Paulvdm Posted November 7, 2019 Share Posted November 7, 2019 Hi I have installed 2 Axpert MKSII 5-48 inverters and have set them up to run in parallel. Each inverter has a single string of 6x 330W Canadian solar PV panels (CS6U-P) connected in series with a VOC of 45.6V per PV panel so a total of 12 panels. Further to this I have a battery bank of 4x Narada 48NPFC50 Lithium ion polymer 50Ah units also connected in parallel as battery backup for night time use so a total of 200Ah. I generally do not have the utility connected to the system so the system runs off the grid. The problem I have comes from the charging of the batteries which takes place during the day via the inverters. The charging voltage is set up at 51.6V the float charging voltage is set up at 51.5V the charging current is set to 40A the low DC cut-off voltage is set to 44V. Due to the fact that my day time load on the system is very small the batteries charge at a rapid rate typically fully charged within about 2 hours of full sun on the Panels and this is where the problem begins to emanate so even though the inverters are set up to a charge voltage of 51.6V and float of 51.5V the charge voltage runs up erratically to anywhere between 60 and 69V in so causing the inverters to get hot as they are trying I think to dissipate watts from this extra high charge voltage. I cannot think that this is a healthy situation for the batteries either. Surely the inverters should not allow this set charge voltage to run away to such a high voltage just because there is a small daytime load on the inverters, if I increase the load on the system during this anomaly say boil the kettle (3000W) then the problem disappears and the battery charge voltage reverts to its set level of 51.6V. Can someone possibly shed some light on this issue. Thanks Paul Quote Link to comment Share on other sites More sharing options...
Jaco De Jongh Posted November 7, 2019 Share Posted November 7, 2019 21 hours ago, Paulvdm said: Due to the fact that my day time load on the system is very small the batteries charge at a rapid rate typically fully charged within about 2 hours of full sun on the Panels and this is where the problem begins to emanate so even though the inverters are set up to a charge voltage of 51.6V and float of 51.5V the charge voltage runs up erratically to anywhere between 60 and 69V in so causing the inverters to get hot as they are trying I think to dissipate watts from this extra high charge voltage. I cannot think that this is a healthy situation for the batteries either. I have seen something similar on an Infinisolar once, the inverter did not see the battery, so kept on increasing its charger volts to almost 70 volts. When the inverter does this again, please see if the battery indication is still present on the display. What is the chances that your batteries switches themselves off when full, that will send the inverter into a mode where it keeps on increasing the voltage in the hope to lift the battery volts. Once you draw a load, the batteries "wakes up" and the charger settles on the battery volts again. I dont think I was clear in the above part, so let me rephrase. Normally there is some kind of communication between the inverter and the Lithium batteries. The lithium battery will tell the Inverter/Charger that its at 100 percent and the Inverter/Charger will cut back or stop charging. If you dont have communication like this, the inverter will continue to charge the batteries. Any BMS would then try and protect the battery from over Charging and switch the battery off (Or that at least is how i understand the process). I suspect this is what is happening in your case. The moment the batteries switches off, the inverter doesn't see it anymore and increases the volts to try and "Lift" the battery volts. Two Questions: Do you have communication between the inverter and your Lithiums? How quickly does the voltage climb from 51 to 69V. If its very quick, It will confirm that the batteries are not connected anymore, I know this is a long shot, but please check that battery indication when it happens again.. Coulomb 1 Quote Link to comment Share on other sites More sharing options...
Jaco De Jongh Posted November 8, 2019 Share Posted November 8, 2019 21 hours ago, Paulvdm said: 2 Axpert MKSII 5-48 inverters and have set them up to run in parallel Here is another guess. If i look at the data sheet for your battery, the recommended charge current is only 10 amps per battery, so that will allow 40 amps for the 4 of them. The 2 inverters combined are capable of charging at a lot higher current than that. Too high charging current may also cause the BMS to shut down. Do you maybe have historical data to shed more light on charging current as well as battery voltage right before this "Over Voltage" condition. Quote Link to comment Share on other sites More sharing options...
Wilfred Posted November 8, 2019 Share Posted November 8, 2019 On 2019/11/07 at 4:07 PM, Paulvdm said: current is set to 40A the That is from each inverter = 80A if I am not mistaken in total. Meaning each inverter push 40A into the batteries. Quote Link to comment Share on other sites More sharing options...
Paulvdm Posted November 10, 2019 Author Share Posted November 10, 2019 Hi Jaco and Wilfred Thanks for your input to my question. I wanted to monitor the situation again during this weekend but due to overcast weather and more load on my system I did not have any hi voltage or overcharging occurring again so i will just have to wait till i get a nice full sunny day again. Wilfred interesting point you make about the 40A charging current possibly becoming 80A as the inverters are running in parallel, I will look into that. Jaco the answer to your question on communication between Inverters and batteries is simply No there is no direct communication between battery and inverter so the battery BMS does it's own thing. With a very small load especially at night sometimes up to 3 of the 4 batteries go to sleep and turn themselves off and as you know as soon as a fair load gets drawn from the inverters the batteries switch themselves back on again. I think a possible solution to my problem will be to reduce the charging current to the batteries so that they don't charge so fast and create a more substantial load on the inverters output. My problem generally occurs when the PV's are receiving midday direct overhead sun. Quote Link to comment Share on other sites More sharing options...
GVC Posted November 10, 2019 Share Posted November 10, 2019 I had the same problem with my Axpert 5kva overcharging my LiFePO4 batteries. I installed a BMV702 battery monitor which sorted out the problem. Quote Link to comment Share on other sites More sharing options...
birdibird Posted November 14, 2019 Share Posted November 14, 2019 Try to flash the new firmware 71.71 posted here: https://powerforum.co.za/topic/4288-axpert-inverter-switching-to-battery-power-at-midday/?do=findComment&comment=62685 Quote Link to comment Share on other sites More sharing options...
Paulvdm Posted November 15, 2019 Author Share Posted November 15, 2019 Thanks B i will look into a firmware update Quote Link to comment Share on other sites More sharing options...
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