On 23 April 2018 at 18:31, V G <***@veegee.org> wrote:
> I wouldn't float charge Li-ion cells.
Nor I, at anything close to normal max terminal voltage.
At lower voltages it c an be "safe" for most values of "can be".
Safe voltage is immensely dependent on more factors than most (certainly
including me) would have dreamed of.
Floating at 4.2V will destroy most LiIon cells reasonably rapidly and risks
plating out metallic Lithium with attendant risk of classic "vent with
flames" version of magic smoke making.
Floating "well below" this range may be safe. 4.0V seems likely to be 'well
below' in m,ost cases.
This paper - "Understanding electrochemical potentials of cathode materials
in rechargeable batteries" may reward the suitably entgusiastic reader but
I offer it mainly to show how much there appears to be to know to do other
than cite rote knowledge and whatever may have been gained by personal
experience.
available via here:
https://www.sciencedirect.com/science/article/pii/S1369702115003181
and possibly directly as a PDF *here
<https://ac.els-cdn.com/S1369702115003181/1-s2.0-S1369702115003181-main.pdf?_tid=fa5da7a3-960e-4ad4-bd45-dbc123adbc86&acdnat=1524469745_a13a4719b6d297279359e4dc38e4ac4d>*
_____________
The best algorithm is voltage limited
> constant current, and terminate charge when current drops to C/10 or so.
> Don't go over 4.200V at any point. Lower termination voltage increases
> cycle life and reduces stress on the cell.
>
'Best' depends on objectives, but that's closish to common "road warrior"
spec.
CV to 4.2V is standard.
CC to C/10 provides nominal capacity or slightly above at the cost of much
lower cycle life than can be achieved with HIGHEr termination CC rates.
Terminating the CC tail at C/2 or C/4 will give almost full capacity and
useful increases in whole of life mAh returned.
_________________
All tables below are from cited Battery University pages:
See
> http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
> for more information.
>
> Yes.
Their table 2 usefully illustrates some of the above.
At a cutoff voltage of 4.0V you get ~~75% capacity initially, increasing to
80% at saturation. In this case "floating" will not fully transfer
available Li ions.
At 4.00V they say capacity is 80%. Charging at C/1 80% is reached in 80% x
60 minutes * = 48 minutes.
Saturation is achieved in a total (they say ) of 150 minutes - or about 100
minutes to add the last 20%.
* I assume that at C/1 rate you'd get 100% capacity in an hour so 80% would
take 80% of an hour = 48 minutes.
LiIon *current* efficiency in charging is typically > 99% so this
assumption is very close to true.
Note that energy charge efficiency is less than current efficiency.
*Charge V/cell*
*Capacity atcut-off voltage*
*Charge time*
*Capacity with full saturation*
*3.80*
*3.90*
*4.00*
*4.10*
*4.20*
60%
70%
75%
80%
85%
120 min
135 min
150 min
165 min
180 min
~65%
~75%
~80%
~90%
100%
Table 2 from
http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries
____
See also
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
From which table 2:
*Depth of discharge*
*Discharge cycles*
(NMC / LiPO4)
*Table 2: Cycle life as a function ofdepth of discharge.* *A partial
discharge reduces stress and prolongs battery life, so does a partial
charge. Elevated temperature and high currents also affect cycle life.
*Note: *100% DoD is a full cycle; 10% is very brief. Cycling in
mid-state-of-charge would have best longevity.
100% DoD ~300 / 600
80% DoD ~400 / 900
60% DoD ~600 / 1,500
40% DoD ~1,500 / 3,000
20% DoD ~1,500 / 9,000
10% DoD ~10,000 / 15,000
and, capacity loss with temperature:
*Temperature*
*40% charge*
*100% charge*
*Table 3: Estimated recoverable capacity when storing Li-ion for one year
at various temperatures. *Elevated temperature hastens permanent capacity
loss. Not all Li-ion systems behave the same.
0°C 98% (after 1 year) 94% (after 1 year)
25°C 96% (after 1 year) 80% (after 1 year)
40°C 85% (after 1 year) 65% (after 1 year)
60°C 75% (after 1 year) 60%
(after 3 months)
and similar figures to 21st table above but not identical
*Charge level *(V/cell)
*Discharge cycles*
*Available stored energy*
*Table 4: Discharge cycles and capacity as a function of charge voltage
limit. *Every 0.10V drop below 4.20V/cell doubles the cycle but holds less
capacity. Raising the voltage above 4.20V/cell would shorten the life. The
readings reflect regular Li-ion charging to 4.20V/cell.
*Guideline:* Every 70mV drop in charge voltage lowers the usable capacity
by about 10%.
*Note: *Partial charging negates the benefit of Li-ion in terms of high
specific energy.
[4.30] [150–250] [110–115%]
4.25 200–350 105–110%
*4.20* *300–500* *100%*
4.15 400–700 90–95%
4.10 600–1,000 85–90%
4.05 850–1,500 80–85%
4.00 1,200–2,000 70–75%
3.90 2,400–4,000 60–65%
3.80 See note 35–40%
3.70 See note 30% and less
_____________________
Also, don't build your own charger, you can find Li-ion chargers anywhere
> for a few dollars.
>
> Generally yes.
But there can be exceptions .
Note that the OP specifically wanted a non-standard voltage .
Those wanting long cycle life may wish to do CCCV charging at say C/1, 4.0V.
Or CC at C/1 and float at 4.0V.
Or ...
And may want to limit vVmin to eg 3.2V to get even more cycle capacity.
Or just CC to 4.2V and no CV tail.
Those doing solar charging in systems that may drop the available voltage
on occasions may wish to make special decisions about when or if to restart
charging if Vcell is 4.2V when voltage becomes available again after a
shadow. The cell MAY have been fully charged or may have been in a CV tail.
In the former case, repeated shadowing would result in charging restarting
each time the sun reappeared. This could lead to the equivalent of 4.2v
floating and raoid cell degradation.
This could be avoided by either a :supervisor) (usually microcontroller)
which kept track of battery state, or a system that never restarts charging
of a 4.2V cell on the same day once charging is terminated (except,
perhaps, when load has been applied).
ie custom algorithms may not be well handled by std ICs.
*A warning*: [A popular USUALLY competent US IC maker] make pseudo MPPT
solar chargers aimed at simplifying solar battery chargers.
A friend designed equipment based on these and had major problems due to
failure of the pseudo MPPT system to handle panel temperature effects well,
and issues with the IC's ability to operate consistently long term. Neither
of these is tightlt connected to the above butthey relate well enough to be
worth noting.
Russell
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