Off-grid 3200w MPPT inverter/charger limits charging at around 2000w

[anti77]

Hello everyone,

My first post, so short intro: I having an electrical engineering diploma, but never worked in the domain, mostly I'm just with theoretical knowledge. Practical knowledge is just for fixing things.

Yet I've installed my first system to help a heat pump with a Sumry SP-3200, that supposed to handle MAX. PV power of 3000w, and Maximum Solar Charger current of 80A to 24V batteries.

In full sun I experience this limiting issue when solar is over 2kW:

So in full sun PV input goes up to ~2KW then drops to ~1.5kw and starts over while the battery Icon flashes once. Battery voltage goes up to 27.2V then drops to 26.6V and starts over. Bulk/Float voltages are set to 28.6V. If I interpret that correctly, the PV is providing more than 2kw but the inverter can't handle it.

Here is the load and production for a day:


Battery bank is ~7kw, seems that in 6 hours of full sun and the light load is still not able to fully charge:

The screenshots should be taken with a grain of salt, as I understood the logger just logs whatever is read at every 5 minutes, so it's not averaged.

I have 6x440w panels. Batteries are 2x12.8v lifepo4 280AH in series with balancer, it should support charging at 160A.

What is going on? Should I just buy a higher capacity inverter as the specs for this one can't be trusted or I misunderstood completely the "chinese" specs?

Is my assumption correct that If I remove 2 panels from the string, then I will get a constant charge and no limiting?

 

[sunshine_eggo]

Bulk and float should not both be set to the same value. People are misled both by ignorant builder specifications and the idea that "LFP doesn't need to be floated." In the case of a solar power system, the float voltage is still needed.

Did you fully charge both batteries to true 100% with all cells at or very near 3.50V before wiring into series?

Can you observe what the individual cells are doing when this happens?

Set bulk to 27.6V and float to 27.0V.

If bulk 27.6V can't be attained, set to 27.2V.

 

[anti77]

Thanks for your answer.

Did you fully charge both batteries to true 100% with all cells at or very near 3.50V before wiring into series?

Yes, a month ago with 14.6V individually charged them until 0A current absorbed, then parallel charged them overnight. One of the batteries acted a bit differently, so I got a balancer for free from the battery supplier that supposed to help with the charge/discharge. Checked with a voltmeter when the issue occurs, both batteries seems to be the same voltage (0.01v difference) when the issue occurs so i don't believe it's the BMS protection kicking in. Batteries are good, as I measured ~6800wh out of them in real life conditions through the inverter (from the 7168wh specified).

Can you observe what the individual cells are doing when this happens?

Unfortunately no, as the batteries are sealed, only two terminals.

Set bulk to 27.6V and float to 27.0V.

I did this now, but I have tried in the past bulk 28.6V and float at 27.0V, it acted the same. The 27.0V showing on the display when the issue starts to occur made me to set the float to 28.6V too. I'll have to wait a few days to have full sun again. Battery manufacturer specifies 14.6V (or 29.2V) charging voltage that is also the maximum.

 

[sunshine_eggo]

I did this now, but I have tried in the past bulk 28.6V and float at 27.0V, it acted the same. The 27.0V showing on the display when the issue starts to occur made me to set the float to 28.6V too. I'll have to wait a few days to have full sun again. Battery manufacturer specifies 14.6V (or 29.2V) charging voltage that is also the maximum.

14.6V is a maximum, is best if fast charges are absolutely necessary, and it requires the cells to be in perfect balance.

I recently demonstrated that 99%+ full charge can be obtained at 13.6V (27.2V @ 24V) or 3.40V/cell:

LFP full charge @ 3.4V/cell

Thought I'd try a little something on a cheap 12V I've been testing... Charge to 20A @ 13.6V (3.4V/cell), 2A tail current. When testing at cell level and dealing with testers that don't have separate sense leads, lead resistance plays a big role, and I would typically observe cells get charged...

diysolarforum.com

 

[anti77]

So if I understood it correctly, there is nothing wrong with the inverter limiting PV at 2kw but the batteries are full already at 27v?

Unfortunately the factory who sells this is not really helpful. I got messages like this inverter supports only up to 10A input from solar panels, so because my PV string is providing 13.24A is the reason the inverter getting into this limiting mode. The specs only mention max 3000w input power and 55-450V mppt range, this current limit is something new. The seller could not explain what is the 80A max charge current mentioned in the specs or the setting 02 which is again set to 80A.

I can't think of a way to increase DC voltage by 1.5 to reduce the current to test this new information.

 

[sunshine_eggo]

So if I understood it correctly, there is nothing wrong with the inverter limiting PV at 2kw but the batteries are full already at 27v?

No. I don't understand why this is a concern to you.

Unfortunately the factory who sells this is not really helpful. I got messages like this inverter supports only up to 10A input from solar panels, so because my PV string is providing 13.24A is the reason the inverter getting into this limiting mode. The specs only mention max 3000w input power and 55-450V mppt range, this current limit is something new. The seller could not explain what is the 80A max charge current mentioned in the specs or the setting 02 which is again set to 80A.

I can't think of a way to increase DC voltage by 1.5 to reduce the current to test this new information.

I think the 3000W is intended to be an upper limit on over-paneling.

Per your link, the MPPT charge limit on the 3200 is 60A, not 80A.

60A * 29V = 1740W, so realistically, you should never see over that going to the batteries.

You MIGHT be able to use > 1740W if the inverter can directly convert PV to AC output, but battery charging is limited to ~1740W @ peak voltage.

Even if it's actually 80A, that's still about 2300W max output to the batteries.

High voltage MPPT often limit how much PV current they can accept. 10A sounds low, but 15-25A is very common.

 

[anti77]

Thanks, indeed a problem in the specs:
- 60A is mentioned as BATTERY & AC CHARGER and
- SOLAR CHARGER maximum charging current mentioned as 80a,
- yet PV input current 10A is not really mentioned anywhere but the seller insists on that.

I would be happy if the inverter could charge with a (limited) constant current, but it cant as the video shows. I can charge the battery from the grid with a constant 60a with this inverter which I don't really need. Indeed manual has the setting 02 mentioning ac+solar charging current maximum of 80a total.

PV input 10a indeed is very low, as I could not find panels with such a low current at Pmax on STC conditions on the market today.

Seller even tries to convince me to buy from him the next inverter from the same line: the SP-4000 has a MPPT PV input limit current of 20a...

I've learned that I have to ask also for the MPPT charging current limit, I will try to sell this inverter and buy one that suits better my needs.

 

[sunshine_eggo]

Are you getting nearly 2000W from your array at high noon when the battery is not full, and/or loads are > 2000W?

 

[anti77]

Panels are not facing directly to the south, so the sun in perpendicular to the panels at around 13:30, that is when the issue occurs. Now I've put a DC clamp meter to the PC string positive wire to see I can catch when it does go over 10A is indeed the moment when the inverter starts to protect itself, just that I can't stay there for hours.

Here is the chart for the battery voltage for the same day as the initial post, I don't believe the battery was full.

 

[sunshine_eggo]

Sounds like that's a real limit. Uncommonly low. panels < 10A are pretty common, but not above 400W. I have 330W panels with Vmp around 8.7A.

Battery is under 27V, so it shouldn't be the limiting factor.

Is the power still cycling between 2000 and 1500W?

 

[anti77]

I managed to make the seller update the specs on their listing and photos to mention this MPPT charging limit of 10A, so others won't make the same mistake. There are 440w panels at sale now that work just under 10A and at around 50v each. As I have mines already installed, I've changed the inverter to a 4200W one and from another brand (that supports MPPT charging current of 18A or 27a - depends which specs I read).

 

[sunshine_eggo]

Always be wary of any high voltage MPPT. They almost always have a PV input current limit. Sorry you learned the hard way... and 10A is absurdly low.