Strange PV charger behavior of Must PV18-5248 PRO in off-grid mode

[mario947]

Hello,

Newbie here. I've just got my first solar system setup recently powering my house during blackouts.
My setup is: Must PV18-5248PRO inverter (5.2kW, 48v) with Must LP16-48100 battery (16s, 48v, 100AH) and 6 x 550W solar panels.
Inverter is set to SUB (Solar-Utility-Battery) mode [program 01].

I followed the manual of the inverter and battery to configure all other params but I think I do not understand it all right.

Let me explain the issue I see there.

Sunny day, battery is fully charged. Utility power is absent. Entire house is powered by PV energy (about 400W) and that is indicated on the screen of the inverter.
But when I switch an electric water heater on (1.2kW) I see next behavior:
- 400W is still provided by the PV and additional 1.2kW is pulled from the battery (I guess this is expected as PV can not "instantly" provide that much power)
- in a few seconds PV power starts to increase
- battery discharges by a fraction during that time
- PV power grows up to 2.8kW powering entire load and starts to charge the battery in the same time (1.6kW to the loads and 1.2kW into the battery)
- in several seconds battery is full again so charging stops (it seems like BMS cuts it off as I even hear the click in the battery)
- PV power drops to 0 instantly so battery powers entire load
- PV power starts to grow up from 0 to 2.8kW
and the cycle repeats again and again and again

I do not know if it is ok that it tries to pump 100% charge into the battery all the time and I'm not able to find any related options in the setup to stop doing it.

Does anybody have any thoughts on this? What did I configure wrong?

Thanks a lot for any help

 

[meetyg]

First, do you have such a setting?


If you do, make sure it's enabled.

Secondly, make sure that your battery charging parameters are set correctly for you battery.
You do not want the BMS to cutoff, so lower your charging voltages appropriately.

It sounds like you have a contactor BMS, which as far as I understand will cutoff both charging and discharging when an overvoltage occurs in the battery pack.

You also might want to lower your charging current. Set it to no more than 0.5C (meaning half of the battery's amp hour rating).

Lastly, you might want to play with setting "Solar supply priority":


If your not using utility at all, I think it would be better to run in SBU mode, rather SUB.
This should blend solar and battery power in a more logical manner.