[kennybobby]

In the CV stage, when the cell voltage reaches, or nearly reaches, the set point CV voltage, then there is very little to no voltage difference left between the charger output terminal and the cell terminal to force current into the cells.

So the current level drops naturally in a somewhat exponential fashion--as electrons are absorbed the cell voltage increases, which reduces the forcing function, so fewer electrons flow.

The cell vendors have prescribed a charging procedure whereby if you follow it, you will fill but (hopefully) not overcharge or damage your cells, and you would expect to get the advertised number of cycles. To extend life some folks use a more conservative approach with lower limits than the advertised procedure--this is the current control to which he says can be done, but is not absolutely necessary.

You, or your BMS, must be diligent to ensure that the limits are never exceeded. So if you hold a CV point then the current should be monitored to shut off the charger before or at the current limit.

There is little to no benefit in trying to absolutely fill the cells to the max. Leave yourself some margin at the top and the bottom, enjoy peace of mind and longer lasting cell life.

 

[kennybobby]

... If I ask the BMS right now, sitting in the garage on a fully charged pack, what is the charge limit, it tells me 53A. That's nowhere near the tail of any exponential curve! If I plugged the charger in right now however, the BMS wouldn't let it charge because the cell voltage is too high.

That's THE GOOD THING™ Your pack is already full and you shouldn't try to charge it--Better to shut off too early than too late. Better safe than sorry, right?

What is the cell voltage to which you are charging?

i have seen datasheets that indicate absorption phase (CV) should continue until the current drops to 1/20 or 1/50 or even 1/100 C. But i know folks have lost cells because that limit was never hit and the charger never cut off.

From cell testing and reviewing literature from vendors i consider the full voltage of LiFePO4 cells to be 3.333 volts. If you charge to 3.6 vpc with 1/50C, then disconnect the charger and come back the next day, generally the cell voltage will have dropped down to ~3.3 to 3.4 vpc. There is just so very little energy to be gained by holding 3.6 versus just shutting off during the bulk CC phase when it hits 3.4.

With 90AHr cells, someone has set your charger cutoff limit to a conservative 53A, probably Davide did this since he knows the dangers of overcharging. He just cut the long tail off the exponential to save wear and tear on your pack. So this leads back to the question, what's your CV setting?

Actually what kind of charger are you using and what is the max current that it can do?

 

[kennybobby]

Until you get a datasheet for those cells and that chemistry to know what the real story is, i would suggest not to let the Elcon charger supply the higher voltage "226V". Start lower and work your way up as you gain confidence that the voltage and current values are "calibrated" correctly.

Use some good meters that you trust and check the charger voltage and current--trust but verify, since any little errors or offsets in their box can wreak havoc in your cell$ and pack. e.g. The advertised "226" may be the 20A-loaded value, which could be higher with only an 8A load.

Your BMS current values are obviously way too high since the charger can only do 20A max, but even that is current limited by the mains supply thru the EVSE. So the EVSE is your limiting supply factor.

The BMS voltage limits seem reasonable assuming the chemistry values are what you said.