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Li-Poly battery changes

Those of us who have been using Lithium-Polymer batteries for a while are probably aware that they are changing. How so?:

1 They are heavier

I have several 2 cell packs of Kokam 1200 mAh cells which weigh around 48 grams. The increasingly common 700/800 mAh packs being used with a lot of R/C 'Toys' (E_sky, etc.) weigh around 47 grams. I have two 860 mAh packs which weigh 51 grams. In the middle are the Mystery cells which are 900 mAh weighing 42 grams.

My theory on this one is that the battery manufacturers are having to ensure that their cells live up to their claims. Kokam claimed 7/10C discharge rate, while E_sky claim 10C, Hi-Model 20C and Mystery 15C.

2 Performance

Kokam were pretty good in their time but any attempt to match the claimed discharge rate gave short flights and a short life. They also had a tendency to 'recover' after a short rest. You could extract the claimed capacity by having several, increasingly shorter, flights.

Mystery cells behave like Kokams. They run out of power about halfway through the expected discharge but will recover after a rest (you can repeat this several times).

The E_sky and Hi-Model cells give much better performance for a longer period, when they will suddenly run out of steam. They do NOT recover, until recharged.

3 Life

Subjected to the rigours of electric flight (deep charge/discharge cycles) the Kokam cells show a marked drop in performance/flight time after 20 cycles, or so, and have a maximum usable life of, perhaps, 50 cycles.

Mystery cells are similar to the Kokams (I had assumed that these were new to the market, but perhaps I'm wrong). Even when new, they don't match the performance of E_sky and Hi-Model cells which seem to be a vast improvement on earlier types (though heavier). So far, the E_sky and Hi-Model cells show no loss of performance over 20 cycles or so. The Mystery cells are already showing signs of fading after just 10 cycles.

There is one other difference: The cells which recover after a rest (Kokam, Mystery, etc.) all show around 3.8 volts per cell even when pushed to the point where they won't fly the model. The newer types (E_sky, Hi-Model) register as low as 3.2 volts when pushed to the same point.

Maker	Capacity (mAh)	Weight (grams)	Discharge rating (C)	Comment
Kokam	1200	48	7/10	short life
Hi-Model	860	51	20
E_sky	800	47	10	surprisingly good
Mystery	900	42	15
Fullriver	800	54	20

I now have a couple of 2 cell packs of Fullriver 800 mAh cells from Indoor Flyer. I have had good results from this manufacturers cells in smaller sizes, so had to try them. These are substantially bigger and heavier than other cells of similar capacity. The trend continues.

Conclusion

Battery manufacturers are improving their products in order to live up to earlier claims. It seems that this is adding weight to the cells. The problem for the modeller is deciding which are the good ones. Simply weighing them may not be a good indication unless you have something to compare with.

Note

The Mystery batteries (2) referred to above were obtained from Hong Kong via eBay at a very cheap price. It seems that I got what I paid for! They had poor performance from the beginning and also produced power glitches. There was a marked decline at around 10 cycles and one pack (actually, the better of the two) had a cell start to swell. They were scrapped at 14 and 15 cycles. I dismantled them to remove the leads and found very poor soldering. E_sky batteries can be obtained from the same source for very little more.