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Li-Poly memory

We are constantly told by numerous sources (non of whom have any vested interest?) that Lithium-Polymer batteries do not suffer from memory effects. Well, not only do they have a dismally short life, but I'm here to tell you that they also suffer from effects that are very, very similar to memory. Hence this page.

The effects noted here became evident when flying small electric helicopters indoors. This is a good way of comparing batteries and their performance, if flown in a restricted space, because you can do little more than hover or perform pirrouettes. Thus the load is fairly constant and the flight time is an indication of a batteries performance during its life and allows you to compare similar and disimilar cells.

I had been using 2S packs of 1200 mAh Kokam cells to fly my 'Hornet' helicopters. When I acquired a 'Piccolo' heli, I expected to use the same packs. The latter is rather lighter and has a less powerful motor. I was surprised to get a poor flight performance and a duration of only about 1/3 of what I expected. There would be a sudden loss of power and the heli would sink to the floor. I checked several times that the battery concerned had actually been charged. When I put a given battery straight back into a 'Hornet' I got the expected performance and a duration which was what I would expect from the remaining charge.

I've noted elsewhere that my first Li-Poly (2S 700 mAh from Falcon Models) battery was bought for me by a friend. This had only been used in lightweight park flyers and would fly the 'Piccolo' well and give good duration. For a while, this was the only battery I had that would fly that model. It is still in use but eventually lost performance and would only give very short flights. Unfortunately, this was before I started logging the number of cycles so I have no idea just how many cycles it has on it, but I suspect it would be less than 40.

So, the lesson appears to be that if you get a Li-Poly battery accustomed to a certain duty cycle and then change to a different one - even if it is lighter - it will give poor results. 'Poor' in this case can mean much less.

I didn't worry too much about this situation at the time, but it is not the end of the story. I have two examples of a small helicopter (Walkera '5G6') weighing around 2 ounces and each powered by a pair of '1215' motors driven by a single Li-Poly cell.. Current draw in the hover is 2.4 amps. I have been flying these on Fullriver 250 mAh cells - thats a good 10C but they seem happy enough and give nearly six minutes flying time..

I also have a similar heli (E-Flite 'Blade mCX') weighing just half that (1 ounce) and powered by a pair of pager motors driven by a single 110 mAh cell. Current draw is 800 mA (7.5C). Flight time is 8 minutes. It seemed obvious to try this with the 250 mAh cells. This gave a weight increase of 3 grams (10%) and a poorer performance. The shock was that the heli would literally fall out of the sky at 3 minutes. This heli has a low battery sensor that reduces power when the battery volts fall below 3, accompanied by a flashing LED. When the 250 mAh cells lose the power to fly the model, the LED doesn't flash.

The obvious next step here was to try a new cell on the Blade and see if it was happy. Things were then made more urgent by the original Blade battery failing at 27 cycles. I located some 200 mAh Fullriver cells which were lighter than the 250 size and gave a weight increase of only 1.5 grams. The first flight was just over 13 minutes. Flights 2 and 3 gave similar times. This seemed fairly conclusive.

It's worth pointing out that we are talking about single cells here. To sumarise, once a Li-Poly has become accustomed to a certain duty cycle, if you change the duty cycle - even for a lighter one (something like a third in the above case) - the cell doesn't like it. Call that what you like, but I call it MEMORY.

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