I have a 8Kw inverter and 2 X 5.5Kw batteries in parallel, If I try and run any appliance over 1Kw the inverter gives a F56 low DC volt error. The 'speedometers' on the main page usually indicate I'm drawing about 2.5Kw at the time of the error occurrence, does anyone have any thoughts as to the cause please, (I have no solar panels)