quoted text clipped - 3 lines] > > Do 3000 mAh SubC NiCds exist? IIRC, I have one siting in my Futura. The highest capacity subC NiCds I've > seen are 2400 mAh. (subC NiMH cells, on the other hand, I've seen go > up to 3700 mAh.) I've never even investigated "Rx" NiMh's as I won't use NiMh;s in the helicopter, just the Tx, so I can't comment. > | and if you're smart, you can install it in such a way that it's > | quickly removable so you can slip a fresh battery in its place, [quoted text clipped - 4 lines] > fast as 4C ... and possibly faster. Though I'd not suggest that for > your RX packs, But that's what we're discussing here and it's why I never mentioed the fast charging aspect. but 2C or 3C (if you're in a hurry) would probably be > just fine. (But having two packs is a better idea.) Isn't it just :-) SignatureBeav Reply to "beavis dot original at ntlworld dot com" (with the obvious changes) | > Do 3000 mAh SubC NiCds exist? | | IIRC, I have one siting in my Futura. I can't find a single vendor selling 3000+ mAh NiCd cells. Perhaps you have full C cells ... | > The highest capacity subC NiCds I've | > seen are 2400 mAh. (subC NiMH cells, on the other hand, I've seen go | > up to 3700 mAh.) | | I've never even investigated "Rx" NiMh's as I won't use NiMh;s in the | helicopter, just the Tx, so I can't comment. The Sub-C NiMH cells should be just as good as the NiCd cells, but I can certainly understand any reluctance. SignatureDoug McLaren, dougmc@frenzy.com `If one studies too zealously, one easily loses his pants.' -- Albert Einstein > The Sub-C NiMH cells should be just as good as the NiCd cells, but > I can certainly understand any reluctance. I can't...you can get NiMH cells to power an electric heli, so I really can't see why someone wouldn't use these to power their radio gear. Of course, I'm not advocating folks use crappy AA NiMH cells, but the GP3300 and the like are more than up to the job. | > The Sub-C NiMH cells should be just as good as the NiCd cells, but | > I can certainly understand any reluctance. [quoted text clipped - 3 lines] | course, I'm not advocating folks use crappy AA NiMH cells, but the | GP3300 and the like are more than up to the job. The reluctance is mostly just inertia. In a helicopter, anything that goes wrong with your batteries tends to result in a very expensive crash, so people are very reluctant to try anything new until it's been very well tested. Traditionally, NiMH cells have been more fragile than NiCd cells. I don't think this is really signifigantly true with the high performance sub-C cells anymore, but the perception is certainly still there. In an electric helicopter the same danger exists, but there's two big differences -- higher capacity batteries give you longer flights, so the benefit is directly and immediately apparent. Secondly, any partial failures of the battery pack will be more obvious (as power will be reduced) and so you may very well get enough warning to land. As for AA cells, you're right -- the high capacity NiMH cells (especially the consumer ones) seem to sacrifice low internal resistances and reliability for high capacity. However, the reduced weight might allow you to put in two or three AA packs instead of one subC pack, which might make it worthwhile (and even more reliable than one sub-C pack) if you can get it padded well enough. SignatureDoug McLaren, dougmc@frenzy.com Failure is not an option. > | > The Sub-C NiMH cells should be just as good as the NiCd cells, but > | > I can certainly understand any reluctance. [quoted text clipped - 8 lines] > crash, so people are very reluctant to try anything new until it's > been very well tested. On top of that, the discharge curve of NiMh isn't what we need. They retain their charge for a longer period, but the drop off as they reach their discharged point is REALLY steep. What this means in the real world, is that you can take off with a battery reading almost full charge but find that half way through the flight, the battery dies ff so rapidly, it's going to be more luck than judgement whether you get the heli back on the ground in one piece. > Traditionally, NiMH cells have been more fragile than NiCd cells. I > don't think this is really signifigantly true with the high [quoted text clipped - 6 lines] > partial failures of the battery pack will be more obvious (as power > will be reduced) and so you may very well get enough warning to land. I notice the lack of "not" in your last line, and I agree, that's one one the problems with NiMh. They're "just" too ready to discharge too quickly as they approach "charge me" time. Nicads have a FAR more gentle curve to "flat" and it's a curve that allows the flyer to FEEL that the heli isn't behaving as it was yet still gives you enough time to land, not lawn dart. Of course, no-one is saying DON'T use NiMh, it's just me saying *I* won't. SignatureBeav Reply to "beavis dot original at ntlworld dot com" (with the obvious changes) > Of course, no-one is saying DON'T use NiMh, it's just me saying *I* won't. I tend to agree. My experience with NiMh's on my glo powered birds has not been good. I'm wondering if anyone can confirm this or not. I've been using a couple of Super Nova Chargers for quite some time now. I really like them for NiCad's and they're supposed to handle NiMh's Ok but you do have to program them to the voltage and milliamp ratings for the pack your charging. I had several occations where the batteries (NiMh's) seemed to charge just fine but only made it through a couple of tanks of fuel before the model started feeling weird. Mostly, it was the tail going screwy on me as the gyro "was not" happy as the voltage dropped off. Twice I landed and checked the battery status, only to find out it was dead as a doornail! :-O Mind you, I was getting 4+ flights out of an "old" NiCad of similar capacity. Does anyone know if the Super Nova's have an issue charging NiMh batteries? I'm back on NiCads now and am having no other problems but I don't feel like I've truly figures out what was going on. Any input appreciated! Fly Safe, Steve R. ... > I tend to agree. My experience with NiMh's on my glo powered birds has not > been good. [quoted text clipped - 13 lines] > I'm back on NiCads now and am having no other problems but I don't feel like > I've truly figures out what was going on. Using Robbe's PowerPeak Ultimate LiPoly charger with NiMH. It shows every time how much charge went in (or out, if you choose to discharge first) even when automatically charging. I'm very happy with it up to now as a beginner (flying, not with electronics :-) Have been well advised by http://www.hoellein.de (they do speak english, ship internationally and specialize in electrical flight models) and I would recommend them anytime as they fly competitions themselves. HTH Marc SignatureSwitzerland/Europe <http://www.heusser.com> remove CHEERS and from MERCIAL to get valid e-mail >> Of course, no-one is saying DON'T use NiMh, it's just me saying *I* >> won't. [quoted text clipped - 10 lines] > the model started feeling weird. Mostly, it was the tail going screwy on > me as the gyro "was not" happy as the voltage dropped off. Probably still happier than a pal of mine who'd "charged" his batts, then went out to the field to fly. Took off and never saw the heli again. A fully loaded X-cell Pro 2K it was too. Twice I landed and checked the > battery status, only to find out it was dead as a doornail! :-O Mind > you, I was getting 4+ flights out of an "old" NiCad of similar capacity. That's why I've not bothered changing. > Does anyone know if the Super Nova's have an issue charging NiMh > batteries? I couldn't tell you Steve. > I'm back on NiCads now and am having no other problems but I don't feel > like I've truly figures out what was going on. I'd stay with Nicads if I were you. Mind you, I'm not you, but I STILL stay with Nicads:-) SignatureBeav Reply to "beavis dot original at ntlworld dot com" (with the obvious changes) >> I'm back on NiCads now and am having no other problems but I don't feel >> like I've truly figures out what was going on. > > I'd stay with Nicads if I were you. Mind you, I'm not you, but I STILL > stay with Nicads:-) No problem Beav! Maybe they are "older" technology but I know them, they work for me and I'm sticking with them. Besides, any other battery technology I might be tempted to try also includes buy a whole new set of battery "chargers" too. This stuff is expensive enough as it! Fly Safe, Steve R. >>> I'm back on NiCads now and am having no other problems but I don't feel >>> like I've truly figures out what was going on. [quoted text clipped - 4 lines] > No problem Beav! Maybe they are "older" technology but I know them, they > work for me and I'm sticking with them. "If it's not broke, don't fix it". That's still as true today as it ever was. Besides, any other battery > technology I might be tempted to try also includes buy a whole new set of > battery "chargers" too. There's that too. One expense leads directly to another and if neither are necessary, I'll keep my money in my sky rocket. > This stuff is expensive enough as it! Isn't it just!! SignatureBeav Reply to "beavis dot original at ntlworld dot com" (with the obvious changes) > Does anyone know if the Super Nova's have an issue charging NiMh batteries? > I'm back on NiCads now and am having no other problems but I don't feel like > I've truly figures out what was going on. Mu SuperNova will stop charging prematurely sometimes with a NiMH pack, often when the pack has not be used for a while. Give the pack a good cycle and it'll charge fine again. > | 6 volts from the same capacity battery means less flight time too. True, both theoretically and empirically. > This is the standard wisdom, though I wonder just how true it is. > After all, a pack with 5 cells will have more total energy than a pack > with 4 cells. Also, true. If capacity is the same, a 5-cell pack contains 25% more energy than a 4-cell pack. But under the same discharge curves, the 5-cell pack supplies it's energy at more than a 50% higher rate. The net effect is that the 6-volt pack depletes it's energy sooner. > When asked to explain it, people tend to dig up ohms law and after a > little math declare that a 4 cell pack will last 20% longer than a 5 > cell pack. Which would be accurate if the load had a constance > resistance, but this is _not_ the case with servos. (Though it > probably is close to accurate with the receiver and maybe the gyro.) Almost true. A 4-cell pack will theoretically last 25% longer than an identical capacity 5-cell pack. And I think this holds true even if the load is not constant, so long as the dynamic loading is the same for both. > In any event, taking everything into account, I suspect a 5 cell pack > probably does get used up faster than an otherwise identical 4 cell > pack under normal use, though I think the difference is a good deal > less than 20%. But I haven't tested it. Practical experience bears this out, although I'm not sure exactly why it's not the expected 20%! One possible explanation is that the 6-volt pack (with its five lower capacity cells) has lower internal resistance and is therefore able to deliver its energy payload more efficiently. Dunno, really. > (Ok, enough rambling.) (and I'm done over-analyzing...) <snip> > | As to NiCad or NiMh? Your choice, but NiMh don't tolerate vibration as > well > | as NiCads, so I don't use them in the flying bits, although I do use > them in > | the Tx. A 3000mAh Nicad will give you plenty of flying time One often overlooked but notable difference between NiCd's and NiMH are their voltage discharge curves. The tests I've seen show that NiCd's tend to gradually drop in voltage as the end approaches, which is manifested as an increasing "mushiness" in servo response. It acts like a built-in low-battery warning alarm - you can tell that it's time to bring 'er in. NiMH's, on the other hand, kick strongly until the very last moments of flight and then die suddenly and completely without warning. This characteristic is especially pronounced if the pack is under heavy loading transits, such as an onboard Rx pack experiences. This aspect of the NiMH's discharge characteristic makes them less suitable for Rx batteries, IMHO, but certainly fine for Tx batteries were the load is flatter and less demanding. So for me, it's NiMH in the tranny, but NiCd's onboard! Just my two cents, Gp | > This is the standard wisdom, though I wonder just how true it is. | > After all, a pack with 5 cells will have more total energy than a pack | > with 4 cells. | | Also, true. If capacity is the same, a 5-cell pack contains 25% more energy | than a 4-cell pack. But under the same discharge curves ... of course, they're not the same discharge curve. | the 5-cell pack supplies it's energy at more than a 50% higher rate. Assuming a constant resistance, energy will go up by 56%, yes. | The net effect is that the 6-volt pack depletes it's energy sooner. | > When asked to explain it, people tend to dig up ohms law and after a | > little math declare that a 4 cell pack will last 20% longer than a 5 [quoted text clipped - 4 lines] | Almost true. A 4-cell pack will theoretically last 25% longer than an | identical capacity 5-cell pack. Er, right. The 4 cell pack will last 25% longer than the 5 cell pack, or the 5 cell pack will last 20% less long than the 4 cell pack. I just got the two confused. This is all assuming a constant resistance. But a servo motor is _not_ a constant resistance. (The receiver and the servo circuitry probably is close, and I don't know about a gyro. But ultimately most of the power goes into those servo motors ...) In any event, if a servo is getting 56% more power, in theory it'll run 56% faster, and will therefore have to run only 64% as long for a given stick movement. 64% as long times 125% the current is 80% the total current drain, so the total current drain would actually be less. (But the total energy drain would be the same, but remember that the 5 cell pack has 25% more total energy.) This all makes a lot of simplifying assumptions -- that the servo motor immediately comes up to speed, that the friction is a constant, etc. Assumptions that are not quite true. | And I think this holds true even if the load is not constant, so | long as the dynamic loading is the same for both. How can it be the same if it's dynamic? :) | > In any event, taking everything into account, I suspect a 5 cell pack | > probably does get used up faster than an otherwise identical 4 cell | > pack under normal use, though I think the difference is a good deal | > less than 20%. But I haven't tested it. | | Practical experience bears this out I can believe that. | although I'm not sure exactly why it's not the expected 20%! Well, I've already said why. At least one reason. | One possible explanation is that the 6-volt pack (with its five | lower capacity cells) has lower internal resistance and is therefore | able to deliver its energy payload more efficiently. Dunno, really. Actually, it's pretty obvious that a 5 cell pack (6 volts isn't very precise) has an overall 25% higher internal resistance than an otherwise identical 4 cell pack. (Assuming that the internal resistance is strictly from the cells themselves, and not the wiring or plug.) | One often overlooked but notable difference between NiCd's and NiMH | are their voltage discharge curves. The tests I've seen show that | NiCd's tend to gradually drop in voltage as the end approaches, | which is manifested as an increasing "mushiness" in servo response. It's not that gradual, even in NiCds. It's really quite sudden, at least in matched cells, though it is a bit more sudden in NiMH cells. http://shdesigns.org/batts/battcyc.html gives some graphs that show this. | It acts like a built-in low-battery warning alarm - you can tell | that it's time to bring 'er in. Really, you should never let a RX pack get _anywhere_ near that point. Low battery alarms are a good thing if you can't keep on top of your battery condition. If things ever get so bad that your servo response time is signifigantly higher, you've already pushed things _way_ too far. | NiMH's, on the other hand, kick strongly until the very last moments of | flight and then die suddenly and completely without warning. | | This characteristic is especially pronounced if the pack is under | heavy loading transits, such as an onboard Rx pack experiences. Perhaps, but getting 50% higher capacity for the same weight far more than makes up for any loss here. | This aspect of the NiMH's discharge characteristic makes them less | suitable for Rx batteries, IMHO There are better reasons to pick NiCd over NiMH than this. But the added capacity may be worthwhile. Personally, I'm still on the fence. | So for me, it's NiMH in the tranny, but NiCd's onboard! For the TX, NiMH. Certainly. SignatureDoug McLaren, dougmc@frenzy.com Which is worse: Ignorance or Apathy? Who knows? Who cares? |
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