Application Note
©2014 OutBack Power Technologies, Arlington, WA 98223 Revision 2/FINAL Page 8 of 14
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 based on load demand. Since the relationship between available PV power and load demand is highly
unpredictable, on sunny days the battery bank may get charged quickly and even turn off the GT inverter while
the sun is still shining; it does so to prevent the batteries from being overcharged. On cloudy days, the battery
bank may not get fully charged from the PV array and a generator may need to finish the charge on the batteries.
If a generator is not part of the system, the premises occupants will face some hard decisions about which loads
to keep up-and-running and which ones to divest or “shed” until more sunny days arrive.
The PV power column is the maximum amount of power that can be back-fed through the inverter to charge the
batteries. The calculation for this guideline presumes some losses on the array and in the GT inverter.
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