February 2018 RCM&E

[Geoff S]

I received my February issue yesterday and what a treat it was! Lots of interesting articles from cockpit and windscreen design/manufacture by Peter Miller to Shaun Garrity's piece on 3D printers. I haven't read it all yet but the skim reads are enough to get me studying further.

One article I did read was the one about puffy LiPos. I'm an electronics engineer and understand volts, amps and what not but have little idea of the chemistry of modern batteries and the introduction certainly helps there.

I take slight issue with one or two statements. I try not to be guilty of the sins of over fast charging/discharging but I am guilty of leaving batteries fully charged. I'm making an effort to leave my batteries on 50% storage charge now that flying opportunities are rather less frequent over the winter.

Mike Freeman, the author suggests that the minimum voltage after a flight should be no lower that 3.7v because the on load terminal voltage could have been lower than the danger point of 3v. A battery (or any DC source for that matter) can be considered as an ideal battery with zero source resistance and a series resistor representing the battery's internal resistance. The fact that the on load terminal voltage is less than 3v doesn't mean that the ideal battery has fallen that low. In fact it's at the measured voltage off-load ie in this case 3.7v so not really in danger at all.

I have an iCharger which displays internal resistance (IR) of each cell in a battery as well as a Wayne Gyles IR meter. A good battery has an IR < 5 milli-ohms/cell. Once the IR/cell gets to 10 milli-ohms then the battery is well on it's way to scrap IMO.

A few months ago some intrepid club members did an experiment with charging a scrap LiPo at a high constant current in the middle of our tarmac runway. Eventually, of course, it burst into flames but it took a surprisingly long time before it did. In fact most of us got bored waiting It seems LiPos may not be quite so volatile as we believe. However I'm still very careful with my charging regime.

However, on the whole, an interesting and informative article.

Geoff

[Trevor Crook]

Agreed. I used to leave my lipos in a charged state so they were ready for the next flying session, but have recently changed to a new regime - I leave them in the state they are in after flying. Typically this means they have 30-40% left, not the perfect storage condition but probably better than leaving them fully charged. I'll try playing with the charger's storage feature in future. Despite all this, my relatively low cost lipos do seem to last rather well.

On another subject, the Super Constellation on the cover is a magnificent model. I couldn't find anything in Alex's article that describes its motive power. As the props are 2-bladers I'm guessing it's i/c rather than electric, if so the motor installation is very neat. Anyone know the details?

[ken anderson.]

hello Trevor. the Connie is owned by a local lad Keith Mitchell and is really impressive in the flesh. Check out our club web site(BVRMC) for some details of the model and the construction...its powered by 4 petrol engines.

 

ken Anderson...ne...1 ....Connie dept.

Edited By ken anderson. on 16/01/2018 19:23:07

[Malcolm Fisher]

I have only partly read anything in my February issue but one item in the article on puffy lipos seemed to be amiss. The writer refers to the anode being negative and the cathode positive. My memory of battery technology from my long ago schooldays has the anode as positive and the cathode negative. Have I remembered incorrectly or are Lipo cells different from other batteries?

Malcolm

[Focae]

Anode is positive

Cathode is negative

[Martin Harris - Moderator]

I'm no expert but I understand that it depends on which is giving off the electrons i.e. this is the definition of the anode. In a lithium based battery, the lithium ions move from the negative electrode to the positive electrode during discharge so it seems reasonable that the anode is negative in this case.

Edited By Martin Harris on 16/01/2018 19:57:57

[Solly]

It's my understanding that ions move from negative to positive inside the battery. But in the external circuit the electrons move from positive to negative, hence the anode is positive.

[Mike Freeman]

Blimey! I seem to have started a bit of a debate!

In a rechargeable battery, like a LiPo, the terminals are either cathode or anode depending on whether the battery is being discharged or charged. As I buy my batteries to power things (ie discharge) I tend to think of the terminals in their discharge condition ie

Cathode is positive

Anode negative

I might be right or wrong but that is the reasoning for the terminology in the article.

Cheers,

Mike

[Geoff S]

Mike, I'm afraid there's no difference when the battery is being discharged or not. Anodes are positive and cathodes are negative in batteries, thermionic valves, or diodes. There is some slight confusion when diodes are being used as rectifiers when the positive voltage appears at the cathode which is why the cathode is often indicated by a red

However, that doesn't detract from a very useful article

Martin Harris says:

I'm no expert but I understand that it depends on which is giving off the electrons i.e. this is the definition of the anode. In a lithium based battery, the lithium ions move from the negative electrode to the positive electrode during discharge so it seems reasonable that the anode is negative in this case.

In a thermionic valve the cathode is heated and gives off electrons which are attracted by the anode which has a positive voltage applied. The flow is controlled by the grid which is negative relative to the cathode. It's the current flow outside the battery that counts and the convention is that the positive end is called the anode. Not that it matters too much.

Geoff

[Martin Harris - Moderator]

Right or wrong? Who knows when you Google but the Battery University is a well known authority.

[John Stainforth]

Surely, by convention, positive current flows from cathode (+ve) to anode (-ve) and whereas as the flow of negatively charged electrons is in the opposite direction, i.e. from anode to cathode.

[Focae]

The anode is positively charged. The cathode is negatively charged.

In electron terms, electrons flow from the cathode to the anode. (Negative to positive).

The problem is, when electricity was discovered, they thought that the current flow was the other way round, from positive to negative.

Unless you are a scientist or electronics geek, we lesser mortals always talk in terms of 'conventional' current flow - positive to negative (anode to cathode).

It really is just one of those things that as a young electrician, we had to accept and not over think.

[Mike Freeman]

Thanks All

My understanding has always been that whilst discharging the anode is -ve and the cathode is +ve and this is the convention I use.

Whilst charging the ions flow in the other direction and the anode becomes +ve and the cathode -ve.

Shall we just call them electrodes from now on?

Edited By Mike Freeman on 16/01/2018 22:51:02

Edited By Mike Freeman on 16/01/2018 22:52:51

[Former Member]

[This posting has been removed]

[Nigel R]

Geoff I don't think you can draw that conclusion about the internal resistance and the voltage. The simple model has many limitations.

You may want to tell Boeing about your overcharging experiment but I suspect they may disagree about the the frailty of lipo construction . ..

Lastly the storage thing. Maximum life certainly comes from perfect storage but I think the style of charge and discharge had more effect on lifespan than storage at full charge vs storage at half charge.

[Tom Thomas]

I would comment, but I'm, very....... very ...... dwunk.......

[OZ e flyer]

I echo your comments focae. Electrical trade training taught us that electron flow is negative to positive (same as was taught in science class), but current flow is positive to negative which is useful to remember when fault finding. Remember it, accept it and move on we were told. Lol. As for storage state of Lipo batteries, I don’t know what is “best” for the battery but, I suspect there is good reason for them to be shipped from manufacturer to retailers in only a partially charged state. Perhaps it is for best shelf life of the battery or possibly it’s just the safest way to transport and store them. I don’t know which it is but I’ll take their lead and store them at about 30% in a good fireproof container. Personally I love my bat-safe storage/charging boxes.

T.P.

[Mike Freeman]

Posted by Tom Sharp 2 on 17/01/2018 00:36:54:

How about terminals

But the terminals are always labelled + & -, the cathode and anode..... I mean electrodes attached will have either a +ve or -ve charge depending on whether the cell is being charged or discharged.

[Bryan Anderson 1]

Posted by Mike Freeman on 17/01/2018 06:37:33:

Posted by Tom Sharp 2 on 17/01/2018 00:36:54:

How about terminals

But the terminals are always labelled + & -, the cathode and anode..... I mean electrodes attached will have either a +ve or -ve charge depending on whether the cell is being charged or discharged.

An anode is always positive, a cathode negative regardless of whether it is being charged or discharged. Check with a voltmeter.

On charge, the current flow is reversed because the electromotive force ( open circuit voltage) of the charger is greater than that of the battery. On discharge, the emf of the battery drives the current through the load. The currents are reversible but the voltage is not.

Using incorrect terminology is potentially dangerous if it misleads people as to polarity.

[Cuban8]

After many years of not taking any aeromodelling magazines, I bought an RCM&E subscription six months ago. Unfortunately, and it's totally from my perspective, I thought that this months issue was the weakest so far. I did enjoy the photography in the Elvington report, the Percival Petrel article was good and best of all, the very detailed piece on Solarfim was very engaging and not a little sad given that the company's workforce has contracted considerably in recent years and clearly, the best times for their product are very much over.

On the down side, the free plans once again, were not to my taste, yet another WOT, for goodness sake, puffed lipos and the danger from engines have all been covered many times before. Did a double take at 'Mignon' as I initially thought that Mr Foss had forsaken the WOT designation this time around..........no so as it turns out! ARTF reviews, by the very nature of the product under test all appear to drift towards being a bit repetitive and not the fault of the authors.

Rest of the issue was OK and I'll work my way through it as and when - sorry. Onwards and upwards though.

Must be a nightmare to come up with a magazine that has to cover so many areas, attempt to appeal to everyone's taste and also remain profitable - obviously, if the magazine was full of just what I like to read about, it wouldn't last long and I fully realise that from time to time the mag will for me, be a quick read through and hope for better stuff next time.

Perhaps a survey of readers' likes/dislikes wouldn't go amiss?

Edit..........as a retired electronics engineer, I find the discussion about anodes and cathodes amusing, we need more stuff to argue about - the down wind turn, the demise of balsa cement, mode 1 versus mode 2, castor against synthetic, ARTF against building, drones (hiss) against real models.

 

Edited By Cuban8 on 17/01/2018 08:58:06

[Martin Harris - Moderator]

I really don't think that it's as easy as simply adopting the convention that an anode is positive - although this is the case for the majority of devices. This extract gives food for thought:

An anode is an electrode through which conventional current flows into a polarized electrical device. A common mnemonic is ACID for "anode current into device". The direction of (positive) electric current is opposite to the direction of electron flow: (negatively charged) electrons flow out the anode to the outside circuit.

The terms anode and cathode do not relate to the voltage polarity of those electrodes but the direction of the current: whether positive charge is flowing into or out of the device. Conventional current quantifies the flow of positive charge. In most cases, positive charge flows into the device via the anode, and positive charge leaves the device via the cathode.

Conventional current depends not only on the direction the charge carriers move, but also the carriers' charge. The currents outside the device are usually carried by electrons in a metal conductor. The flow of electrons is opposite to conventional current because electrons have a negative charge. Consequently, electrons leave the device via the anode, and electrons enter the device through the cathode.

The anode and cathode have slightly different definitions for electrical devices such as diodes and vacuum tubes where the electrode naming is fixed and does not depend on the actual charge flow (current).

In a recharging battery, or an electrolytic cell, the anode is the positive terminal, which receives current from an external generator. The current through a recharging battery is opposite to the direction of current during discharge; in other words, the electrode which was the cathode during battery discharge becomes the anode while the battery is recharging.

Note that this doesn't imply a change of polarity - and for the record, I always thought that the definition of an anode was that it was the positive terminal until I started looking into this!

I suspect that the conclusion to be drawn is that although Mike was correct (I haven't read the article) the terms anode and cathode should only be used when discussing cell chemistry and for all practical purposes, positive and negative terminals should be quoted.

P.S. I was taught my basic electronics when earth was often regarded as positive and my "Principles A" lecturer taught us to remember that earth falls through a funnel - hence the origin of the diode symbol! (He was a rather eccentric Czech who'd stayed on after coming over during WW2 to work on electronics research...but as my motorbikes were old enough to have positive earths it made sense to me.)

Edited By Martin Harris on 17/01/2018 11:31:09

[Nigel R]

"Perhaps it is for best shelf life of the battery or possibly it’s just the safest way to transport and store them."

Shelf life is best around 30% to 50% charged.

As for transport, it is now FAA regulations to have 30% or less, and of course rest of world follows suit. It's all about reducing the stored energy being shifted about - they don't want unexploded incendiary devices filling up aircraft, for some reason...

[The Wright Stuff]

Well, the March issue has arrived, and I'm still reading the February one.

I have to say, the article on engine safety, and particularly the use of suitable mounts delivers an important message. But please, if you are going to write "Wankel" on an engine stand to be photographed in a national magazine, please write it out in full...

Edited By The Wright Stuff on 13/02/2018 13:33:21

[MaL]

Posted by Bryan Anderson 1 on 17/01/2018 08:44:48:

Posted by Mike Freeman on 17/01/2018 06:37:33:

Posted by Tom Sharp 2 on 17/01/2018 00:36:54:

How about terminals

But the terminals are always labelled + & -, the cathode and anode..... I mean electrodes attached will have either a +ve or -ve charge depending on whether the cell is being charged or discharged.

An anode is always positive, a cathode negative regardless of whether it is being charged or discharged. Check with a voltmeter.

On charge, the current flow is reversed because the electromotive force ( open circuit voltage) of the charger is greater than that of the battery. On discharge, the emf of the battery drives the current through the load. The currents are reversible but the voltage is not.

Using incorrect terminology is potentially dangerous if it misleads people as to polarity.

 

 

potentially,,,,ha ha ha!

Just seen another post that explains what i was going to say in a much better way so....rest of post deleted.

/0.02p

Edited By MaL on 13/02/2018 15:59:33