These are notes on modifications to make to the IBM branded 2RU APC SmartUPS 3000VA UPS to make it operate as a hybrid solar inverter/ups.
Disable the charging circuit
Since I am charging from solar I do not want the UPS to be able to charge the externally attached battery array, on this model this is very simply done by inserting a diode on the RED and BLACK wires coming from the front of the ups (charging module) to J4 on the main PCB to prevent reverse current flow to the batteries. The diode is required or the charging module wont be powerered at startup which puts the unit into a fault mode.
Coltrolling the output
Initially I was using a solid state relay to disconnect the feed in to the UPS when I was getting enough power to charge the batteries and run my equipment from solar, however this has been causing issues with RCDs (residual current devices) due to the massive inrush current when the UPS is re-connected to the supply. As such I have reverse engineered enough of the circuit to understa