On 31 May 2013 00:38, RussellMc wrote: > Have TI gone utterly & irresponsibly mad or is this a fantastic new way t= o > charge Lithium Ferro Phosphate cells? > Any real world experiences OR general comments on this would be valued. > (Very long - this is essentially my musings as I investigate the subject > and discover various useful things. ) > > TI claim that you can charge LiFePO4 cells by CC charging to a higher tha= n > usual voltage (eg 3.7V rather than the usual 3.6V for LiFePO4) and then > step transitioning to a lower float voltage with NO intermediate CV mode. > It SEEMS logical that this might apply to LiIon as well but TI offer no > ICs for LiIon that work in this way. Note that the post charge phase > float voltage is VERY low , indicating a strong attempt to keep well clea= r > of the Lithium plating issues found at higher float voltages. > > This goes against **ALL** other advice, IC specs and charger circuits > that I have seen. > > > Doing this with Vcv <=3D 3.6V is fine enough - with or without a CV stag= e. > It's the extra voltage and no CV mode that is radical. > The implication or statement from all other sources is that exceeding the > normal Vmax of 4.2V for LiIon or 3.6V for LiFePO4 by even a small amount > is liable to be damaging or fatal. > > TI have a number of charger IC's for LiIon with similar specs, pinoutsand= target uses. They only have a few that are suitable for LiFePO4. NONE > of the LiIon / LiPo specific chargers uses this method. They may be > depending on the Olivine matrix in LiFePO4 that gives it is ruggedness an= d > decreases energy densities, to provide enough protection against the > excesses of this method. > > > The usual Lithium Chemistry charging method is to charge at CC (constant > current) until Vmax is reached and then to hold the cell at Vmax while > the current ramps down in a non linear fashion under cell-chemistry > control until some target %age of Imax is reached. > > The TI method claims > - 100% charge at 1 hour > - compared to 85% at 4.2V > - a gain in 15% of total battery capacity > - or about 18% more capacity relative to 4.2V (100/85% =3D~1.18) > > Does it produce 100% in one hour? > - Does it damage the battery. > > See "Battery university warnings" at end. > > _______________ > > The TI "claim" is in the "hardest" form possible - not just on paper but > in the Silicon of a battery control IC. > The BQ 25070, data sheet here: http://www.ti.com/lit/ds/symlink/bq25070. > pdf > > Says in its data sheet, dated July 2011: > > - The LiFePO4 charging algorithm *removes the constant voltage mode > control usually present in Li-Ion battery* > *charge cycles*. > > - Instead, the battery is fast charged to the *overcharge* voltage and > then allowed to relax to a lower > float charge voltage threshold. > > The removal of the constant voltage control reduces charge time > significantly. > > During the charge cycle, an internal control loop monitors the ICjunction= temperature and reduces the charge > current if an internal temperature threshold is exceeded. The charger > power stage and charge current sense > functions are fully integrated. The charger function has high accuracy > current and voltage regulation loops, and > charge status display. > ________________ > > Are they mad ? > > This table is based on table 2 from battery university at http:// > batteryuniversity.com/learn/article/charging_lithium_ion_batteries > > Columns headed BU are in the original. > Columns headed RMc were added by me. > Rows for 4.3, 4.4, 4.5 V were added by me. > > Their table says that > - If you charge at constant current till Voltage Vcv is reached > - Then the % of full capacity in column 2 is reached. (% cap at end of > CC) > - And then, if you hold the voltage at Vcv until Ibat falls to about 5% > if Icc (usually 5% if C/1 =3D C/20) > - Then capacity in column 4 will be reached. (Cap full sat) > - They say total charge time in minutes is in column 3 > > My additions are not overly profound, and make a few assumptions which ma= y > be invalid. > > 5 Minutes CC: I assume that in initial CC mode capacity increases linearl= y > with time. This is probably very close to true for current capacity and a= s > in early stages Vcg is relatively constant, it is probably a n adequate > assumption for energy capacity as well. > > 6 Time in CV =3D 3 - 5. > > 7. Mean rate in CV =3D (100 - col.2) / ( (col.3 - col.5 ) / 60 ) > This is just to give me a feel for how fast the post CC mode balance need= s > to be made up. If there IS no post CC CV-mode then it needs to be zero an= d > in fact it has fallen to &% of CC rate by the time Vcv =3D 4.2V. > > While TI use 3.7V for Vovchg (as opposed to regular 3.6V) for their magic > trick, extrapolation of the table would seem to suggest that about 4.5V > would be need for a LiIon call and perhaps about 3.8V for a LiFePO4 cell.= .. > > It may be however that significant things start to happens just above 3.6= V > / 4.2V and that the extra 0.1V is all it takes to up the rate by (100 -85= ) > / 55 =3D 28% compared to the CC rate which terminates at 4.2V. > > For this to be true then 15% charge needs to occur s Vbat rises 0.1V, > this occurs in about 9 minutes (60 - col5.4.2V row entry) so delta charge > rate is 15%/ (9/60)hr =3D 15%/15% =3D 100% =3D C/1 rate - which it would = have to > be. [This "coincidence" occurs because 15% of the capacity remains to be > supplied when 15% of one hour remains.]. > > I've added TI's crash charge method to the table on the 4.3V row. > > * 1 BU* * 2 BU* * 3 BU* * 4 BU* * 5 RMc* = * > 6 RMc* * 7 RMc* > * Vcv* *% cap at end of CC* *Charge time* *Cap full sat* *Minutes C= C > * *Time in CV* *Mean rate in CV* > *3.8* *60* *120* *75* *36* *84* *28.6* > *3.9* *70* *135* *76* *42* *93* *19.4* > *4* *75* *150* *82* *45* *105* *14.3* > *4.1* *80* *165* *87* *48* *117* *10.3* > *4.2* *85* *180* *100* *51* *129* *7.0* > *RMc 4.3* *RMc : 90 > TI : 100* *174 > 60* *not used * *54 > 60 > * *120 > 0* *5 > -* *Set at 5%* *RMc 4.4* *95* *117* * > * *57* *60* *5* *Set at 5%* *RMc 4.5* *100* *60* * > * *60* *0* *5* *Set at 5%* > > > > > > > > > *Battery University warnings and comments from the above referenced page:= * > > This is fine - you "just" lose 15% of face-plate capacity of about 18% > less capacity than you could have > > Some lower-cost consumer chargers may use the simplified =93charge-and-ru= n=94 > method that charges a lithium-ion battery in one hour or less without goi= ng > to the Stage 2 saturation charge. =93Ready=94 appears when the battery re= aches > the voltage threshold at Stage 1. Since the state-of-charge (SoC) at this > point is only about 85 percent, the user may complain of short runtime, > not knowing that the charger is to blame. Many warranty batteries are bei= ng > replaced for this reason, and this phenomenon is especially common in the > cellular industry. > > > This is of more concern > > Li-ion cannot absorb overcharge, and when fully charged the charge curren= t > must be cut off. > > *A continuous trickle charge would cause plating of metallic lithium, *an= d > this could compromise safety. > > To minimise stress, keep the lithium-ion battery at the 4.20V/cell peak > voltage as short a time as possible. > > > The TI bq25070 floats the battery at 3.5V - below the range of "safe" - i= eso very safe as to slightly lose capacity with time. > > Once the charge is terminated, the battery voltage begins to drop, and > this eases the voltage stress. Over time, the open-circuit voltage will > settle to between 3.60 and 3.90V/cell. Note that a Li-ion battery that > received a fully saturated charge will keep the higher voltage longer tha= n > one that was fast-charged and terminated at the voltage threshold without= a > saturation charge. > > > > > > ______________________________ > > Related: > > bq25070 data sheet > > http://www.ti.com/lit/ds/symlink/bq25070.pdf > & http://www.ti.com/lit/ds/slusa66/slusa66.pdf > > bq20z80-V101 "Gas gauge" > > http://cs.utsource.net/goods_files/pdf/12/121917_TI_BQ20Z80DBTR.pdf > > bq25060 LiIon charger IC > > http://www.ti.com/lit/ds/symlink/bq25060.pdf > > > > > -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .