MPP LV2424 with LiFePo4 battery

[humeno]

I have been investigating my issues with LV2424 shutting down after cloudy days. It seems there are a few issues going on and wanted to see if others who are using the same inverter have ran into similar.
First, the setup is completely off-grid. There are no utility tie at all. I have 4x 200W panels on the roof and the inverter is connected to 2x 100AH LiFePo4 batteries in 2S1P so that they produce 24V nominal.
LV2424 does not have LiFePo4 specific preset so I am using the User defined settings.

• CV - 28.8V (double of the 14.4V charging voltage specified by the battery)
• DC Cutoff - 24V
• Battery priority for Solar

With these settings, the inverter will discharge the batteries on cloudy day until it is completely shut down and will not restart until I pull the batteries out and charge them with a car for a few minutes.

What I am noticing is that the it appears that the batteries are not charging all the way. The charging indicator will turn solid (fully charged) when the battery voltage is reading 26.6 - 27V. At that point, according to the battery datasheet, the batteries are only at 20-25% SOC. This tells me that even though I am setting the custom charging voltage higher, the charging profile and the charge cutoff is set to what's appropriate for an AGM battery which needs to stop charging at 13.5V or so.

Is this a known limitation of this inverter? Do I need to switch to an AGM batteries (or use a different inverter)? Unfortunately, I am discovering these too late to return any of the equipment as the 30 day return period is already passed.

 

[sunshine_eggo]

Extremely common issue. Batteries as received are rarely top balanced and one or more cells will hit "full" cut-off before the others.

It's important to individually charge them to 14.4V and confirm if they are sufficiently top balanced.

Once confirmed, charge both to 14.4V in parallel and hold for two hours.

Then place in series.

If you didn't do this prior to install, the issues you are experience are very very normal.

 

[humeno]

Oh, interesting. I have not done that since the original install a few months ago. I will pull both batteries and bring them to full charge and try it out. Does this explain the system stops charging at 27V?

 

[meetyg]

Oh, interesting. I have not done that since the original install a few months ago. I will pull both batteries and bring them to full charge and try it out. Does this explain the system stops charging at 27V?

Check your float voltage setting too.
There is a common firmware bug:

MPP Solar "premature float bug"

Hey all, So I assume from searching the forum (as well as a general google search), that the so called "premature float bug" still exists with all MPP Solar units. Question: Is this as big of a deal if using LFP? Or is it more of an issue with Lead Acid? And does it apply only with solar...

diysolarforum.com

In short, it doesn't allow the charge to go up to absorption voltage, but skips right to the float voltage.

Anyways, I would agree with @sunshine_eggo that you need to make sure your batteries are well balanced first.

 

[humeno]

O.K. Conditioned the batteries by charging them all the way using separate chargers out of the solar system. OCV was up to 14V on each of them. I also changed the float voltage to 29V. I previously assumed that was irrelevant for the LiFePo4 chemistry. The highest I can set the DC cutoff was 24V so that is what it is set to. We are looking at 5 days or so of thick clouds. Let's see how this works out.

 

[sunshine_eggo]

27V float preferred. 27.6V if you're trying to address a balance issue.

 

[humeno]

Well, the system appears to have gone offline at around 48hours mark. Pretty much right on cue since the battery is 2400WH and 60W load was drawing from it. Theoretically, that will give around 40 hours of runtime but I suspect some amount of power was being supplied even on these rainy couple days. The sun is forecast to return on Friday. The test is if the system comes back online at that point.

 

[humeno]

O.K. Partial success with the new settings. When I arrived this morning, the system was shut down with the Inverter screen showing no display. The batteries were not at cutoff and were at 25.3V. Clearly, the imbalance issue is addressed and also the inverter cut off before draining the batteries to their cutoff voltage. These are both positives. However, the system still required battery disconnect to recover. As soon as I reconnected the batteries without further reconditioning or charging, the inverter booted back up and started charging the batteries from PV.
So, what settings do I need to change so that the system will recharge and recover when the sun comes up after cutoff?

 

[humeno]

Now, I think I have a verified solution path using the LV2424 so I am going to document it for posterity. To recap, the issue is that this inverter shuts down completely when the battery voltage goes low enough into cutoff. When it does, even when the sun comes out, it will not charge the battery to recover. What I did was to add a charge controller to the system so that battery is being charged through the charge controller that is independent of LV2424. I can still use the LV2424 to take 120V input from the generator when the battery is dead and it is raining. When the sun comes up, the charge controller will charge the battery independent of LV2424.
I implemented this change Saturday and it had been raining until this morning. I could tell that the whole system was off for the past two days. The sun came back late yesterday and it was mostly sunny today. This afternoon, I saw the system was back up without any intervention. So, by adding the charge controller, I saw that it can recover from dead battery autonomously.
Long term, I am thinking that I may retire the LV2424 and put a higher power split phase inverter along with another bank of batteries but that will be a project for down the road a bit. For now, it works.