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Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

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Old 07-16-2006, 02:34 AM   #1
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Default Don't charge LiPo if under 3 V per cell?

I just read in a thread about Vampower Pro batteries being good in the eyes of a poster and clicked on his included link to their web page. The page stated that you shouldn't charge a LiPo if the voltage is under 3 volts per cell. What are you supposed to do if under .... throw them away?
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Old 07-16-2006, 02:46 AM   #2
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I dunno, I had a common sense lipo that spent the night and the better part of a day in a tree. When I was able to retrieve it I only had about 1.2 v per cell. It charged it up under a watchful eye and now have about 10 cycles on it sense then and it shows no visible signs of deterioration at all. I will be interested to see over time how it holds up against the others of the same type and brand that didn't go through that.

I've heard some say that 3v per cell is the point of no return on lipos but not in this case.

Am I just lucky or what?
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Old 07-16-2006, 03:02 AM   #3
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the FMS 4s says it can do it also
I have read the number of cycles can be reduced but I haven't done the electric thing long enough to tell you if it's true
but I have one pack that got drained so I will be able to tell you someday

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Old 07-16-2006, 03:28 AM   #4
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This is from the manual for the AstroFlight #109 Lipo charger:

"If ANY cell in your pack is discharged below 2.5 volts,
permanent and irreversible chemical damage will slowly and completely
destroy the cell within 30 days. If a cell is exposed to this low state of
discharge overnight it can sometimes be saved by applying a slow
“trickle” charge until the cell voltage reaches 3.2 volts or more and
remains at this voltage after charging current is removed. By “trickle” we
mean a charge rate between 5% and 10% of cell rated mahr capacity.
For example on a 2000 mahr cells, “trickle” means 100 to 200 milliamps.
I have recovered cells as low as 1.4V by trickle charging for one to two
hours. It doesn’t always work, but I have recovered cells most of the
time. The longer the cell has been exposed to low voltage, the lower the
chances of recovery. After recovery I recommend fully charging the cell
to 4.2 volts, and then running a discharge test at a “1 Cell” rate to
measure its capacity. “1 C” means discharging a 2000 mahr cell at 2
amps. If the battery capacity has deteriorated to less then 75% of its
rating when new, it is time to discard that pack.

IF ANY CELL IS DISCHARGED TO BELOW 3.2 VOLTS BUT
ABOVE 2.5 VOLTS it is in a stable voltage region and may remain in
that condition for a long time without damage. But the internal
resistance of the cell may have become much higher than normal. If you
try to charge this cell at a normal “1 C” rate, its voltage may jump up by
as much as one volt or more. But if the charging current stopped, the
voltage will drop back the original value. This cell is not ACCEPTING
the charge. Stop at once. If you continue to charge at a “1 C” rate,
the cell will puff up and be ruined and may even start BURNING. Always
monitor the voltage behavior of any lithium battery pack for the first
minute to be certain that the charge is progressing normally. In multi cell
packs that have no taps this is about the only way of detecting a problem.
If your cell has a separate connector with battery taps, use a battery
balancer like our Astro Blinky #106. It will detect a low voltage cell even
if the total pack voltage looks OK. If you do not have a balancer or a
balance connector on your pack you can still detect the problem by
observing the abnormal voltage rise when attempting to charge at the “1
C” rate. Good condition packs will show a very small voltage rise of less
than 0.1 volts per cell. A three cell pack in good condition will only
show a 0.1 to 0.3 volts rise. If you observe a large jump in voltage, stop.
Something is wrong with your pack."

Hope that helps.

Mike N

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Not just the parts you like.
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Old 07-16-2006, 05:55 AM   #5
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Got a scare this evening when I was just coming in to land, and the cutoff started slowly cutting back. No problem landing, but I thought I had a CC10 in the plane, set for hard cutoff, not soft. Thought, its gone. Got home and tested 9.25V. No problem, charged fine. Then I remembered that I had swapped the ESC to an Eflite20. Apparantly, the Eflite must have soft cutoff. Works pretty well, as I've never actually flown to LVC from a low batt condition, only from excessive draw, where the battery is not discharged.
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Old 07-16-2006, 06:37 AM   #6
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My post contained incorrect information. Please accept my apology.

- Jeff
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Old 07-16-2006, 12:05 PM   #7
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Originally Posted by jdetray View Post
Lipos should never be discharged to the point that they drop below 3.0V per cell under load. For the newest generation of lipos, the recommendation is even more conservative: never discharge below 3.2V per cell under load. The newer lipos hold their voltage better, but near the end of their useful charge, the voltage drops rapidly, so 3.2V per cell is a safer value.

While lipos discharged below the recommended voltage can sometimes be recovered, it's very likely that some damage has occurred.

Bill G:
A lipo that measures 9.25V at rest (that is, not under load) has very likely been over-discharged. If it is 9.25V at rest, it was almost certainly below 9.0V under load.

Here's a table of lipo resting voltage versus state of charge.

4.20v = 100%
4.03v = 76%
3.86v = 52%
3.83v = 42%
3.79v = 30%
3.70v = 11%
3.6?v = 0%

Capacity below 3.7v "resting" is not usable for flying, it's where the battery voltage dump begins.


- Jeff
Please give source of your information. I goes against most published material regarding Lithium Polymer technology. To state that the voltage should never go below a certain point under load without stating the load is meaningless.

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Old 07-16-2006, 12:23 PM   #8
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Originally Posted by Red Scholefield View Post
Please give source of your information. I goes against most published material regarding Lithium Polymer technology. To state that the voltage should never go below a certain point under load without stating the load is meaningless.
Thanks Red. It is disinformation, about dropping below min voltage under load. Castle ESCs originally SPECIFIED cutoffs BELOW 3V per cell, UNDER LOAD. This is because the cell voltage immediately returns to a safe level, after the cutoff. Where this can be a problem, is under a light load, without a significant voltage drop under load. Red knows that, I know that, Jtetray apparantly doesn't. Looking a CBA charts, and just good 'ol common sense, will tell one that lipos commonly drop below cell min voltage, to meet their burst ratings.
The lowered LVC setting dilemma was actually due to the common beating of lipos. With a properly rated lipo, the user does not need to set cutoff at a lowered "under load" voltage. This is obviously due to improved C ratings, from the original 5C lipos.
It is tough for an EET like myself, and a batt expert like Red to swallow these meaningless schoolings, but we'll deal with it. I'll always credit a source when possible, for the purpose of credibility, while some want it to appear to be their own genious. See it all the time on the forums.
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Old 07-16-2006, 05:09 PM   #9
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I agree about the "misinfo" BUT

it is a known fact that a lipo discharge is logarithmic not linear that is why it is dangerous to discharge below a certain point. I think that is a better point rather than give numbers (which can very from chemistry to chemistry)

In more clear terms: "the lower the voltage the faster it drops in a discharge" does that make sense? Or am I just adding to the confusion?

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Old 07-16-2006, 05:26 PM   #10
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Hi Red,
Apparently I have given wrong information, and I apologize to the forum.

My understanding was that it is never advisable to allow lipos to be discharged below 3.0V per cell (3.2V for newer-generation cells) under any conditions. In this context, the size of the load is not relevant. Big load, small load, no load -- you don't want your lipo voltage to fall below the minimum per-cell value, period.

I really hope you'll set me straight so I can avoid giving bad advice in the future.

Again, I apologize for the mis-information.

Thanks,
Jeff
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Old 07-16-2006, 06:43 PM   #11
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Originally Posted by jdetray View Post
Hi Red,
Apparently I have given wrong information, and I apologize to the forum.

My understanding was that it is never advisable to allow lipos to be discharged below 3.0V per cell (3.2V for newer-generation cells) under any conditions. In this context, the size of the load is not relevant. Big load, small load, no load -- you don't want your lipo voltage to fall below the minimum per-cell value, period.

I really hope you'll set me straight so I can avoid giving bad advice in the future.

Again, I apologize for the mis-information.

Thanks,
Jeff
The minimum open circuit voltage of 3.0 volts is probably a good and safe value for LiPo cells. The minimum cutoff voltage under load is dependent on the load. You will find that if you stick with the 2.5 volts under loads above 2C you will never see the battery open circuit voltage drop below 3 volts. At higher loads we normally encounter, say in the 10C range the voltage the voltage drops rapidly in the first 10% of the discharge to around 3.4 volts and then is fairly linear down to where the discharge curve begins to take a dive at 3.2 volts, from this point on to 2.7 volts about 15% of the capacity is delivered. So you can understand the logic of specifying 3.0 volts cut off under load, you really don't have that much left and continuing the discharge is pushing your luck for insignificant payback.

"The problem with today's modelers isn't so much what they don't know; the problem is what they think they know that just ain't so."

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www.hangtimes.com/redsbatteryclinic.html
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Old 07-16-2006, 09:34 PM   #12
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Hi Red-

Thank you for setting me straight. I'll leave future advice on this topic to experts such as yourself!

- Jeff
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Old 07-16-2006, 11:20 PM   #13
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Default Drop dead cut off

There is a way you can tell if the cell/pack has been irreversibly damaged. Early on , we pressed Kokam to define the minimum cutoff voltage at which there was a 3 sigma (> 99%) chance that a cell would not recover. The usual period of dead silence followed. So as usual, we ran tests using production cells to try to find out. The following facts were found:
1. In some instances, a cell could be taken below 0.9V and, if pre-charged at a low current, would recover. Cycle life is reduced because salt formation begins even in a small way on any over discharge.
2. Here's how you tell if you killed the buggar: If the cell returns to < 2.7V under no load after them load is removed or has been removed for a little while, the call has been harmed.
3. If the cell pops back to > 3.3V under zero load, it probably has not been harmed.
4. This kind of engineering evidence guides us in design of FMA chargers. The CP4S is smart enough to check the cells individually before it launches charge to determine if a cell has suffered damage, then to initiate a slow charge until the cell is above a preset voltage before it initiates full charge. In an estimated 90% or more of cells that have been harmed, the cell/pack will be recovered and be suitable for continued use. That is quite a cost saving. Service life may be shortened somewhat over what it would have been if the cell were cut off properly above 2.7V using DPM.
5. NOTE: if you do not use the CellPro/BalancePro DPM that cuts off on individual cell voltage, then it is best to cut off on pack end-to-end voltage at 3.3V/cell to try to avoid unbalance.

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Old 07-17-2006, 12:50 AM   #14
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Originally Posted by Red Scholefield View Post

"The problem with today's modelers isn't so much what they don't know; the problem is what they think they know that just ain't so."
Ain't that the truth? A pretty evident sign on many posts on this and other forums.

I do thank you for clearing the fog RS.

I also thank Fred Marks for adding some common sense to this discussion as well.
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Old 07-17-2006, 12:59 AM   #15
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Originally Posted by jdetray View Post
Hi Red-

Thank you for setting me straight. I'll leave future advice on this topic to experts such as yourself!

- Jeff
Keep learning. Experts eventually need relief and someone to take over and spread the faith.

Red S. AMA 951
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www.hangtimes.com/redsbatteryclinic.html
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Old 07-17-2006, 05:04 AM   #16
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Originally Posted by Red Scholefield View Post
The minimum open circuit voltage of 3.0 volts is probably a good and safe value for LiPo cells. The minimum cutoff voltage under load is dependent on the load. You will find that if you stick with the 2.5 volts under loads above 2C you will never see the battery open circuit voltage drop below 3 volts.
Red, I've been trying to explain this principle since a day after lipos came out .


Originally Posted by Red Scholefield View Post
"The problem with today's modelers isn't so much what they don't know; the problem is what they think they know that just ain't so."
As a credit to Red, he's one of the few who HASN'T met the criteria of his statement .
At first, I though he wrote "moderators".


I have CC ESCs that list 3s reccomended cutoff at 7.2V. I wouldn't do that in a light load, but would in something like an EDF, if BENCH TESTING WITH A WATTMETER warranted it safe. I put off the wattmeter buying for a year and a half. Way too long. I had a good guess with voltage parameters, but guessing gets old quick.

There are books literally labelled EE/EET 101 that are essential to having a reasonable understanding of electrics, whether used in rc flight, or anything else, for that matter.
This time, the "101" analogy was not meant to be a joke .

Didn't mean to be harsh Jdetray, actually its more principle with me. I'm not trying to set the rules here, but personally I try to quote with correcting info, specifically and only when the poster asked for it with a question. Otherwise, I may state it, but don't quote the original poster's source. There are folks here who reply to that action a lot worse than I do .
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