Outback FX-R Series Technical Note on Input Modes

Application Note

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Using the 17,200 watt hours (17.2 kWh) in Table 1, and a battery depth-of-discharge (DOD) of

80%, we would need a battery bank with a capacity of 21.5 kWh to back up the loads for 24

hours. Quite often a DOD of 50% is used in battery-based off-grid systems since a shallower

discharge extends the life of the battery. However, 80% DOD can be used for backup

applications since the battery bank will only be cycled a few days per year, or a week or two at

the most, and going with a deeper DOD means a smaller battery bank. In the example above,

50% DOD for a 17.2 kWh energy demand would equate to a 34.4 kWh battery compared to

21.5 kWh for 80% DOD.

The next step is to translate energy demand from kWh into battery amp-hours (Ah) since that is

how battery storage capacity is commonly measured. Using the load profile above and a 48-Vdc

nominal battery bank, divide 21,500 Wh by 48 Vdc. The result, 448 Ah, is the minimum size

battery bank for this application. Since the energy demand is based on a 24-hour rate, then the

battery Ah for the same discharge rate of 24 hours should be used as the battery capacity (Ah)

will vary depending on how fast it is discharged (see table below). Using the OutBack batteries

listed in the table below, two strings (four 12 Vdc batteries in series for each string) of the

EnergyCell 220GH batteries could be used for a total of 432 Ah and be slightly under our

estimate. If we wanted to be more conservative, then we could choose to use three strings of

the EnergyCell 170RE batteries for a total of 471 Ah.

Table 2

Discharge

Hours

EnergyCell

170RE

EnergyCell

200RE

EnergyCell

200GH

EnergyCell

220GH

Rated Ah Rated Ah Rated Ah Rated Ah

1 89.1 103 120 133.5

3 114.2 132 148.5 166.2

4 120.6 139 154.8 173.2

8 137.0 158 168 188.8

12 145.3 168 176.4 198

20 153.8 178 191 214

24 157.0 181 189.6 216

100 170.0 200 200 220