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Author Topic: Will this work?  (Read 2568 times)

Unsinkable 2

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Will this work?
« on: February 08, 2016, 01:24:55 PM »

Will this work?





I need a minimum 11.1v for my brushless motor (up to 18v)
I need a good 6v to power my winch servos (minimum 4.8v but at a lesser torque, I want 6v)
I prefer to have 2 smaller 6v batteries joined in series than one big 12v (easier to position in the hull and lower down!)
I want to charge the batteries at the same time with one connection.


 SO, will this work?........


I get 2x 6v gel cell batteries and join them together to give 12 v
I cut the red wire on my ESC and connect it to the 12v supply and plug it into my throttle channel on my receiver
I get a 12v to 6v reducer and plug it into a spare channel on my receiver


I think (hope :embarrassed: ) that gives me a constant 6v for my servos and should I need to use my motor it will run at 12 volts


If I am completely wrong or if there is a simpler way please enlighten me....... U2
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microgyros

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Re: Will this work?
« Reply #1 on: February 08, 2016, 01:54:02 PM »

There are many "will this work?" posts asking for guidance. Understandably, they often omit relevant details. You might have an esc that has a 6volt, 3amp bec, for example. You might have an auto-calibrate esc which counts Lipo cells with no NiCd setting. You may have a low power motor or a high power motor.

I'll leave the main tennis match to the regulars.  :-)
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inertia

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Re: Will this work?
« Reply #2 on: February 08, 2016, 02:13:06 PM »

I didn't read past "gel cell".
In general terms brushless motors need quite high currents to perform properly and that's not possible with lead-acid batteries. They make good ballast and they can be useful to weigh things down in the workshop, but unless you're running a very low current motor in a workboat (which needs their weight as ballast) then don't look at them as a power source. This is the 21st century.
I would suggest a ten-cell NiMH or 3S LiPo pack for the motor, with the ESC's BEC output for the receiver supply. Fit a completely separate 5-cell NiMH pack for the winch, using the circuit I drew in the thread about servo testers. http://www.modelboatmayhem.co.uk/forum/index.php/topic,53832.msg556275.html#msg556275 Reply #15.
DM
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Tug Fanatic

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Re: Will this work?
« Reply #3 on: February 08, 2016, 02:36:37 PM »

I agree with Inertia.

As a rule of thumb I would not use gel cells where the maximum rate of discharge exceeded the amp rating of the battery eg if you have a 12v 7Ah battery then I would not want to use it in an application that might draw more than 7A. If you do (& this holds true for a current draw of less than 7 amps) then the life of the battery will be reduced and you will find that the battery you think is 7Ah actually is holding a lot less. You will also find that the voltage from the battery is not the 12.?v that you expected. The faster you draw the current out the smaller the capacity and the lower the voltage available. This only gets worse over time.

The example 12v 7Ah rating is only true for a very low current draw situation - I believe this to be 1/20th of the capacity eg 0.7A current usage for a gel cell type 7Ah battery.

Personally I would resist using gel cells or any type of lead acid battery at all unless it still left a lot of ballast to be provided by some other means. They are heavy and have a low energy density. Even where the weight is not a problem you may well get a nicer handling model if you can put the ballast where you want rather than where a great lump of a battery forces you to have it.
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Netleyned

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Re: Will this work?
« Reply #4 on: February 08, 2016, 02:47:49 PM »

If this motor is to be used as a 'Get out of trouble/get you home motor in your Atlantis, then why o why a brushless?
A 555 brushed would be more than enough for the job. This would enable you to steer clear of Lipos and the inherent
management problems in a yacht (Getting wet, removal for charging etc.)
NiMH's could then be used and laid flat above the ballast keel.
A weighty SLA mounted at around the waterline would not be too clever in a yacht that needs its ballast as low as possible.


If it's for an entirely different project you may disregard my ramblings.


Ned
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inertia

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Re: Will this work?
« Reply #5 on: February 08, 2016, 02:53:59 PM »

Personally I would resist using gel cells or any type of lead acid battery at all unless it still left a lot of ballast to be provided by some other means. They are heavy and have a low energy density. Even where the weight is not a problem you may well get a nicer handling model if you can put the ballast where you want rather than where a great lump of a battery forces you to have it.

And this is from a self-confessed tug fanatic! I rest my case after such erudite evidence from an expert witness.
DM
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Tug Fanatic

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Re: Will this work?
« Reply #6 on: February 08, 2016, 03:28:32 PM »

And this is from a self-confessed tug fanatic! I rest my case after such erudite evidence from an expert witness.
DM

Expert not. I am mainly a model aircraft guy but in boats tugs are just my thing. I have been around for a long time and I have learnt a lot from others and their mistakes. I have also made a few of my own.

Tugs are the case where I seem to need a lot of extra ballast so I would not be so worried in a model tug. They also have the capacity to hold relatively large unstressed batteries. If you look at a tug hull it is just designed for large lumps of battery but many other types on model hull are not. Keeping the centre of gravity low and distributing ballast to make the model sit and ride better are rarely a bad idea.

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Unsinkable 2

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Re: Will this work?
« Reply #7 on: February 08, 2016, 04:01:56 PM »

Microgyros, you are indeed correct I did miss out some information not intentionally just lack of understanding on my behalf......


Inertia I didn't realise that about gel cell batteries, I assumed a big bulky battery would have more to give than a 10 pack of nicads....... Again just lack of understanding on my part.


So, a 10 cell NiMH for the brushless motor connected to the ESC. this is then plugged into the receiver. So far so good :}


Now then I have 4 sail winches, they have a working current of 0.3a to 0.35a and a stall current of 0.86a to 1.05a. Is the suggested separate battery pack per servo or enough to power them all for a decent sailing time?


The reason I am trying to avoid Lipos is because I want to charge my batteries whilst still in the model and I don't like doing that with Lipos! There may be a lot of space in the hull and 3 openings but 2 are blocked by servos and the large centre opening will have strings in the way. (Although if needs must I will fit a removable lipo under the strings) I'm just trying to find the best method to suit my situation and preferences. I'm beginning to understand and with all your help I'm sure I'll get there. Thanks! :-))


Netleyned you are quite right it is only a 'get me home motor' for my 'Alantis' so why a brushless. Well, when I started the build I had one in my workshop sitting there doing nothing. It was shiny, new, and got designed into my plan. I am learning as I go along (especially with boats) that just because it's a more modern invention that it isn't necessarily better, but it's been built in now and I don't want to start changing what's already been built. That's why. That's also why I ask all these questions now so that next time............ U2





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Tug Fanatic

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Re: Will this work?
« Reply #8 on: February 08, 2016, 04:24:05 PM »

Hang on a minute - slow down.

What exactly is the motor - what is its kv and what prop do you intend to connect to it?

Your winches draw a lot less current that I had expected. Hopefully one of the sailing guys will come in here. Is it fair to say that they would all tend to be working at the same time in which case we have a working load of around 1.3amps and a stall current of say 4amps? That does not require a particularly large battery. Higher capacity C cell NiMh batteries would easily do the job. 

You do not need to think of LiPo type batteries at all unless you want to or weight is very critical. In model flying we have nearly all moved over to LiPo type batteries but then weight - or better the lack of it or the ability to control it - is critical.
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microgyros

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Re: Will this work?
« Reply #9 on: February 08, 2016, 04:52:47 PM »

Don't forget LiFe batteries if you have developed a phobia.
A yacht racing neighbour replaced 5 Nimh cells with two 20 gram cylindrical Life cells and now all his pals use the flat 2s LiFe batteries for 6volt sail winches. The complication is you may need a 10-20 menu driven charger for LiFe rather than the simple Lipo charger
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Unsinkable 2

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Re: Will this work?
« Reply #10 on: February 10, 2016, 08:22:53 PM »

Tug Fanatic, you are quite right, they aren't as greedy as I was lead to believe and I have now managed to solve the problem and run them off a single 6v NiMH battery pack just as people have been telling me (on various other posts I made)


I hold my hand up I made a mistake which lead me to believe I had a power deficiency problem and in describing it the way I thought it was lead to 'normal response answers' which made no sense to me...... IT DOES NOW!


 :embarrassed: :embarrassed: .... See Alantis thread.... Oh and thanks to all who listened to my nonsensical ramblings when I didn't know any better.......
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tsenecal

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Re: Will this work?
« Reply #11 on: February 10, 2016, 10:36:51 PM »

The reason I am trying to avoid Lipos is because I want to charge my batteries whilst still in the model and I don't like doing that with Lipos!

never charge any form of battery in the model.  ALL batteries out-gas something while charging.  gel-cell and lead-acid specifically out-gas hydrogen.  more explosive than any lipo i have ever charged.
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Colin Bishop

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Re: Will this work?
« Reply #12 on: February 10, 2016, 10:53:40 PM »

Sealed lead acid will not normally vent gas, nor will NiMH. I have been charging batteries in my models for over forty years hand and have never had a problem. I have to charge them in the models as they are usually built in and inaccessible unless you remove most of the other internals as well.

Colin
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microgyros

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Re: Will this work?
« Reply #13 on: February 10, 2016, 11:00:07 PM »

...A yacht racing neighbour replaced 5 Nimh cells with two 20 gram cylindrical Life cells and now all his pals use the flat 2s LiFe batteries for 6volt sail winches...

How about this for a power deficiency problem?
The aforementioned neighbour had fast charged, never pulse conditioned, a pair of 6volt sub-c NiMH batteries. This had burnt out a resettable fuse but both 5 cell batteries had a combined cell resistance in the order of 0.5 ohms, tested at 1 amp. It should have been around 0.1 ohm. I was told that one pack was just over one year old and it is used once or twice a week.
   I checked the battery manufacturer spec,checked my equipment, discharge-charged the packs twice, ensured the packs were not cold. After two cycles there was no significant improvement.
 He remarked the holding strength was boosted with a new battery. After showing two other people their drama was down to similar battery condition issues I don't hesitate to recommend a move away from NiMH chemistry. At least maintain them as required, test before use but at least use an on-board battery checker. These bar graph style indicators only cost two or three pounds and a voltage dropout would have been made clear as day.   
   
My own battery story.
I wasn't even charging the blighters. Years ago I re soldered ex-equipment tagged nicads squares into a flat pack. I had no low-temperature shrink pvc but just 105C polyester heatshrink. Used a rather aggressive hair drier but heard a hissing so moved them outside. One cell had internally shorted. :o
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