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Sub C Batteries

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Doug Ireland04/11/2007 22:56:00
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Gents,

I was reading another thread on the forum (running JR Servos on 6v) and nasa_steve said to use "Sub C" Rx packs. I followed a link on the thread and found this "Tornado" battery as described below. My question is what are Sub C batteries, how long do you charge them and at what rate?

Since I read the article, I'm a bit worried about running 5 Cirrus 13kg and a JR 591 servo off a 2400mah NiMh pack.

Tornado 4100 4.8v Sub C Nimh

Tim Mackey05/11/2007 13:43:00
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http://tbn0.google.com/images?q=tbn:0FQTm2pec_rbaM:http://www.onlybatteries.com/webimages/images/13665.jpg


Sub Cs ( above ) are, if you like an analogy, " next one down from a big torch cell "

....which are, unsurprisingly "C" cells.                   see below.

http://tbn0.google.com/images?q=tbn:wWlpGr-WY-R1AM:http://www.electronicplus.com/images/products/MN1300.jpg


They are normally slow charged at around 10/th C, so if the capacity is 2000m/a charge 'em at 200m/a and so on. Many are quite happy being fast charged at C rate EG: 2000m/a in this case.

The cirrus servos are digital, and quite torquey too, so depending on their job, and how well the linkages are set up etc, could well be pulling quite large currents. If possible, put a DVM set to current measurement range, in line with the positive supply from the battery, and give the sticks a good hammering. Then try and "stall" one or two of the servos as well and see what current is being demanded.

IMO 2400 m/a is a bit marginal for what sounds like a fairly big and heavy plane.

Remember that although all these servos are useable on 6V if required ( even the JR ! ) doing so will INCREASE the current drawn, with the benefit purely of  marginally faster reponse times only.

Tom Sharp05/11/2007 18:47:00
387 forum posts
Gives some good answers this Timbo Fella. When's he going to write a book?
winchweight05/11/2007 18:57:00
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Nah..... BIG torch cells, are D cells. Middle sized torches use C cells and r/c fliers use sub C which is smaller than a C cell (hence the name), but bigger than a AA.
Tim Mackey05/11/2007 19:08:00
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Yeh true.....but I think he got the idea
Doug Ireland05/11/2007 19:13:00
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Timbo et al, thanks for the replies. But this still doesn't tell me what rate I should charge a 4xxx NiMh (Tornado) rx pack at. The only charger I have is the one I got with the new JR 2720. This one has a max rate of 110mah for the rx pack. I did buy a charger for the 7.2vdc pack that powers the ignition but I don't think it is adjustable downwards.

Confused,

Doug.

Tim Mackey05/11/2007 19:23:00
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Yes it does. Normally set it to 10/th of the C, however if you want to use the trickle output of the wallwart charger of your radio set, then thats fine......at 110ma output you can leave a 1000 ma pack on for at least 10 hours, so do the maths. 2000ma = 20 hrs..... and so on.

If you are going up to 4000 m/a packs its time you got a more flexible charger, such as pro peak prodigy or similar. The important thing with the charger output is current NOT voltage.

Many people worry that using a high voltage to charge a low voltage cell / battery they will somehow cook / otherwise destroy the cell/s but as i said it is CURRENT you need to limit. You could even use the 7V jobby IF the current is limited as above, which is also easy to achieve with a series resistor, the cells will not charge beyond normal peak voltage of around 1.5V each.

HTH

Doug Ireland05/11/2007 19:49:00
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Thanks Timbo, but please indulge a slow witted person here... I have a Pro-Peak Varipluse 2500VP charger but the destructions are a bit vague to say the least.! I've always been a bit wary when it comes to charging accumulators and am totally baffled when you and others start talking about "C" ratings. The upshot is, what is the max rate that i can charge a 4.8vdc 4800mah pack with and for how long?

Tim Mackey05/11/2007 20:58:00
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It depends to some extent on the "size" of the battery IE sub C or triple AAA / AA???

Assuming "normal" AA size recvr packs I would not suggest you exceed .5C which means half the capacity ( 4800 mah /2 = 2400m/a ). Even this ( 2.4A ) is high, and it would be better to avoid "the maximum rate that you can charge at" and stick to an overnight charge of around 500m/a ( .5A )

The following is an extract.

Some equipment manufacturers consider that NiMH can be safely charged in simple fixed (low) current chargers with or without timers, and that permanent overcharging is permissible with currents up to C/10 h. In fact, this is what happens in cheap http://www.batteryuniversity.com/

including this extract.....

 Charging nickel-metal-hydride

Nickel-metal-hydride chargers require more complex electronics than nickel-cadmium systems.
A nickel-metal-hydride charger must respond to a voltage drop of 8-16mV per cell. Making the charger too sensitive may terminate the fast charge halfway through the charge due to voltage fluctuations and electrical noise.
Nickel-metal-hydride should be rapid charged rather than slow charged. Because of poor overcharge absorption, the trickle charge must be lower than that of nickel-cadmium and is usually around 0.05C. This explains why the original nickel-cadmium charger cannot be used nickel-metal-hydride.
Avoid high temperature during charging. Discontinue the use of chargers that cook batteries.

  • A charger for nickel-metal-hydride can also accommodate nickel-cadmium, but not the other way around. A charger designed for nickel-cadmium would overcharge the nickel-metal-hydride battery.
  • nickel-based batteries prefer fast-charge. Lingering slow charges cause crystalline formation (memory).
  • If not used immediately, remove the battery from the charger and apply a topping-charge before use. Do not leave nickel-based battery in the charger for more than a few days, even if on trickle charge. Choosing a well-engineered charger will return the investment in longer lasting and better performing batteries.
Doug Ireland05/11/2007 22:35:00
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Timbo, thank you very much. Thats all I wanted to know. I will charge the new pack at the 0.5 rate you suggested.

In answer to your original query regarding the servos I'm using; yes they are in a large airframe, the Hanger 9 CAP 232G

Tim Mackey05/11/2007 23:05:00
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Thats a pretty big and heavy model. Now I dont want to open a whole new can of worms for you, but I would like to suggest that you seriously consider something a little better suited to it than a standard radio /rcvr battery setup. There are several setups available which will not only provide a much better degree of radio operation ( such as opto isolated servo wiring etc ) but also a dual battery / redundancy system for you radio power supply. I am not sure which radio you are using, but for instance the better MPX rcvrs have a dual supply system, and the following link is to a german firm specialising in quality R/C switches and the like. Have you considered the implications of long servo leads for instance? In view of this , maybe 6V is the way to go, or even the excellent powerbox12 combined with LiPo power.

I employ a modelbau digiswitch and Lipo supply on my humble 90 4T 64" span 8lb Spitfire!

Apologies if you are an experienced large model pilot, and this is teaching grannie to suck eggs, but if not, then please consider your investment, and potential for disaster if you employ simple "club flyer 60 sized" R/C systems in what is certainly a step beyond that !

http://www.modellbau-deutsch.com/e/powerbox_systeme/powerbox_12/details.php

PS taking into account the current exchange rates, these devices are excellent value. If you want to purchase in the Uk, then Als hobbies are agents.

Doug Ireland06/11/2007 00:22:00
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Timbo,

By no means are you demeaning but thank you for the deference. This (the Cap) is my first large aircraft for quite some time. I've been away from the hobby for so long that I've lost track of the advancements and am at a loss as to what to use. I assembled the model with the reccommended parts and have flown it with only one slight mishap to date but feel I could have done better with the knowledge I've gleaned from these pages.

I'm driving the CAP with a JR X2720 and the standard synthesised rx. As this is the first time I've used a petrol engine I use a huge 7.2v battery (hence the Pro-Peak Charger) that also drives the choke servo. I've read threads from elsewhere in this forum that suggest I'm on borrowed time before I reduce my pride and joy to matchsticks by using battery packs of such short duration.

In conclusion, I will take yours and others advice and ask about opto-isolator systems?

Tim Mackey06/11/2007 10:19:00
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The powerbox style devices utilize opto isolation.

This works by - simple explanation here I hope - utilizing a light beam ( usually infra-red) to transmit the signal from the receiver output, to each servo lead. This beam travels across a very small gap within the electronics unit, and therefore completely isolates any actual wiring involved at that point. THis prevents any unwanted interference /spurious feedback etc from arriving back at the receiver and causing mayhem. This "noise" is often produced and carried along extended servo wiring such as in large models. The better units  ( powerbox included ) also feature a small  amplifier to boost the signal being sent to the servos. Most also employ a dual battery system, with automatic switching if one fails. Some allow two seperate batteries for receiver and servos.

6v supply ( either by 5 cell nickel packs or 2s LiPo regulated ) is normally deployed to avoid the inevitable volt drop across long and relatively thin wiring, especially when the current is quite large as in heavy digital servos driving large control surfaces. Some of the very large stuff use 3 or more powerful servos ganged together just to power the huge rudder!! The current drawn by this alone would far exceeed ALL the servos on a conventional model.

Now of course there is a very reasonable argument to KISS, and introducing more complicated electronics into the system can well....complicate things! and for most average club models the old simple standard receiver and nickel pack works fine. When however you get into bigger and heavier stuff, this sort of device can be a wise move, and when you consider the total cost of the machine, the extra money involved seems worthwhile IMO. Using LiPo packs also means that much more power is available for longer at the same or lower weight, with added benefit of a regulated power supply to your electronics, which will last right down to the point where the system will inform you that the pack needs re-charging, yet at this point there will still be plenty of power available to supply the radio

Sorry if this seems to be adding to the confusion / choices but it is worth knowing about this stuff, and then you have better information in order to make an informed choice. 

Doug Ireland08/11/2007 10:10:00
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Hi Timbo, I'm particularly interested in being able to use a separate battery pack to run the servos. Could you reccommend a suitable unit (cost is not really an issue)?

I'm aware of the benefits of using opto-isolators as we use them in most of our comms equipment at work.

Tim Mackey08/11/2007 12:25:00
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JR specifically do not recommend opto isolation for their equipment, but do retail a little gizmo which enables seperate power to the recvr / servo. Here's a link to their statement on such matters, so I am reluctant to recommend a specific 3rd party device.

http://www.macgregor.co.uk/faq/General/optoisolator.htm

I personally have not used the JR "multibox", as I am more inclined to believe that a good dual battery supply system with true redundancy is of more value.

I suggest if you wish to pursue this avenue, with JR gear, then an email to Powerbox ( linked previously ) outlining your model / R/C equipment, will probably result in them recommending something appropriate.

I currently use several of the digiswitch devices, and they are excellent pieces of equipment, and have in the past ( on a large model which I no longer fly ) used the following device with again, excellent results.

http://www.modellbau-deutsch.com/e/powerbox_systeme/powerbox_40_24_competition/start.php

Doug Ireland09/11/2007 07:53:00
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2088 forum posts
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Hi Timbo,

I've just bought a Powerbox 40/16 and two 2800 LiPo's. Not cheap this big aeroplane lark is it?

Tim Mackey09/11/2007 09:22:00
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20919 forum posts
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Hope you got the EVO version as the EXPERT does not accept LiPo.

Yes it can get pricey.....but then you did say cost was not a problem

I never bothered with their fancy LiPo packs, a bit overpriced I felt, and I certainly didn't need the included charger and fancy mounting box. Any good quality 2s LiPo will suffice, but then again with the exchange rate as it is, I suppose the price is not too horrific. I think you have made a wise choice, and I am sure you will be delighted with the gear, and also your flying will improve!

Why they all cry?? Because, I say, you will no longer be subconsciously worrying about your radio power supply as point of failure, and a relaxed flyer is a better flyer

Keep posting so we know how you are getting on with it. 

Doug Ireland09/11/2007 10:15:00
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2088 forum posts
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As usual I jumped the gun! Just sent a frantic email to Al's trying to cancel the two LiPo's having just read the destructions for the Powerbox. You're quite right, the EXPERT version does not accept these fance batteries. I thought I was being smart in buying batteries with the correct connectors already on them, not too sure what to do now though.

Tim Mackey09/11/2007 10:20:00
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20919 forum posts
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15 articles

I suggest you continue with the LiPo route, and change the box to the EVO. remember that NMh have a self discharge issue, and also perform rather poorly in winter. liPo will maintain its charge almost indefinately, and is ready for a flight whenever you are!

The EVO version is not that much more expensive. I am sure Al will sort you out.

Doug Ireland11/11/2007 17:00:00
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hi Timbo,

It was a bit too late to change from EXPERT to EVO so I will have to go with what I've got but I did manage to cancell the LiPo batteries. I have two brand new 5 cell (sub C) 3000 something NiMh units that I will press into service but I've no idea what type of plugs I need to make them compatible with the Powerbox. Would you know what they are?

From what I can gather from the features of the Powerbox, one of the advantages is that it uses both battery packs at the same time and switches over in the event of a failure of one of them.

I'm aware of the shortcomings of NiMh but after reading your article on powering our equipment on LiPo's, it left me even more confused. Although these battery systems are superior to NiMh and NiCad they do seem to need a bit of looking after!

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