Li Po Batteries

[Happy Days]

Oh Skyhawk, with out realizing it you’ve walked into a mine field. :cry:

You see, as Aidan said a few postings ago, the power train, (Prop, motor, speed controller, and battery) all work “hand in glove” with each other. You can’t just pick & mix. Here’s the problem I’m talking about.

Your Metal-hydride battery pack has a nominal voltage of 9.6volts. The brushless Electronic Speed Controller that you’ve brought is designed to work with a 3cell Li-po,………which has a nominal voltage of 11.1volts. Doesn’t sound much of a difference does it? But it IS.

To get the speed controller to work with your metal hydride battery pack you would need to fit 2 more cells into the plane to get the voltage close enough for the speed controller to work. But I’ll bet you a pound to a pinch of salt that you haven’t enough room, nor would the plane fly correctly balanced  even if there was room.

(It’s a pity you brought these models without asking for some advise before hand.)

I think the best thing you could do is to continue with your brushless conversion, and get some Li-po’s

To safely store and charge them at home get yourself a fire proof Li-Po bag like this one. Much safer that way.


Keith
p.s. Ignore the rubber band in the picture, I didn't see it through the view finder :oops:

[skyhawk newbie]

Thanks Aidan....

 Just ordered 3x ZIPPY Flightmax 1300mAh 3S1P 20C and 3 charger bags...

will have to look into getting a charger now......

Sean

[skyhawk newbie]

For Keith...


I know from reading this...  http://www.dmitrynizh.com/tw742.htm
that the Prop, motor, speed controller are the right ones...
I'm just hopeing now that the Li-Po's I've just ordered and bought will work with this combo......

3x ZIPPY Flightmax 1300mAh 3S1P 20C and 3 charger bags...

See Aidan's link.......


Sean

[Happy Days]

Put your mind at rest Sean, the batteries you’ve ordered will work in the power train you have got, ……….don’t Panic!  :lol:
Provided you’ve checked the physical size of the battery to ensure it’ll fit inside the model, all the electrics will work. (I’m just wondering if you’ve also ordered any connectors, to connect the esc to the motor, and the battery to the esc?)
The only slight question will be over the propeller. (As Aidan mentioned.)

From a gliders’ point of view there are two types of propellers.

One type is like that on most planes. The propeller rotates all of the time to keep the plane flying.

With gliders however the propeller rotates for only a short period, just to get the glider up to height. Then the pilot switches the motor off and the glider “Glides.”

From the Pilots point of view, switching the motor on, is like the driver of a car using first gear to get the car to the top of a steep hill,………..then turning the engine off and rolling down the other side!
(If you were thinking that by using the electric motor it would turn your glider into a fast aerobatic aeroplane,……………It wont.)

Anyway, coming back to the question of propellers, the problem with using a fixed propeller is that when the motor is turned off (After the plane has gained height) the propeller blades continue to stick out into the air flow. This acts like a kind of air break (producing lots of drag) and slowing the glider, making it loose height.
To stop this problem, powered gliders normally use what are called “folding propellers”. These work in the following way.

a/ When the motor is switched on the propeller blades stick out and work just like any other propeller.
b/ When the motor is switched off the blades of the propeller fold themselves back and lay against the side of the fuselage. This way the blades keep themselves out of the air flow and allow the glider to fly more efficiently.

I don’t see these type of folding props in the pictures of Dmitry’s web article. If you’ve ordered the fixed props like those you’ve seen in the article it will handicap your planes performance, but it will still fly. ( Replacement props are quite cheap for your type of set up.)

I believe you said you’re going to visit a local flying club today Sean. I’m sure you’ll get a lot of good information there.

Let us know how you get on.

Keith

[Ron]

Hi Sean,

Having spoken to Barry from the local club it looks like there won't be any flying at the field today due to the weather.
But might be worth a look as you're close.
Don't think I'm going to manage it today.

But on the subject of LiPo's, they have great advantages and some disadvantages - like most things.
They have great capacity for their weight and can deliver high currents.
But they can be ruined easily, especially by discharging below a certain point. This is why LiPo controllers cut off sharply and don't allow you to run the batteries down in the same way that you can get away with for NiCads.
Also, the max charge point is not easy to detect just from the voltage alone. So chargers have to monitor the voltage curve during charging, and watch for the critical slowing of voltage rise at the top of the curve.

But don't worry, the chargers and controllers take care of all this stuff.

I would just say however, that in general I believe it's better to use LiPos in a regular way. In other words, charge until the charger shuts off, and discharge until the controller shuts off, then immediately re-charge.
Putting LiPos on charge from a 'half charge' state can upset some chargers.

Ron

[Ron]

PS....
Google Earth says the flying field is somewhere over there...
 :lol:  :lol:  :lol:

[Aidan]

I would just say however, that in general I believe it's better to use LiPos in a regular way. In other words, charge until the charger shuts off, and discharge until the controller shuts off, then immediately re-charge.
Putting LiPos on charge from a 'half charge' state can upset some chargers.

Personally, I avoid discharging the battery until the controller's low voltage cut-off (this is usually abbreviated to LVC) when I can. Occasionally I misjudge and do hit LVC but it's not hte plan! The consensus is that the lifespan of the cells is much better when you don't discharge them below about 20% on a regular basis. Still, on cells costing less than €10 it's not really worth worrying about. I'd would be very worried about any charger that get's confused by part discharged lipos! I'm inclined to think the charger is one piece of equipment that you shouldn't skimp too much on!

If you don't already have one, you should get yourself either a Multimeter (you can get pretty cheap one's in places like Maplin) or a purpose designed power meter for electric power systems. A power meter (mine's an Astro Whattmeter) will typically show you the voltage, current and power simultaneously on a screen while you run your power system and will be a big help in matching up power systems and troubleshooting. You can also use it to check the voltage of your cells or you can use a multimeter for this (a multimeter isn't suitable for checking current as they usually can't handle more than 5A or 10A).

When a lipo cell is fully charged it should not exceed 4.23V per cell and when it's fully discharged it shouldn't be below 3.7V per cell.
The "3s" in the spec of the battery pack you ordered means there are 3 cells in series in the pack. So the total discharged voltage for the pack is 3.7V x 3 = 11.1V. The max safe voltage for the pack is 12.69V. Most chargers will stop the charge around 4.15V or 4.2V per cell to be on the safe side.
The voltage will drop below 11.1V while it's in use and just after you finish using it but it should soon come back up to at least 11.1V. If not, the cell was over-discharged.
The things to be aware of with lipos are:

1. only charge with a lipo charger and make sure it's set for the correct number of cells and the correct current. If you're unsure about this don't take any chances, anyone on here will be able to point you in the right direction.
2. make sure not to short them out
3. don't over-discharge them
4. Don't use them if they've puffed up - just discharge them to zero and dispose of them per manufacturers instructions.

I mentioned the risk of fire just to give you the full picture. It's not common but it's well worth being cautious with your lipos just in case.

Aidan

[Happy Days]

HHHMMMmm, pity there is no flying today Sean……..But there is a gathering of indoor fliers in Cork tomorrow

It is that time of year again and the Cork Model Aero Club Indoor fly-in will be held on Sunday Oct 24th 1.00pm till 6.00pm in the Hall in Middleton.
All Welcome, tea, coffee and cake will be available. A small fee if any will be charged. Helicopters, planes and freeflight welcome.
For more info contact Ralph McCarthy at 087 8322791 or rmccarthy@cit.ie
Looking forward to seeing everyone.

A lot of the indoor boys also fly gliders and, in addition all indoor models are electrically powered, so you will be able to get plenty of advise there.

See if you can make it............You'd learn a lot from them. :?:

K.

[Aidan]

HHHMMMmm, pity there is no flying today Sean……..But there is a gathering of indoor fliers in Cork tomorrow

It is that time of year again and the Cork Model Aero Club Indoor fly-in will be held on Sunday Oct 24th 1.00pm till 6.00pm in the Hall in Middleton.
All Welcome, tea, coffee and cake will be available. A small fee if any will be charged. Helicopters, planes and freeflight welcome.
For more info contact Ralph McCarthy at 087 8322791 or rmccarthy@cit.ie
Looking forward to seeing everyone.

A lot of the indoor boys also fly gliders and, in addition all indoor models are electrically powered, so you will be able to get plenty of advise there.

See if you can make it............You'd learn a lot from them. :?:

K.

Yep, and we might manage to convince you to try indoor next!

[Ron]

Hi Aidan,

Yes you are right of course.
My experience of chargers getting confused is with, probably cheap, commercial Li-Ion chargers.
And one hand power tool actually ruined a battery by letting it discharge too low.
The good ICs made for battery charge/discharge control in small battery equipment, look for the charge voltage curve rather than the absolute voltage, so it shouldn't matter what state of charge the battery is in to start with.
And, to the best of my knowledge, this is how cell balancing is done, as each cell will have a slightly different voltage at the critical point on the curve.

Anyway, yes I agree, don't skimp on the charger.

Yes the weather might make me take up indoor flying  

[Ron]

Where is the Hall in Middleton...... :?
Apart from being in Middleton of course  :oops:

[Happy Days]

Aidan, please allow me to correct you slightly re Li-Po voltages.

3.7V is the cells nominal voltage.
It’s fully discharged voltage is 2.7v although most people would say 2.75 to be on the safe side.
And to be even more safe most LVC’s cut the power at 3v/cell.

Yes, I know I’m a pedantic old bastard :evil: ……………….but I can’t help it! :lol:

K.

[Ron]

You and me both Keith.
I used to be just a pedantic bastard, but now I'm an pedantic old bastard :lol:  :lol:  :lol:

Ron

[Aidan]

Aidan, please allow me to correct you slightly re Li-Po voltages.

3.7V is the cells nominal voltage.
It’s fully discharged voltage is 2.7v although most people would say 2.75 to be on the safe side.
And to be even more safe most LVC’s cut the power at 3v/cell.

Yes, I know I’m a pedantic old bastard :evil: ……………….but I can’t help it! :lol:

K.

I have to disagree with your disagreement Keith!

Those voltages refer to the loaded voltage not the resting voltage which is what I was discussing. The resting voltage of a discharged cell is 3.7V. They should normally be stored at about 3.85V which equates to approx half charge. Full charge is 4.23V.
Under load the voltage will "sag". How much depends entirely on the size of the load, the capability of the cells, the charge state and the temperature. If you're at the limit of the cell's output the sag can be pretty significant.

LVC limits early on in lipo use were typically around 2.7V but that was because the cells available could only barely deliver the currents needed and sagged A LOT! A typical 6C or 8C lipo operating at it's limit might trip the LVC at 2.7V under load but would come back up to about 3.7V at rest. However newer cells don't sag nearly as much so an LVC set at 2.7V would allow the cells over-discharge enough to damage them.
An LVC of about 3V is still normal practice but for high C rate cells it's often suggested that this be raised to 3.2V or so.
As far as I know there is no "correct" LVC value. It all depends on the resting voltage you end up with and that depends on all the factors listed above... and more.

Well - I've got to get off my ass now and go finish a new plane for Middleton and charge all my lipos for some indoor fun.

By the way - in case it helps persuade you to come along tomorrow - Ralph usually brings cakes! :clap:

Aidan

[Happy Days]

What, What, WHAT! You disagree! :shock:  :shock:

Hmmmmm :?: …….. I like a person that stands their ground.

Well Aidan, purely as an exercise, and from one gentleman to another;
To quote from your posting.....

When a lipo cell is fully charged it should not exceed 4.23V per cell and when it's fully discharged it shouldn't be below 3.7V per cell.

It has always been my understanding that the term ‘Discharged battery voltage’ refers to the potential across a battery’s terminals, at it’s lowest safe working voltage, while connected to a load of 1C. Therefore, to quote from  Wikipedia

“The voltage of a Li-poly cell varies from about 2.7 V (discharged) to about 4.23 V (fully charged).”

What you refer to as “Resting voltage” I’ve understood to called the “Nominal voltage.”

Maybe the confusion has occurred in translation?


Either way, whether we choose to agree or disagree about this matter  I assure you that I fully respect you point of view and I’ve found this discussion very stimulating indeed. Thank you.

(Actually, we used to have an encyclopaedia in our house, but we found we didn‘t need it…………my wife knows everything!) :lol:

Alas I won’t be at the fly-in tomorrow. I used up all my brownie point by flying off Mt. Lienster this afternoon.
Enjoy your day and I’ll look forward to meeting up with you again at some future venue.

K.