Add battery back-up power option to existing grid-tied PV and solar systems: Page 7 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.
generator features, the result is a system that achieves higher performance at lower effective cost.

Notable features of the OutBack GSLC175-AC-120/240 AC-Coupling solution include:

  • UL-1741 end-to-end—when used with an OutBack battery rack, the entire system is specifically for this application, ensuring fully-compliant operation.
  • Split-Phase design—more easily integrated into standard household wiring without costly, inefficient transformers
  • Dynamic Stability—more stable output provides a cleaner signal to the system’s grid-tied (GT) inverter during load spikes and variations, ensuring it remains on-line producing electricity.
  • Universal design—works well with other brands and models of GT inverters.

Grid instability, extreme weather and earthquakes are all factors to consider, and geographic and seasonal vulnerabilities will affect the choice of which loads are most critical. Following are a few guidelines on how to size the system and interact with the two types of inverter systems.

Guideline Number One: The daily critical load watt-hours shouldn’t exceed 80% of the watt-hours available from the battery bank. An off-grid system is typically designed to discharge the batteries no more than 50% per day to extend the life of the batteries. However, this assumes that the backup system will only be used a few days or perhaps a week or two per year, so discharging batteries to an 80% depth will not significantly reduce their life below what is considered the normal life cycle for a battery. Cycle-testing of OutBack’s Energy Cell batteries has shown that 600 cycles are possible for 80% depth of discharge (DOD) on sealed AGM batteries, which translates to 600 days of backup power if sized to provide a day’s worth of power.
 
The rate or speed of which the batteries are both charged and discharged will affect their overall capacity. The slower the rate of charge or discharge, the more capacity in the battery. The following table shows the typical capacity for one to six strings of batteries. One would take 80% of these numbers to

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