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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 03-06-2011, 01:07 AM   #1
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Default Lipo low voltage

I have heard different stories about this question, but not sure what to believe. Question: 'What is a safe voltage you should end up with after a flight?'
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Old 03-06-2011, 01:16 AM   #2
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From Thunder Power RC LiPoly Safety sheet

You must check the pack voltage after each flight before re-charging. Do not attempt to charge any pack if the unloaded

individual cell voltages are less than 3.3V.

For example:

Do not charge a 2-cell pack if below 6.6V
Do not charge a 3 cell pack if below 9.9V

The standard practice is to leave approx. 20% capacity which should result in the cells being approx. 3.7 volts each.


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Old 03-06-2011, 01:22 AM   #3
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Here you go...

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

I try never to drop below about about 80% discharge. Most of the time I am in the 3.8v/cell range.

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Old 03-06-2011, 10:07 AM   #4
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Well it depends a bit what you mean by "safe". I'll go for
"The off-load voltage measured after the flight should be at least 3.7V"

That's not only safe in the sense that the pack won't immediately puff or burst into flames but you should also get a reasonably long life from a pack treated that way.

But absolutely no-one has done any really definitive scientific testing so it's always going to be a bit of a matter of opinion.

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Old 03-06-2011, 02:38 PM   #5
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Well, there are exceptions ... so here's mine ... doesn't mean its OK or advisable. Understand that the following is a research learning EXPERIMENT ... afterall, that's how we all learn whether for better or worse.

One of my 4-yr old PKZ 3S 2200 packs had the following cell voltages just a few seconds(3) before LVC … 3.40v, 3.42v, 2.61v. So, at LVC of 9 volts cell #3 could have dropped as low as 2.20v (3.38v, 3.40v, 2.22v = 9.00v). Even after 3 hours the resting bounce back recovery voltages were … 3.55v, 3.57v, 3.08v (another reason why LVC of 9.6v is preferred)

Using my Vision Peak Ultra Charger I first gave cell #3 a little boost to 3.56v via the 4-pin LiPo connector using my custom-made individual cell patch cord so as to match the resting voltages of the other two cells. Then, charged all 3 cells together via larger discharge connector to 12.55 volts. Then topped off each cell, plus giving each cell a little extra juice; especially #3 just in case it was weakened from previous LVC of 2.22v. The resting(5 minutes after charging) voltages of each cell after topping off with a little extra juice were … 4.23v, 4.22v, 4.27v. Then as before bench discharging to 9v LVC of programmed ESC. Individual cell voltages taken with E-flite Cell Checker just before before LVC of 9.6v were now … 3.22v, 3.15v, 3.42v. The used mAh capacity at LVC was 2177mAh or 98.95 percent of 2200 rated capacity. Thus, what I thought was an inferior LiPo was rejuvenated. After resting for 1 hour the bounce back recovery voltages of each cell were … 3.42v, 3.30v, 3.53v. So, what I thought was a weak cell was afterall, AOK! So, it appears I wouldn't have had to give cell #3 any extra juice(reserve mAh capacity), but always balance charge each cell to 4.20 resting volts(or 4.10-4.15v depending on time restraints and individual preference)

Since then, I have done three more bench tests with this pack and all the cells appear to be of equal strength. By individually topping off each cell I can control the bottom-end LVC of each cell within 0.10v of each other. So for example the cell voltages thru 16 minutes were very close having topped off each last night at 4.21v, 4.20v, and 4.19v (resting voltages after a couple minutes). This morning the resting voltages were 4.19v, 4,18v, 4.18v. Seventeen minutes and thrity seconds into depleting capacity the voltages were 3.21v, 3.13v, 3.14v with 2000mAh or 90.9% of rated 2200 capacity.

This is an EXPERIMENTAL research so as edf has said more than once it is better in actual practice to have stopped 30 seconds sooner(assuming cruising at less than half-throttle) or around 3.29v, 3.24v, 3.25v. I think you can appreciate how valuable is a Cell Checker and that if you get to know your lipos and treat them well, they will last a long time. And all this being done with a 4-yr old Vision Peak Ultra, an E-flite Cell Checker, a custom-made patch cord for topping off each cell, common sense, and TLCC (TendenLovingCareConditioning).

edit ... and all done in a concrete silo bunker in North Dakota ... just kidding.
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