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View Full Version : 452 Cycle Life for FMA KOK 2000 at Heavy RC Discharge


Fred Marks
11-17-2005, 04:03 AM
The data attached is the first realistic test daat for life cycle seen in any of the forums. Nathan Gwozdecki ran life cycle teats on the popular FMA Direct Kok 2000 HDR pack in 3S configuration.


Salient observations:

1. All done under carefully controlled lab conditions using precision test equipment.
2. The data is very conservative with the cells not cooled during charge or discharge but cooled by fan after discharge and before charge as one should do in the field. Any cooling in the airplane would extend life.
3. All runs were at a constant 17.5 amps discharge reflecting the average current drain for many applications in helicopters and with the many new brushless (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=1904) motors that draw some 15 to 25 amps and are replacements for Speed 400 brushed motors. Discharge was cut off at 3V/cell based on individual cell voltage. cell voltage characteristically rebounds to the neighborhood of 3.65 to 3.8 V when load is removed and the cell cools a bit. This amounts to about a 5% rebound of capacity.
4. All charges were done at 3C to 94% (1785 mAh) to expedite time. Even so, the tests took almost 14 solid days.
5. The test pack is a 3S CellPro pack with tap and connector so that cell voltage could be recorded ala CellPro.
6. Handling and plugging/unplugging of the pack was minimized.
7. Cycle life as defined by international standard 80% of base capacity was 450 cycles. The pack was run to complete death just to see what would happen. Results are interesting: Capacity declined slowly and steadily to 475 cycles, then plummeted. Dissassembly of cells confirms the rapid formation of salts (dendrites) as the cell dies.
8. The peak temperature rise for eacy cycle is recorded and averaged 65 Deg F over the life of the pack. It is interesting that, as cell capacity falls gradually and run time shortens very sligtly, rise temperature falls very slightly as well. Since discharge current was always a constant 17.5 amps, this implies that internal resistance was not degraded and, if anything, increased only slightly. The data recorded by Rod shows that the TP 2100 (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=2460) runs 161.5 Deg F at the 17.5 amp discharge , so it will be history very shortly as test runs begin.
9. The need for and achievement of cell balnce is also graphed. Cell voltage was brought to wihthin plus and minus 5 millivolts even after 450 runs. This is a powerful statement of the desirability of cell balancing as done by FMA BalancePro systems.
10. The discharge profile matches the flight profile for a typical trex heli or any of the new generation of brushless motor (http://www.rcuniverse.com/buynow/keywordclick.cfm?bid_id=1905) propulsion systems that demand an average 200 watts.
11. Life cycle cost for a stock KOK 2000 3S pack is $83.50/450= $0.18/flight compared to $1.41 per flight per the 60 cycles for a TP 2100 that RC Tester and trex report. Nathan will validate that figure very soon as it won't take long to run 60 cycles.

Jspencer
11-21-2005, 04:26 PM
So what internal chemical or physical change happens to the batteries to make them lose capacity and "die" eventually ?.:confused:

rcers
11-21-2005, 05:28 PM
Fred -they are great cells, but the wrong form factor for most of my planes.

We need long and skinny (replacing two wide A cell packs).

Mike

Fred Marks
11-21-2005, 05:49 PM
So what internal chemical or physical change happens to the batteries to make them lose capacity and "die" eventually ?.:confused:

The eventual failure mode for all chemical batteries is the formation of salts or "dendrites". If a pack is handled carefully as described for the cycle tests posted earlier, those salts simply beging to form as the constituents of the cell are "used up" by repeated cycling. Once they start to form, the salts are crystalline and in the very rough form of a sphere. Very rough, with pits and peaks that create greater surface area than a smooth sphere. As the crystal grows, the surface area increases with the cube of diameter and the dendrite grows almost exponentially.

Uniquely for Ni Cds, the dendrites can be "burned" or zapped by short pulses of high voltage. This is not an accepted practice for Li Pos because of the tendency of Li to ignite if such a hot spotis created.

The causes of salt formation leading to cell degradation are:

1. Normal wear out as the cell ages. 452 cycles at 133 Deg F, 3C charge, deep discharge to over 98 % of capacity, and recharge to about 94% capacity (Vs 90% ideally) is quite good. Please recognize that operating with < 130 Deg F, discharge to 80% DOD, charge at 1C, and charge to 90% capacity would yield upwards of 1000 cycle life. At the other end, discharge at over 160 Deg F, charge at 3C, 100% DOD, and charge to 100% capacity will likely mean a life cycle of 60 or so. All this is controlled by the end user. It's kinda like my aged body! I began forming dendrites a long time ago!
2. Over temperature. Formation of salts is accelerated at temperatures above 140 Deg F. As temperature approaches 185 Deg F or thereabouts (depending on the electrolyte used) the electrolyte can begin to bubble and then to boil. This causes plate separation, swelling as gas is emitted and the immeditate formation of salts and internal shorts that can become unstable and lead to thermal runaway.
3. Storage under improper conditions, particularly elevated temperatures.
4. Creation of dead shorts for a duration long enough to cause heating.
5. "Hangar rash"- dents, bumps, bends, improper pack assembly and the like that compress the plates and cause uneven pressure points on the internal structure.
6. Over voltage during charge has essentially the same effect as overtemp or dead shorts.

If you consider it, it is not at all difficult to maintain the conditions used for the life cycle test reported.