Add battery back-up power option to existing grid-tied PV and solar systems: Page 8 of 14

August 05, 2013 //By Mark Mays
Add battery back-up power option to existing grid-tied PV and solar systems
Mark Mays of OutBack Power explains how to add a battery back-up power option to existing grid-tied PV and solar systems.
estimate how much power is available to the loads for a 24-hour period. This is reflected on the 12-hour discharge rate in the next sizing table.

Table 1 – Power for various rates of discharge on the OutBack Energy Cell 200RE AGM battery.


Under normal operating conditions when the grid is present, an OutBack BB inverter/charger will keep the batteries in a “float” charge which is a small charging rate that replaces the energy lost inside the battery due to self-discharge. However, when the grid is lost, the inverter is no longer in control of the charging current going into the battery. The BB inverter is in the “invert” mode providing AC power to the GT inverter so it can stay online and provide energy to the critical loads. Any remaining power not needed by the loads will flow back through the bidirectional H-bridge circuit of the BB inverter into the batteries in an unregulated charge.
In most cases the power generated by the PV array that is passed through the GT inverter goes to both the loads and the unregulated charge onto the batteries, and, if the system is sized properly, the unregulated charge onto the battery bank will not exceed the maximum charge rate. However, in a worst-case scenario where the critical loads are all or mostly off, the current from the GT inverter should not exceed the maximum charge rate onto the battery bank. OutBack’s remote-operated circuit breaker (ROCB) design can easily take the GT inverter offline when the batteries are detected as being full.

The following table shows the maximum PV power per string of battery so the maximum charge rate is not exceeded, as well as the associated available power for a given 24 hour period. This system sizing should keep the system in balance depending on the combination of solar radiation available to the array and how much power is needed

Design category: 

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