Stupid question... Why 1.2v on rechargeables?

TMorita

Enlightened
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Sep 5, 2006
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Hello Toshi,

Keep in mind that some of the discussion revolves around loaded voltage, and other parts revolve around open circuit resting voltage.

I believe if you have a Li-Ion, or Li-Po cell or pack with an open circuit resting voltage of around 3.7 volts, it wouldn't be recommended for "flight."

Tom

If you read mdocod's original post:

mdocod said:
For some reason, that I have never understood. rechargeable cells have a label voltage that represents something in the ballpark of the output voltage that could be expected towards the END of a discharge into a typical load.

He said "the label voltage represents ... the output voltage that could be expected twoards the end of a discharge into a typical load."

So he is referencing not the open circuit voltage, but the voltage into a load.

Toshi
 

mdocod

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I really should have used the 3.6V li-ion standard label for this argument, jeez, didn't realize that I was going to get myself so far dug into a hole. TMorita, I did say for typical consumer loads in the 0.1C to 0.5C range, we have no examples to look at to show something on the low end of my spectrum, and the only one that comes in the middle of my listed example is the Emoli which you have grossly interpreted incorrectly. At 1A it is operating at 0.37C which is right in the middle of my proposed example, 3.7V clearly occurs at about 2.5AH delivered capacity out of 2.684AH available capacity at that load moments before falling on it's face voltage wise, Over 90% of the run was delivered above 3.7V. And Over ~95% of the Watt-Hours were delivered above 3.7V. You are also coming to your %%% conclusions by taking the point that the cell reaches 3.7V and comparing the amp-hours delivered to the cells label capacity, which is always inflated or based on a lower load. Either way overall your interpretations make my points look very bad. I wish we had some 0.1C and 0.3C examples of these cells, they would clearly prove me correct in those areas. Had I simply chosen 3.6V (which is common label voltage of li-ion cells) instead the argument would hold up, or if you simply consider WATT-HOURS instead of amp-hours delivered then it turns more in my arguments favor, interpretation can be done a hundred ways for data to come to the conclusion you want, we have both proven to be lousy interpreters seeking to prove we are right. We are both wrong and it depends on the specific cell and the specific load and whether you choose watt-hours of amp-hours or whatever. I'm going to admit that my interpretations have been flawed in attempting to prove my worth(ego interference), will you do the same?


In nearly every example of a 0.5C or lower load run in the charts the cells are basically DONE with their discharge at 3.5V. The point I was originally trying to make was that rechargeable cells are for whatever reason labeled below their typical output voltages compared with primary cells by throwing out some example numbers that i remembered from studying charts and graphs. Had I known someone was going to come along and pick the statement and numbers apart just to prove someone wrong I suppose I would have been more careful in my wording and chosen to use either watt-hours, open-circuit voltage, or lower loads for my argument. It was supposed to be a general idea statement that made it easier to understand the difference between rechargeable labeling and primary labeling and now we have gone and made this an impossible mess to unravel for anyone reading this thread, lol.
 

VidPro

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Erm...I have a bit of experience with Li-ion batteries, having used them with R/C helicopters.

Toshi

RC, its the discharge LOAD they have in rc , that makes such a huge variation in use of the batteries. 5-10min discharge rates would be rarer for this flashlight group, but likley for the RC group.

so as usual everybody is right based on thier experience, the experience/use is just different.
also the voltage depression/resistance even on li-ion/li-poly varies LOTS. EX: 2 same sized cells one "cheap" and one "great" cell, and the variation in v-drop under load, and Lifetime (both cycles and years) of the cell can be huge.
 
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