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Float Charging Lithium-Ion Batteries at Lower Voltages
BravoV:
--- Quote from: Siwastaja on April 26, 2019, 04:43:58 pm ---But, contrary to intuition and common belief, over about 80%, strange things happen. The cells I have tested show very little difference at all (i.e., 80% and 100% are equally bad), and I have read a paper in which a Panasonic cell (IIRC) actually showed better calendar life at 100% compared to at 80%. I really don't know the mechanism behind this (haven't looked at it). Do note that this is based on actual production chemistries available now (visible on both LCO and NCA), but is not a fundamental physical law, so if you read this post after years, things may be different.
Now, between 80-100%, charging current does more damage (than at, say, between 60-80%), so if a lot of cycling happens, then limiting yourself to 80% will be a benefit. But for storage, or very low-ripple, low-cycling floating, 80% is not any better than 100%. So you can as well use the near-full capacity; or go even lower to really increase the life.
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Interested at above note, mind elaborate which "exact" brand and type are you talking about ?
How about current popular 18650, say like Panasonic NCR18650GA, as I used these intensively.
Siwastaja:
--- Quote from: BravoV on April 27, 2019, 06:26:20 am ---Interested at above note, mind elaborate which "exact" brand and type are you talking about ?
How about current popular 18650, say like Panasonic NCR18650GA, as I used these intensively.
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Talking about basically all available cells from big brand manufacturers such as Panasonic, Samsung, LG, Sony; they seem to perform in a somewhat similar way. They are of LCO (now starting to get obsolete), NCA or NMC chemistry. None of these exhibit strong calendar life increases when going from 100% to 80%, as expected by some people.
My point is, there is no strong benefit in calendar lifetime between 80-100%, in any cell I have measured or read about, only relatively small and uncertain differences to either direction. Actual calendar life benefits start way below 80%, so if you need long calendar life, keep it low enough.
SilverSolder:
Poking around in an older hybrid car, it was noticeable that the control system keeps the battery somewhere in the range of 35% to 55% charged under all circumstances. (NiMH on that vehicle).
The 13 year old vehicle had >200,000 miles on it and the battery still performed like new...
thm_w:
--- Quote from: MadScientist on April 24, 2019, 11:54:49 pm ---...
Li doesn’t normally actually need float charging as the self discharge rate are very low so I’m not sure why you think you need float charging in the first place
If the application is long term standby , then Li is a poor choice because of my first paragraph
Low value CC charging subjects the Li to multiple micro charging cycles and studies have shown that any charging cycle in regards overall degradation. Is nearly as bad as a more complete charge. , so most li wait for a significant discharge point to be reached , then recharge and switch off
I’d examine the technical reasons why you think you need float charging in the first place
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Do you have citation for this?
Sure any charging cycle matters for degradation, but a 10% dicharge and 10% charge ten times is better than a full 100% discharge and charge, in terms of cell wear. So you are telling me there is some point between 0% and 10% charge where the behavior reverses, and wear increases. Its possible, I'm just curious where that would be?
This study used various pulsed charges, and found almost no difference in capacity: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.876.2485&rep=rep1&type=pdf
But maybe the frequency is too low.
http://mail.lancaironline.net:81/Lists/lml/Message/56976-02-B/Li-Ion%20Battery%20Life.pdf
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