MPPT charging algorithm

[LucianM]

Hi all,

I have a problem understanding how my MPPT charger works.

This is my setup ( 24V off-grid system):
- two 410W solar panels
- EaSun SMH 3k inverter with MPPT charger
- home built LiFePO4 7KW battery with JK BMS

According to the manual, the charger should use a 3 stage charging algorithm


The charger is in USER mode, bulk charging voltage is set to 27.4 V, float voltage to 26.5 V.
Also there is a small DC load connected to the battery, 15 to 100W. (battery heater, surveillance cameras )

However, this is what I see in a sunny day when the solar power is relatively constant:

1. Charger works normally in bulk mode, until the bulk charging voltage of 27.4 V is reached. At this point the battery is charging with more than 500W.

2. When 27.4 V is reached, the charger stops charging completely instead of switching to constant voltage mode.
SOC is about 98%, the battery can take more charging.

3. After 15-30 min battery voltage drops to ~26.7 V (above float voltage), then the charging resumes but this time pushing 10-50W instead of 500W.
It continues like this for the rest of the day, even though the panels can deliver 500-700W.

I don't understand why does it stop completely when it reaches 27.4V and why it's resuming charging with limited power.
When it resumes charging it's not using constant voltage of 27.4 V, nor it's in float mode because float starts at 26.5 V.
Instead it seems to aim for a small 0.4 A current, gradually increasing over a few hours to max 2 A, then gradually decreasing until the battery gets full.
Could somebody explain this?

 

[shvm]

When 27.4 V is reached, the charger stops charging completely instead of switching to constant voltage mode.

Check BMS logs to see what it says around this time. Probable Cell OVP being triggered
Ignore this, I was wrong.

After 15-30 min battery voltage drops to ~26.7 V (above float voltage), then the charging resumes but this time pushing 10-50W instead of 500W

It's not above float voltage. There is a small discrepancy between what the MPPT controller senses and what the BMS shows.
This discrepancy should also be present in reported values of current by BMS vs Charger when in float.

 

[BentleyJ]

The behavior you are describing is normal, although without the Absorption part of the cycle. Once the battery is near 100% its own internal voltage will resist current flow and the amperage will mostly all be going to service the loads with almost nothing going to battery. At 26.5V that is only 3.31V per cell.
The inverter cannot hold the exact float voltage setting, there is some hysteresis in the system by design to prevent it from rapid cycling at the set point.

As a test, I would increase Bulk to 28.0V and Float to 26.9V. With a higher bulk voltage the charge controller should switch over to Absorption and finish the job. When eventually switching to Float there will be a brief OFF time while the batt voltage falls to the set point.

 

[shvm]

When eventually switching to Float there will be a brief OFF time while the batt voltage falls to the set point.

This charger will just discharge the battery using available load to bring it down to float voltage.

 

[BentleyJ]

Check BMS logs to see what it says around this time. Probable Cell OVP being triggered.

Maybe, but I wouldn't say probable. The cell voltages are too low (below 3.4V) to determine with absolute certaintly if there is an out of balance condition but the screen shot posted indicates the cells are within 9mV of each other.

 

[shvm]

Maybe, but I wouldn't say probable. The cell voltages are too low (below 3.4V) to determine with absolute certaintly if there is an out of balance condition but the screen shot posted indicates the cells are within 9mV of each other.

My bad. I don't how to explain it but I kept seeing that 3.362 V as 3.62 V in my head
I need some rest .

But thanks to you, I now have a more plausible explanation.
27.4 is 3.425 V/Cell. For LFP , this will pretty much fully charge the battery.

When it resumes charging it's not using constant voltage of 27.4 V, nor it's in float mode because float starts at 26.5 V.
Instead it seems to aim for a small 0.4 A current, gradually increasing over a few hours to max 2 A, then gradually decreasing until the battery gets full.

And then balancing happens, (means OP has 2A balancer) until they're balanced.

 

[shvm]

Adding to this, OP you're gucci. No need to change anything.

 

[LucianM]

Check BMS logs to see what it says around this time. Probable Cell OVP being triggered
Ignore this, I was wrong.

It's not above float voltage. There is a small discrepancy between what the MPPT controller senses and what the BMS shows.
This discrepancy should also be present in reported values of current by BMS vs Charger when in float.

I don't think it's float. There is a discrepancy but the other way, at 26.5 on MPPT the BMS shows 26.30 something. So on 26.7 on BMS the charger actually shows 26.9 V.