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Old Jul 27, 2005, 02:47 PM
A man with too many toys
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LiPo Slow Charge – what’s that?

I have seen it mentioned in several threads that slow charging could be beneficial to LiPo batteries under certain circumstances. I really don’t understand the How, When, and Why of slow charging.

1. First exactly what is slow charging, 0.5c, 0.1c ?

2. When do I need to slow charge my packs?

3. Exactly what does slow charging do for a pack? I get the impression that some think it will balance a lack but I think not.

.
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Old Jul 27, 2005, 02:53 PM
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Simply Normal & Abnormal Lipo Pack Conditioning

Slow Charging ( for noraml packs)and balancing ( abnormal packs) r two different ways to condition a LIPO pack.

A discharge capacity check should tell U what to do.
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Old Jul 27, 2005, 03:04 PM
A man with too many toys
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Quote:
Originally Posted by hall woo
Slow Charging ( for noraml packs)and balancing ( abnormal packs) r two different ways to condition a LIPO pack.

A discharge capacity check should tell U what to do.

Condition a LiPo Pack? I have no idea what that means?


Does it increase the capacity?

Does it lower the internal resistance?

I use a TP205 all the time so balance is not a problem.



.
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Old Jul 27, 2005, 03:11 PM
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Of course, Hall Woo may know something I don't about slow charging lipos, but to the best of my knowledge, the only time you need to slow charge your batteries is if they were discharged to under 3.0 volts/cell open circuit. I have read that it is beneficial to charge the over-discharged cells at 1/10c until they reach about 3.7 volts/cell, then charge at the normal 1C (or .7C as some do).

I don't consider slow charging lipos on a regular basis to be necessary, or safe. Safety dictates that we keep an eye on our lipos as they charge, and NOBODY is going to watch their batteries the whole time if the charge takes 10 hours.

IMHO of course

Dan
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Old Jul 27, 2005, 03:34 PM
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When we charge our LiPo`s at 1C then we charge them as hard as they can be charged. If you have the time (charging at home for next days flying) you may extend the battery-life if you slowcharge at 0,5C or lower! The chemical process inside that detorates the LiPo is much related to the charging. Also the batteries have less chance to become unbalanced when not only fastcharging.
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Old Jul 27, 2005, 03:58 PM
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OK, let's talk. As far as the best info that's been published here is able to establish, the ONLY time that slow charging LiPos is mandatory is to help bring back cells that have been over-discharged. This may at least salvage some cells for you, but it's by no means sure. (Hasn't worked too well for me). Slow charging LiPos to "condition" them (as is common standard practice with NiCAD and NiMH cells) is a myth. The LiPo cell does not benefit. Slow charging to "balance" them, (again as is standard practice with NiCad and NiMH) is also an utter myth, and is in fact dangerous to the cells, and possibly you and your property, unless both are completely fireproof.

NOW, having said that, I am on a campaign to get people to charge their LiPos at well under 1C, using a "taper-charge" protocol instead of a "constant current" protocol. By which I mean setting up a charger based upon a regulated VOLTAGE equal to or just a bit higher than the full-charge voltage of the pack, and current regulated with a resitor to 1C or less when the pack voltage is at 3.0 volts per cell. As the pack approaches full charge, the rate will have tapered off to nearly zero. This is pretty easy to do, although no commercial charger does it at this time, so far as I know. The downside is it takes all night to charge a pack. The upside is that the charger is incapable of massively overcharging the pack, which is what can happen if a constant-current charger fails to shut off correctly. Just check your constant-current charger next time. It will have the voltage to the pack cranked up to 5.0 or even 6.0 volts per cell to maintain the charge current, since current is delta-V over R.

I have an old, old, OLD pack 3S3P of the ORIGINAL KOKAM 1020 cells that now has well over 150 flights on it, always charged this way, and drained at about 2.5-3C, (which is about all those old cells could deliver when new). It still delivers all the capacity needed for the motor glider it is installed in. I do believe that this gentle charging regime may have something to do with this. Cells that I work harder simply have never lasted as long.
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Old Jul 27, 2005, 04:07 PM
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I agree with 4 out of 5 of the answers...good stuff!
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Old Jul 27, 2005, 04:31 PM
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Minimizing Voltage Gap Between Cells

If one were to monitor the individual cell voltages thru the " TAPs" as the LIPO pack charges, U'll see a smaller MV ( millivolt) difference at a SLOW charge rate say 100 ma 1/20C than at a FAST charge rate 1C say at 2000 ma (2 amp) for a 2100 mah 3S pack.

By keeping this slow charge rate as low as possible, the pack will charge more EVENLY but still won't be at a perfect balance of say 4.155 for each cell
as an example. Each cell will have probably a 20mv ( .020V )difference,(normal cells) not a 200mv difference ( abnormal cells )

So, there is nothing magic about " SLOW" charging, just a way to keep them rejuvenated.

My 2S 2100 packs used in a speed 300 Predator will do two sorties in an enduro combat with 6 other Combat Wings on a single SLOW charge. I 'm usually the last plane to land
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Old Jul 27, 2005, 05:02 PM
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Not controlled testing ,merely my observations. 3S LiPolys which are always charged at nominal 1C and never balanced when discharged on a CBA show a more raged (steps) discharge line. Cells which are (where) charged at 0.7C 90% of the time and 0.3C 10% of the time should a smother discharge line especially if the 0.3C charge was performed right before the CBA discharge. A series of 0.3C charges and 3C max discharges right before a CBA discharge resulted in a even smother line.

I believe that even though slow charging does not balance LiPoly cells that it is beneficial in that to me it improves the cells ability to deliver volts under load better. I now balance charge most of the time so I have no idea if any of this applies to the newer LiPoly cells.

Charles
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Old Jul 27, 2005, 05:30 PM
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RC MAN

SLOW charging does not lower the internal resistance nor does it increase the capacity

But it lets a pack charge to its full nominal capacity, prevents a premature charger shut-off.

The internal resistance of a LIPO pack is dependent on the tab length joints of each cell center tap and width of tab and also the integrity of the internal tab interface
I don't see how SLOW charging can damage the tabs with it low current, might with excessive hi-discharge currents or direct SHORT
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Old Jul 27, 2005, 07:54 PM
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Quote:
Just check your constant-current charger next time. It will have the voltage to the pack cranked up to 5.0 or even 6.0 volts per cell to maintain the charge current, since current is delta-V over R.
I guess I don't understand what you are saying here.

I sometimes monitor individual cell voltage through taps during a constant current charge, and the highest voltage I see is about 4.23 volts on one cell and that happens at the very end of the charge.
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Old Jul 27, 2005, 09:58 PM
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Lower charge rates are generally healthier for all types of cells just as lower discharge rates are generally healthier for all types of cells. However, beware of unsupported claims that slow charging produces miraculous results.
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Old Jul 28, 2005, 11:11 AM
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YEP, SLOW charging won't bring back a DEAD or DEEP discharged battery pack, say a pack that has been lost in a plane stuck in a tree for a couple of weeks.
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