It must be stressed that the use that we are making of all batteries used in electric flight applications goes far beyond anything that they were designed for. Some of them stand up to this extremely well, but life will always be short. As a rule of thumb, 50 cycles is about all you can expect before there is an obvious drop in performance - yes, I've changed that.
For some years I then used a total of 3 packs of 1700 mAh SCR cells which, again, gave similar flight times. Each pack weighed 16 ounces. There's a distinct pattern here, increasing capacity meant increasing weight and no increase in flight time.
Then, disaster, no more 1700 SCR's. I tried a pack of 2400 mAh 'N' cells (18 ounces) with disappointing results. This could have had something to do with the fact that I could never get them to accept more than about 1700 mAh on charge. Flight time was down and, more important, there was a marked reduction in power. I soldiered on with this and the last of my 1700 SCR packs for some time, getting similar performance.
I looked around for a replacement and was offered a pack of Tornado cells labelled 'RC 1920+' in large letters. I was encouraged, initially, by the fact that it only weighed 14 ounces. After accepting 2250 mAh on the first charge, this has never since taken more than 1750. In use it has about the same power and flight time as my tired 1700 SCR pack. For the record, the 1700 pack has 152 cycles on it and the 2400 a mere 70. When you consider that the new pack has only 5 cycles on it and is already showing signs of reduced performance, the outlook is depressing.
Comparison of early 1100 mAh pack with current 1920 (?) mAh pack.
The early pack had better performance.
The problem that this particular model presents is that spare parts are impossible to obtain and the next flight could be its last. In those circumstances I'm not inclined to spend around £60 - £70 on a suitable Lithium-Polymer battery.
Many of these cells were quite new and only had a few cycles on them. The commen denominator appeared to be that all of them were bought some time ago as part of a large order of Vanson cells from Maplin. They had all been removed from the packages and given one slow charge to initialise them and then stored in that condition. It seems that this was a mistake and that I should have left them uncharged in the packs. Another lesson learned. NiMH seem to like being used regularly, if you charge them, use them.
All of the above were 750 mAh cells. I have packs of 1600 and 2000 mAh cells in transmitters which seem to work normally - a different duty, of course. I have one pack of 8 x 2000 mAh cells used in the 'Streaker' with a Speed 400 motor (now my largest model) which have lost a lot of power, but which still fly the model and give reasonable flight times.
My regular charger, the HE Cell type described previously has LED's to indicate the charge state. A red LED shows that it is charging at constant current. When it switches to constant voltage, the red LED goes out and a green comes on. This goes out when charging is complete. It is difficult to check exactly what is happening because the charger pulses and an ammeter gives confusing readings. However, I did establish that charging does not actually stop when the green LED goes out, but continues for some time at a reducing rate. It pays to leave it on for a while.
I had been using custom built chargers for my smaller cells, so built a similar device for the 1200 mAh Kokam cells that I used for the Hornets. Now I knew that I was getting a complete charge but still got poor flights. OK, buy two more 2 cell packs. These gave normal charging and normal flights, so it was the old cells. Next move was to buy two more packs which were on offer from Indoor Flyer at reduced price.
For the record, my first two 1200 mAh 2S1P packs were bought at different times and one was clearly marked as 'Kokam', while the other looked similar but had other markings. This second one was never as good as the 'Kokam' pack. The next two packs were bought together and, again, one pack was marked 'Kokam' and one wasn't. Right from the start there was a marked difference in flight time. I'm not sure what can be done about this as the packs are now being 'sold-off' at reduced price.
The recovery effect seems to have received very little attention except in the instructions of a few toys. The temperature effect seems to have received no attention except in terms of very low temperature.
NiMH cells show the same effect but will lose power - and need a rest - when they get too hot. For best results, they need to operate in a fairly narrow 'window' where temperature is concermed.
This could be the first time this has appeared anywhere, but Li-Poly cells show the same effect! My 'Hornet' helicopters are equipped with my warning light and both will lift off showing low power and with the light on. After a couple of minutes struggling around in ground effect, they will then climb away and the light will go out. This can reach a point where I am struggling to prevent the model climbing into the ceiling indoors. Needless to say, it gets worse with age and low temperatures (just like me).
Now, apart from the effects noted above, all of these different types of cell appear to have the same tendency to dislike change. If they are regularly used in a certain way (duty cycle) they will give poor results if something is changed. You can call this effect anything you like, but I call it MEMORY. Don't try to tell me that Li-Poly cells don't suffer from memory effects. More.
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