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Old Jun 06, 2005, 02:17 PM
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meteor's Avatar
Ontario, Canada
Joined Nov 2002
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Li-Po over-discharge/under-voltage issues...

Just read some rather scary assertions in a few threads, which are potentially dangerous, or ambiguous at best!

For example: "Any discharge where the cell voltage is above 3.0V/cell under no load is considered a safe discharge level"


Well, I can categorically state that I have permanently damaged cells where the open-circuit voltage recovered to far above 3.0V. (10C constant-current discharges to 2.7V on some Enerland and EtecHP cells caused significant, permanent, capacity loss)

In a recent destructive test I discharged an 800mAh cell to 1.5V, and it recovered to over 3.3V, open-circuit!



This "capacity depletion" idea is quite meaningless, as you can brutalize a new 10C cell with a 12C discharge and get far less than the stated capacity, and get "good" voltage recovery, but the heat you just created has essentially killed the cell!

I'm not sure where this "capacity depletion is critical, not voltage" observation comes from, but I think it needs to be taken with a huge grain of salt!



Thoughts?
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Last edited by meteor; Jun 06, 2005 at 02:39 PM. Reason: grammar
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Old Jun 06, 2005, 03:16 PM
Southern Pride
everydayflyer's Avatar
Haralson County GA. USA
Joined Oct 2004
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I agree and my findings with high C discharge rate LiPoly is that any discharge to the point which results in an open cell voltage below 3.7 is cause for concern. Heat is also a major concern or should be. The fact is that as cells are able to discharge at greater rates and for a larger percentage of thier totlal capacity they are more self destructive. The greater the capacity the more energy is available and the more heat that can be created. Just because a cell can deliver 12 -20C cont. while maintaing high volts does not mean it should be operated at those levels if you expect to get resonable likfe from it.
A 10-12C LiPoly when operated at 5-6 C may very well last 200 or more cycles while the same cell operated at 10-12C most likely will only last 60-80 cycles. If the cells exceed 130F then they will expire even sooner.

THe references to a 2.5 LVC and 3 V open being OK were for 3-6C cells. Many are just unaware of the difference in the newer cells.

Charles
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