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Old Jun 29, 2003, 09:00 PM
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Question

How Lipo's are rated


If I have a 1000 mAH (1AH) Lithium battery what will be the final voltage when the battery is considered flat.

I might assume it is 3 volts as this is as low as you can take these without causing possible damage.

I know 12 volt lead acid car batteries are considered flat at around 10 volts. So a 150 AH car battery should give out 150 AH before the voltage drops below 10 volts. That is unless it has power consumed at a high rate then its derated.

What makes me ask is there are some very extreme posts about how longer duration people are getting for these cells (lipo). And are they overdischarging them in order to achieve this.
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Old Jun 29, 2003, 09:36 PM
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I don't think they are overchargeing I know the differance between 4.23v and 4.1 charge isn't that much I read a artical
that about chargeing to 4.4v but the Battery EE's of course. But at
what risk. Those are the mistakes you never hear about. CU
Old Jun 29, 2003, 09:55 PM
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Sorry if you missunderstood jmbig.

You cannot overcharge lithiums without ruining them I believe.

I never charge to over 4.2 volts per cell. I know 4.25 is the absolute max according to every source I have ever seen. This gives me some safety margin if my Multimeter is not as accurate as I would like.

Its what the voltage of a cell will be after it has used its rated capacity that interests me.
Old Jun 30, 2003, 09:20 AM
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The cells have little capacity below 3.0 volts.

However, I have noticed that due to voltage depression, you can discharge one at say 5c to say 3.0 volts and get, say, 600 mah. The voltage will go back to 3.5 or more when the load is removed. Then discharge it again at, say, 1c and get another 100+ mah to 3.0 volts.

If you are using 2s packs, the standard low voltage cutoff on your esc will occur right around the time you have used virtually all the charge.

John
Old Jun 30, 2003, 04:55 PM
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Thanks John

Heres some more of my possibly flawed logic.

1C continous discharge = 1 hours flying
2C continous discharge = 1/2 hour flying
5C continous discharge = 12 mins flying

If you have a cell that is capable of 5C contiunous and you are using it to its full potential then the most it will deliver is 12 mins.

Granted if you use bursts of power to gain altitude then glide you will prolong the flight times.

When using the old Li Ions that where only capable of 3C you could only get 20 mins plus of flightime becuase thats as fast as the current could be sucked out of the cell.
Old Jun 30, 2003, 07:13 PM
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>> Heres some more of my possibly flawed logic.

Not at all flawed. Relating the discharge in C to flight time is a smart way to look at it. One of the things that was noted about the new Kokam 340mAh Li-poly cells, which are rated at up to 20C continuous, is that if you pull 20C (6.8A) continuous out of them for the full flight, you will only get a 3-minute flight!
Old Jun 30, 2003, 09:58 PM
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I would think that once a battery pack meets the motors current and voltage needs then the only accurate way to compare it to other chemistrys is to get the Watt/Hour figure and divide that by the weight.

e.g 8.4volts x 0.6AH (600mAH) = 5.04 WH

Now if a pack weights say 150grams thats
5.04WH/150g = 0.0336 WH per gram.

I'm not prepared to work any real world examples but I will say that Lithium Polymers will be far superior to any Nicad or NiMH.

The power to weight ratio (energy density) of LiPo's is why they are going to make such an impact in Electric flight. As any good electric builder knows the 3 most important things in flight are Weight Weight and Weight.


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