I would like solar to run the always present load and charge the battery until it is full, then stop charging and run the load only. When sunlight levels decline, the battery should gradually take over, and then the cycle should repeat when sunlight levels become sufficient again.
I have selected the “Smart Lithium (LiFePO4)” battery preset and then adjusted both the “Absorption Voltage” and “Float Voltage” to 14.6V to match the battery cells.
With full sun and loads up to around 70W, the system performs as expected, but if the load is increased to 80W, the charger and battery current appears to fluctuate, with the the battery’s CHG FET turning on and off repeatedly.
Are there settings I could adjust to reduce or eliminate these fluctuations? While the battery remains full, the FETs are cycled repeatedly, increasing potential for wear. I have also noticed flickering of lighting attached to inverter output - perhaps this is due to voltage ringing at the input.
That might be a bit of an issue. One could argue that 14.6V absorption is a bit high already, but for sure also using 14.6V for float is too high.
Thats the BMS trying to protect the cells. Very likely due to overvoltage, but if your BMS has bluetooth (or some other communication) you could check the individual cell voltages
I think your voltage is set too high. The BMS is protecting one of the cells, and probably the overvoltage limit is set to 3.60 or 3.65. When one of those cells reaches this voltage, the BMS stops charging because of this.
It might be better to use lower voltages. There is absolutely no reason to charge with these high voltages. Try 13.80; a battery will get fully charged with 13.80V too, if you use the right absorption. (tailcurrent at around 0.05C or even 0.033C).
The cells in this pack are designed for charging to 3650mV and I have configured over voltage protection for 4300mV, so this charging voltage won’t cause the BMS to prevent charging.
The BMS has an internal taper (tail) current sense that determines when the battery is fully charged and this is the trigger for switching off the CHG FET.
At this point, I’d like the charger to continue delivering current to the load, which is does for loads up to around 70W, but if I increase the load, it appears that the battery now supplements the charger current. When the BMS detects current draw from the battery, it turns on the CHG FET to eliminate the voltage drop and heat dissipation that would otherwise occur when relying on the CHG FET’s body diode to handle this current (back to back N-Channel FETs).
However, it appears that the voltage then fluctuates and current flows into the battery and the BMS switches the CHG FET off again, before the cycle repeats.
Is there a defined current limit for the Float Charge mode?
Battery bms is a safety device and should not be used as an on/off charge controler.
Think of the battery as a buffer,.
if load is less than pv - energy will charge the battery up to mppt charge settings( these should be lower than bms high voltage settings).
If load is greater than pv - battery will provide deficiet.
I would drop overvoltage protection to battery bms max voltage, presumably 3.65v? And mppt to say 3.5v and then see if any overvoltage disconnections occure, if none occure then if wanted increase mppt charge voltage step by step.
If you look at soc charge curve, you will see the last bit of voltage increase 3.35-3.65v gives very little extra stored energy
Unknown your battery, but if lifpo4, 4.2v overvoltage is dangerously high.
@jezza
with 4s LFP I recommend:
3,60V | 14,4V OV protect
3,45V | 13,8V bulk/absorbtion
3,40V | 13,6V float
I use this cell voltages on my 16s packs.
Wiht a 4A active blancer, these values ensure single cells do not get overloaded and BMS never has to protect and open charge MOSFETs.
From 3,45 to 3,60V you are not gaining much energy on LFP’s SOC-S-curve. Just drive them between 3,00V und 3,45V.
3,65V is the maximum voltage for LFP.
4,20V is the maximum voltage for Li-Ion not LFP.
