fixing undervoltage protection on unknown pure sine wave inverter ZXB-800WC

Hello, I'm looking for help in fixing pure sine wave inverter 230V (on case there is no model) ZXB-800WC. I believe it's a 24V volt inverter, but internal circuitry cut off battery on 10.x V and sound alarm ad 11V so it's looking like 12V inverter. But at 12V it's not possible to get 800W using built fuses, also fan's are 24V, and last sine wave inverter module it's not working at 12V only on 24V. Main IC on low voltage side (i believe there is a problem) PWM driver - KA7500B op-amp - AS324P-e1 Could someone shine light where is voltage sense for cut off? What might be wrong?

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