| E-TEC 3S 1700mAh | ||||||||||||||||||||||
| Date: 30th January 2005 Battery Type: E-Tec 1700 Li-Po (3 x series Lithium-Polymer battery pack) Weight: 135g Dimensions: 42mm x 68mm x 27mm Manufacturers Rating: 11.1V 1700mAh, max discharge current 20A continuous (30A peak) Note: All tests are carried out in a controlled 24C ambient for consistency. Author: Mark Hopkins |
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| Mechanical. The battery is in a 3 x series Li-Po configuration, with the cells stacked horizontally on top of each other, and covered with twolevels of blue heatshrink. The two out-put wires (+ve & -ve) exit from one end, and appear quite adequate to carry at least 10A without too much of a problem. The cells were neatly soldered directly to a pcb which was fitted to the terminal end of the pack. As per standard for the model market there was no protection of any kind fitted to the battery, (over voltage, under voltage, over current, over/under temperature, cell imbalance, etc) also the three cells are stacked with no air gap for the middle cell, this cell is therefore likely to get hotter than the other cells during use and will most likely be the first cell to fail from the pack. This type of 'cell stacking' is generally avoided in high current applications for this reason. Electrical The voltage of each cell was measured before commencing charge and was found to be well within acceptable parameters with less than a 10mV maximum difference between the highest and lowest cell. The pack was then charged to 12.6V (4.2V per cell) with a current limit of 600mA, in a 24C ambient, and with a termination current of 50mA at 12.6V. At end of charge the cell pack was still well within acceptable balance parameters. The pack was discharged at C rate (1700mA) with a 8.5V cut-off voltage. The capacity at this rate was 1.89Ah, 21Wh, the cell pack reached a maximum temperature of 36.7C. The battery was then charged as before then discharged at 10A and 20A respectively, the following results were recorded: At 10A the capacity was 1.86Ah, 19Wh, the cell pack reached a maximum temperature of 69.8C. At 20A the capacity was 1.42Ah, 13Wh, the cell pack reached a maximum temperature of 73.9C. At the end of the discharge tests the cells exhibited the following voltages after a 30 second rest: Cell 1 - 3.07V Cell 2 - 3.07V Cell 3 - 3.08V |
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| Inside of the battery | ||||||||||||||||||||||
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| Pack Performance at a Glance | ||||||||||||||||||||||
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| Discharge Graphs | ||||||||||||||||||||||
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| Conclusion The battery seems well put together although the lack of protection which seems to be standard in the model market is, as usual a concern. The cell in the middle of the pack is likely to fail early if used at excessive currents. The battery performed extremely well at C rate and the voltage held up surprisingly well at 10A too, however the temperature of the pack did peek rather high at this current. The maximum temperature normally recommended for these type of cells is 70C, and as I can find no reference of the manufacturers recommended maximum temperature, 70C has to considered a sensible threshold. In a higher ambient this would definitely be exceeded and would almost certainly reduce the performance of the battery pack, or potentially leave it in a dangerous condition. The 20A rating I feel is a little optimistic, the voltage collapsed to around 8.75V almost immediately at this current reducing the Wh figure significantly and with a maximum temperature above what is usually considered to a reasonable safety threshold. The battery would most likely operate comfortably up to around 6A or so, but exceeding this level will gradually decrease the performance proportionately, and also increase the chance of overheating. Additional Info These tests were carried out under lab conditions using constant current dischargers in a controlled environment, but many other factors come into play in a model. (Ambient temperature, air flow across the battery, gear ratio, pitch curve, etc) A typical example of one of these variations, is that a model requires power to fly measured in watts, (volts times amps) so a high Ah reading can sometimes be a little misleading especially at higher currents where the voltage of the battery droops. This is why the watt hour figures are essential when assessing a battery pack for flight conditions and the Wh per kg figures will tell you exactly what you need to know, as can clearly be seen in the above table. The above pack was tested in NEW condition and it is common knowledge that a battery's internal impedance will increase as the battery ages, this means that the Wh figures at higher currents will gradually reduce with time/cycles even if the Ah figures are still relatively good at lower discharge currents. This applies to any model aircraft battery currently on the market, although some chemistries are more susceptible than others. Mark Hopkins (Research & Development Engineer/Designer and ISO 9001:2000 Auditor for PAG Advanced Battery Systems Ltd) |
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