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Old Jun 10, 2006, 09:06 AM
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Curious about Lipo 4.2V limit

I know that Lipo voltage should not be allowed to rise above 4.2V and Lipo chargers limit their voltage per cell to that.

But, can we apply more than 4.2V to Lipo cell? If that is the case then may be we can charge 1C all the way to 4.2V instead of current taper off during the constant voltage phase.

This is what I dreamt of to test this theory. Since LBA6 can disconnect the charger from the pack if any of the cell rises near the 4.2V limit. Lets charge a very small 3S or 2S pack through the LBA6 with 1C contant current (may be a charger set in NiCd mode to supply 1C current). In an ideal world, the pack would be charge at 1C all the way to 4.2V and the LBA will disconnect it from the charger.

Like I said, I'm just curious and have no intention of testing out my theory (unless I can build a bunker for the pack). I have great respect for LiPo, I know what it can do .

Do all this make sense?
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Old Jun 10, 2006, 09:58 AM
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running lipo cells beyond 4.2V only shortens their life putting them on a higher discharge energy level and generating more heat thru their hi-internal resistance

that's what making a 20C cell is about, better internal electrode interface to the lipo substance to lower internal resistance
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Old Jun 10, 2006, 10:59 AM
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I don't know how a lipo would take a nicad charge since most are pulse. It may work. But then again.
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Old Jun 10, 2006, 11:30 AM
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Lipo chemistry causes the cell to irreversibly damage itself at voltages above 4.30V. Some have experimented with increasing the capacity slightly by charging at a higher voltage than 4.2V but the cycle life went down quickly.
http://www.batteryuniversity.com/partone-12.htm
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Old Jun 10, 2006, 04:34 PM
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Quote:
Since LBA6 can disconnect the charger from the pack if any of the cell rises near the 4.2V limit
LBA 6 terminates the charg if a cell reaches 4.3 volts not 4.2. Many chargers do sty in the CC mode until the cells reach 4.235 volts which is considered the max. voltage at 1C charge rate for good cell life.
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Old Jun 10, 2006, 07:44 PM
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Thanks hoppy for the link, very informative.

However, my thinking (just exercising my brain :-) ) is not to charge the pack so that each cell goes over 4.2V, but to apply a 1C charge to the pack (not single cell, difficult to measure cell voltage) and stops when a cell reaches 4.2V. So cells are not charged over 4.2V.

May be my question should be does Lipo chemistry acts differently between ~4 to 4.2V that we should not apply 1C current between the voltages?

Am I going round in circle here?
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Old Jun 10, 2006, 08:55 PM
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There are LiPoly chargers which charge at the 1C rate until the cells rech between 4.2 and 4.235 volts per cell at that time they change fro constant current to constant voltage and the voltage of the cells drop. This is normal. A Pb charger works much the same way. If the charge terminated at the end of the CC stage the cells would be approx. 80-85% fully charged. Some chargreers Stay in the CC stage longer than others. Most LiPo chargers are controlled by firmware (a computer program) on a chip.
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Old Jun 10, 2006, 10:30 PM
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Quote:
Originally Posted by Mheeli
Thanks hoppy for the link, very informative.

However, my thinking (just exercising my brain :-) ) is not to charge the pack so that each cell goes over 4.2V, but to apply a 1C charge to the pack (not single cell, difficult to measure cell voltage) and stops when a cell reaches 4.2V. So cells are not charged over 4.2V.

May be my question should be does Lipo chemistry acts differently between ~4 to 4.2V that we should not apply 1C current between the voltages?

Am I going round in circle here?
OK, I see what you are saying - You want just a CV charger. Pump whatever current will go in at 4.2V from start to finish. Interesting - Maybe charles can tell you what the voltage on his charger measures when he is doing a 3C charge.
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Old Jun 11, 2006, 02:38 AM
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My superbrain charges to 4.3 and then just plain shuts off with possible trickle charge.

I don't know if it's good or not, but the cells settle at exactly 4.2 volts.
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Old Jun 12, 2006, 12:43 AM
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Isn't the superbrain the one that has blown up more than a few lipos? I remember there being a big controversy over a charger a while back. Wasn't that it?
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Old Jun 12, 2006, 12:56 AM
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I hadn't heard about any blown lipos. The original 969 had the user figure the math and added capacity without a voltage taper or cv charge. Kinda like a glorified timer charger for lipos. The 977 is said to be lipo safe and still does not use a conventional cc/cv charge.
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Old Jun 12, 2006, 07:42 PM
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hi,

i have a theory too that contains an imaginary limit for voltage in a li cell
that i whould like to take adventage of, in my fast li charger.
and i m planning to test it with high C li packs.
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