Is CV charging (adsorption) even necessary?
- Thread starter scott harris
- Start date
Johncfii
Solar Enthusiast
- Joined
- Nov 24, 2020
- Messages
- 265
About CC/CV charging ......
There seems to be some belief that a constant current stage of charging, followed by a constant voltage stage is some carefully engineered optimum way to charge a battery. This is actually not the case. CC/CV charging was widely described and adopted for charging lead acid batteries simply because that was the cheapest way to build battery chargers in the days of older technology. (And it still is cheaper than the alternative.)
In the olden days, every battery charger (of any size) was simply an iron core transformer, of suitable size, and suitable secondary voltage, followed by a simple half-wave rectifier. A more sophisticated charger could have full wave rectification (for better efficiency) and perhaps some kind of inductive filtering to smooth ripples. (Only the most expensive would have any form of output voltage regulation.). When the battery was connected to the charger, it would deliver up to its rated amperage, and the internal impedance of the transformer would cause the charger output voltage to sag. As the battery charged, battery internal impedance would increase, IR losses in the transformer would drop, less power would be dissipated as heat in the transformer, and more would be delivered to the battery, which would be manifest by slowly rising charger output voltage (more watts delivered to the battery). The output voltage would increase until the battery charged to that point where the charger terminal voltage was nearly the same as the open-circuit voltage of the charger. This is when this old-fashioned charger would enter the CV stage of charging. This CV stage continued, and current tapered, until you decided to unplug the battery from the charger, perhaps the next morning.
These days, with cheap current and voltage regulating IC’s, and inexpensive PWM IC’s, more sophisticated power supplies are affordably available. If you have a power supply of sufficient size, that is voltage AND current regulated, you can use it and disregard any “need” for two-stage charging. Regulate for the maximum voltage that you choose to apply to the battery (or any lower voltage, if you want to come back and adjust it again later), at either the maximum current your supply can deliver, or at the maximum rate you want to push into the battery. Your battery won’t know any difference as long as you don’t exceed specified voltage limits or current limits. Charging current will still taper off normally, as the battery reaches a higher state of charge.
It’s just a matter of pushing electrons into the battery without busting those safe limits. But there is good case to be made for being gentle with your expensive batteries, and setting a conservative current limit.
The BULK and ABSORPTION phases that are likely described in the manual for your SCC are delineated for exactly the same reason .... your SCC is not current regulated, other than perhaps letting you restrict the upper current limit. Current regulated would just be more expensive, and impossible in times of low sunlight.
There seems to be some belief that a constant current stage of charging, followed by a constant voltage stage is some carefully engineered optimum way to charge a battery. This is actually not the case. CC/CV charging was widely described and adopted for charging lead acid batteries simply because that was the cheapest way to build battery chargers in the days of older technology. (And it still is cheaper than the alternative.)
In the olden days, every battery charger (of any size) was simply an iron core transformer, of suitable size, and suitable secondary voltage, followed by a simple half-wave rectifier. A more sophisticated charger could have full wave rectification (for better efficiency) and perhaps some kind of inductive filtering to smooth ripples. (Only the most expensive would have any form of output voltage regulation.). When the battery was connected to the charger, it would deliver up to its rated amperage, and the internal impedance of the transformer would cause the charger output voltage to sag. As the battery charged, battery internal impedance would increase, IR losses in the transformer would drop, less power would be dissipated as heat in the transformer, and more would be delivered to the battery, which would be manifest by slowly rising charger output voltage (more watts delivered to the battery). The output voltage would increase until the battery charged to that point where the charger terminal voltage was nearly the same as the open-circuit voltage of the charger. This is when this old-fashioned charger would enter the CV stage of charging. This CV stage continued, and current tapered, until you decided to unplug the battery from the charger, perhaps the next morning.
These days, with cheap current and voltage regulating IC’s, and inexpensive PWM IC’s, more sophisticated power supplies are affordably available. If you have a power supply of sufficient size, that is voltage AND current regulated, you can use it and disregard any “need” for two-stage charging. Regulate for the maximum voltage that you choose to apply to the battery (or any lower voltage, if you want to come back and adjust it again later), at either the maximum current your supply can deliver, or at the maximum rate you want to push into the battery. Your battery won’t know any difference as long as you don’t exceed specified voltage limits or current limits. Charging current will still taper off normally, as the battery reaches a higher state of charge.
It’s just a matter of pushing electrons into the battery without busting those safe limits. But there is good case to be made for being gentle with your expensive batteries, and setting a conservative current limit.
The BULK and ABSORPTION phases that are likely described in the manual for your SCC are delineated for exactly the same reason .... your SCC is not current regulated, other than perhaps letting you restrict the upper current limit. Current regulated would just be more expensive, and impossible in times of low sunlight.
Last edited:
Yes, big bloated problems. Between 3.4v and 3.65v the big Eve 280AH cells will take less than 5AH to put in that extra .25 volts.
That is exactly what you should do to charge LiFePO4 to full. Any lower current than 0.05C @3.65V is damaging the cell.
It may be that I just missed it, but I've never heard of this for LFP. I'd agree 100% on FLA/AGM/GEL.
Based on that statement, the act of top balancing @ 3.65V and 10A is damaging all cells, e.g., 8P 280Ah cells = 2240Ah
10A/2240Ah = .0045C
trevor.the.van
New Member
Yeah, also, if I have my battery hooked up to a solar MPPT charger, and there isn't enough sun to reach a higher C rating, it will happily charge at the lower rate for what it has - that would seem problematic for ALL solar based LiFePo4 batteries. I use a Victron MPPT which is reasonable quality.
That is correct, the longer the current is applied, the greater the damage. Research the single phase lithiation reaction that occurs at saturation voltage to find out more.
Sadly, my Google-Fu is weak. Those terms and variations thereof did not yield anything that didn't require in-depth reading from links that were often seemingly not relevant.
Let me ask for some clarification.
Is the issue of low current AT the saturation voltage and not below it, i.e., you can charge at .03C or .02C or whatever below 3.65V provided you are under the "full" mark, i.e., the battery is below the SoC at which it would terminate at 3.65V and 0.05C?
I took your statement and mentally applied it to low current charging everywhere, not just at 3.65V. If you were speaking specifically at 3.65V, which it now seems you were, I would agree solely on the basis of the manufacturer's recommendation - if they say it's full at 3.65V with a tail current of 0.05C. My own tests have shown they they accept almost no input at lower tail currents. I was using a 20A hobby charger that terminates at 2A on Eve 280Ah cells. Out of curiosity, after the 2A termination, I ran it from 2A to 200mA. It took on less than 200mAh of additional capacity before the charge terminated.
Yes, low current charging when the battery is fully charged is what causes damage.
In very simple terms when a LiFePO4 cell is fully charged there is no more room in the lattice for Lithium Ions, any further pushing of current will permanently bond the Lithium, causing reduced capacity.
At a less than fully charged condition this doesn’t apply.
There are many papers on this subject online.
edit: where this causes an issue is when parallel top balancing very mismatched large cells, where the highest cells will spend significant time at saturation while the lowest cells become fully charged.
In very simple terms when a LiFePO4 cell is fully charged there is no more room in the lattice for Lithium Ions, any further pushing of current will permanently bond the Lithium, causing reduced capacity.
At a less than fully charged condition this doesn’t apply.
There are many papers on this subject online.
edit: where this causes an issue is when parallel top balancing very mismatched large cells, where the highest cells will spend significant time at saturation while the lowest cells become fully charged.
Thank you. My mistake was in applying it to lower states of charge.
sparrowhawk
Christian Prepper
So given this information it sounds like a multi step approach to top balancing with pauses in the charging process to allow time for cells to balance with each other before the final push to 3.65 volts.
mikefitz
Solar Wizard
- Joined
- May 28, 2020
- Messages
- 2,971
I have suggested on previous posts that high voltage 3.65 per cell and long absorption times are unnecessary and perhaps damaging. I have also suggested the concept of top balancing multiple cells in parallel at 3.65 V until the current falls to zero, is flawed. Even a modest understanding of how a lithium cell operates supports the thought that these accepted procedures, where a fully charged cell being 'forced fed', is damaging.
The guy from the 'off grid garage' carried out some tests on a 100Ah single cell charged at constant current values ranging from 5 to 40 amps with a charge termination at 3.5 volts with no constant voltage stage.
Mike
The guy from the 'off grid garage' carried out some tests on a 100Ah single cell charged at constant current values ranging from 5 to 40 amps with a charge termination at 3.5 volts with no constant voltage stage.
Mike
Good luck trying to inform the general population of this forum that parallel top balancing is a bad idea.
The general response is “we know it is damaging to the cells, but nobody can provide information on precisely how much damage is being done so we will assume it is insignificant”
The general response is “we know it is damaging to the cells, but nobody can provide information on precisely how much damage is being done so we will assume it is insignificant”
Or individually charge the cells, or use an active balancer on a series pack.
It is never a good idea to parallel cells of differing SOC.
RCinFLA
Solar Wizard
- Joined
- Jun 21, 2020
- Messages
- 3,560
40 amps of charge rate on a 280 ah cell is still a fairly low charge rate with low overpotential voltage (about 35 mV). With low charge current having low overpotential, the cell is fully charged when it hits 3.47v. The greater the charge current, the greater the overpotential voltage and the higher the terminal voltage must be to fully charge. With higher charge currents, eventually you reach the 3.65v electrolyte damage rate cap and have to just dwell at 3.65v until cell current drops off to top off charge.
At any charging terminal voltage above 3.45v, if there is little to no cell current flow then cell is fully charged. The magic cell voltage for LFP is 3.43v but there requires a minimum of 10-20 mV overpotential voltage to create any cell current flow, therefore the 3.45v number.
When you charge to 3.65v, at 3.65v absorb point, you begin to trade cell overpotential voltage, as charge current drops off, for surface charge most associated with anode graphite layer. This is like a supercap in series with cell. This extra surface charge voltage will remain for a few days if not discharged by a terminal load. The actual energy stored in surface charge is very low and it only takes about 30 seconds of 1-2 amp discharge to bleed off most of the surface charge on a 280 AH cell.
Keep in mind most BMS's don't enable balancing until a cell exceeds 3.4v so even though you can eventually fully charge a cell at 3.45v you will not get much balancing time in.
At any charging terminal voltage above 3.45v, if there is little to no cell current flow then cell is fully charged. The magic cell voltage for LFP is 3.43v but there requires a minimum of 10-20 mV overpotential voltage to create any cell current flow, therefore the 3.45v number.
When you charge to 3.65v, at 3.65v absorb point, you begin to trade cell overpotential voltage, as charge current drops off, for surface charge most associated with anode graphite layer. This is like a supercap in series with cell. This extra surface charge voltage will remain for a few days if not discharged by a terminal load. The actual energy stored in surface charge is very low and it only takes about 30 seconds of 1-2 amp discharge to bleed off most of the surface charge on a 280 AH cell.
Keep in mind most BMS's don't enable balancing until a cell exceeds 3.4v so even though you can eventually fully charge a cell at 3.45v you will not get much balancing time in.
You can also look at it as:
The single push to 3.65v gives the least amount of time the cells are fully saturated.
I have tested extensively, and 3.45v is indeed the magic number. If you charge to 3.45v, and allow current to taper, you are over 99% SOC. Also, from a capacity standpoint, discharging below 3.0v is not useful, and it causes a significant temperature rise in the cell.
I have played with charge and discharge current voltage and amps, over 100 combinations with both 105ah and 280ah EVE cells.
Voltage is a very poor SOC indicator unless you allow cells to settle for 24 hours.
I have been convinced that an active balancer is very useful, but really only to top balance cells, you should disconnect it during normal use.
Similar threads
