Recovering after battery discharge

[humeno]

I have a DIY system that runs off MPP Solar LV2424 inverter with 2x 100AH LiFePo4 batteries in 2S1P configuration. They are fed by 4x 200W solar panels. The system is pure off-grid and there is no utility service to the shed. I am observing that once the battery is discharged, the inverter shuts down and will not start charging the batteries until the batteries are charged to some degree even when it is sunny outside. I set the inverter to battery priority and AC cutoff to 23V so that there will be reserve on cloudy days. Is there any other settings I need to change so that the inverter will charge the battery when the sun comes up after going into DC cutoff?
Appreciate in advance for any suggestions.

 

[sunshine_eggo]

If the BMS is cutting off the discharge, you're screwed. The battery has disconnected itself from the unit, and your unit needs a battery attached.

There is very little juice left below 24V. I would increase your AC cut-off to 24V - maybe higher. You need to survive the night and stay above the BMS cut off voltage to keep the battery connected to the MPP. Once sun returns, it should start charging the battery even if AC output is off.

 

[humeno]

No dice. It seems there might be something with this inverter capabilities. Will create a separate thread specifically about this inverter.

 

[RCinFLA]

Your inverter low voltage shutdown should be set to trigger before BMS does.

Inverter low voltage shutdown setting can be tricky because inverter load and battery cabling voltage drop can cause premature tripping under heavy inverter loads. Make sure you do not have too great of DC voltage drop on inverter input terminal at your maximum inverter load. You may need larger gauge battery cables if you have trouble.

The inverter uses battery power to run its charge controller and battery charger overhead power. For that inverter, overhead is about 40-50 watts. If you run batteries down to point they will not power inverter, you get into a 'Catch-22' situation on recharging from PV input to AIO inverter.

 

[sunshine_eggo]

Have you tried higher voltages? It's critical that you raise it about the BMS cut-off value.

24.8V?

 

[humeno]

Battery spec sheet states the cutoff voltage is 11V. The cutoff is set to 24V at this time so it should be at 12V for each battery. My suspicion is that even after the load is cut off at 24V, the inverter itself continues to discharge the battery until it reaches the cutoff. I am theorizing this as when I attempt to recover the system after charging the battery a bit using the car, I get a bit of spark with the inverter turned off with the power switch in the back. I also think there is a charging side issue as well (on a separate thread).

 

[sunshine_eggo]

Battery spec sheet states the cutoff voltage is 11V. The cutoff is set to 24V at this time so it should be at 12V for each battery. My suspicion is that even after the load is cut off at 24V, the inverter itself continues to discharge the battery until it reaches the cutoff. I am theorizing this as when I attempt to recover the system after charging the battery a bit using the car, I get a bit of spark with the inverter turned off with the power switch in the back. I also think there is a charging side issue as well (on a separate thread).

Battery spec sheets can't always be trusted. Reality is that a single cell hitting the cut off is more likely to engage BMS protection rather than the total voltage, and you have to deal with the potential issues of two separate 4S 12V rather than a single 8S 24V. If only one of the two batteries disconnects, the system is broken until you can intervene.

It's now clear that they're out of balance, you're not fully charging the batteries, and they're behaving imbalanced at the bottom end as well.

See my post to your other issue.

Recommend you set absorption to 27.6V and float at 27.5V.

Have you tried 24.8V, or did you simply dismiss it?

 

[Warpspeed]

Eleven volts is getting dangerously low, its the kind of voltage that encourages gradual cumulative battery sulphation over time.
Agree 12v minimum would be better, and you lose almost no capacity.

If you have grid power available, the way I have solved the problem of insufficient battery capacity on gloomy days is to run a dc switching power supply continuously off the grid set to minimum battery voltage (plus one diode drop). This draws very low grid power, only a couple of watts under zero load.

When the battery voltage falls low, the diode between the switching power supply and the battery conducts, preventing the battery voltage from falling any further. It has proven to be an excellent way of avoiding undervoltage disconnects. The switching power supply is not there to charge the battery, only to take over when the battery has given all that it can give.

 

[sunshine_eggo]

Eleven volts is getting dangerously low, its the kind of voltage that encourages gradual cumulative battery sulphation over time.
Agree 12v minimum would be better, and you lose almost no capacity.

LFP batteries.

 

[Warpspeed]

LFP batteries.

Sorry, I missed that.
But the same overall principle apples.