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Old Oct 11, 2003, 03:34 PM
Registered User
Two Rivers, WI
Joined Mar 2003
565 Posts
If 3s voltage is too high?

This may sound stupid- Most people are having trouble finding a spot between 8-9 cell NMH and lithiums. If 12 volts is too much and Lipo don't have a memory couldn't I just watch my charger and quit at say 11 volts? don't let it peak to full charge. Is this a problem or a downfall I'm missing? Of course they would not be full capacity- but most have too much anyway-8000 MAH's
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Old Oct 11, 2003, 05:44 PM
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gtstubbs's Avatar
Perth Westen Australia
Joined Jun 2003
443 Posts
This is my guess as to some of the implications of your proposition.


Capacity

See attached graph

Source http://www.buchmann.ca/Chap4-page8.asp

The charged capacity of a cell (ignoring losses etc) if the charge terminates at 3% of rated current is a function of amps into the battery over time.

The green area (assuming the curve is correct) represents the capacity of the battery at 100% of charge based on termination at 3% of rated current

If you terminate the charge at about 3.7 volts / cell then you get about 11 volts for a 3s pack.

The charged capacity of the cells (and hence the pack) is represented by the red area.

If the curve function was known then you could by calculating the red and green areas under the curve arrive at the % capacity for an 11-volt pack charge.

My eyeball calculation method suggests at 11 volts you would have 50% or less capacity in your pack.


Amp Draw and ESC cut-off.


If you only charge to a pack voltage of 11 volts and then hit the pack with WOT runs you will drag the battery terminal voltage down and depending on your ESC setting may cause a cut-off.


“Digital devices are especially demanding on a battery. Momentary pulsed loads cause a brief voltage drop, which may push the voltage into the cut-off region.”
Source http://www.buchmann.ca/Chap5-page3.asp

If you charge to 4.2 volts the terminal voltage is less likely to drop at WOT to a level that may trigger the ESC.


Discharge Depth


“Most rechargeable batteries prefer a partial rather than a full discharge. Repeated full discharge robs the battery of its capacity. The battery chemistry which is most affected by repeat deep discharge is lead acid. Additives to the deep-cycle version of the lead acid battery compensate for some of the cycling strain.

Similar to the lead acid battery, the Li-ion battery prefers shallow over repetitive deep discharge cycles. Up to 1000 cycles can be achieved if the battery is only partially discharged. Besides cycling, the performance of the Li-ion is also affected by aging. Capacity loss through aging is independent of use. However, in daily use, there is a combination of both.”
Source http://www.buchmann.ca/Chap5-page3.asp




On balance for capacity, cutoff and life span reasons managing the throttle may be the way to go.
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Old Oct 11, 2003, 06:06 PM
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Two Rivers, WI
Joined Mar 2003
565 Posts
wow!

Now that's an answer! Thank you
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Old Oct 11, 2003, 06:46 PM
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JTinCT's Avatar
New Milford CT 06776
Joined Sep 2000
621 Posts
gtstubbs, I'm having trouble with a solution to world peace issue I've been working on
JT
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Old Oct 11, 2003, 07:13 PM
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Perth Westen Australia
Joined Jun 2003
443 Posts
JTinCT

The solution is the Gubenator of California.
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Old Oct 11, 2003, 07:17 PM
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Albuquerque Intl, New Mexico, United States
Joined Jun 2003
193 Posts
A 3s pack charges to about 12.6v, but delivers more like 11.1v. Still a little high, but the inexpensive GWS-ICS 100 ESC doesn't seem to mind - Brian
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Old Oct 17, 2003, 04:52 PM
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gtstubbs's Avatar
Perth Westen Australia
Joined Jun 2003
443 Posts
I found another graph in the The Kokam USA
Lithium Polymer Battery System manual that seems to show that my estimate of less than 50% capacity was far to optimistic, see attached modified graph. (red line)
Source
https://www.fmadirect.com/support_docs/item_1115.pdf

The graph suggests that charging to 3.7v would only put in 5%-10% of the capacity of the pack.

One of the interesting things about this graph is that it demonstrates fairly clearly the futility of charging LiPo’s over 1C.

This can be seen from the coincidence of the capacity curves for 1,2,3C charge rates at 1 hour and the fact that each curve at that point is within 2-3% of all others.

The manual also says in respect of this graph.

“The above chart is very important, so study it carefully. The following charging guidelines emanate from the chart:

1 Never exceed a 2C charge rate and accept that cycle life will be maximized by charging at 1C.
2 never exceed a maximum charge voltage of 4.235 VDC.”


This is a good manual in relation to the use of LiPo's, pack construction, wiring and support materials etc and should be read by all LiPo users.

There is another manual that contains further info, the Kokam/FMA Lithium Polymer Cell application manual. Also worth a read.
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Old Oct 17, 2003, 05:49 PM
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dave morris's Avatar
orlando fl
Joined Oct 2000
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the solution most used is to change the prop load and/or gearing to adjust the total power to what you want using either 2s or 3s
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