7.9 APPENDIX I: V3 BBU BATTERY RUNTIME CALCULATIONS EXAMPLE
The following example explains how to calculate the required Battery Capacity for a V3 BDA/BBU System
based on the Total Power Consumption, Device Operating Voltage and Backup Runtime requirement (e.g. 12
or 24 hours). Once we choose a battery option, we can calculate the actual Backup Runtime.
Power (W) = Power Consumption in Watts
DC Voltage (VDC) = DC Operating Voltage of Device
Current (A) = Total Current in Amps
Battery Runtime (hr) = Duration of Power provided at 100-percent capacity in hours
Safety Factor (%) = Percent margin required to account for battery aging (refer to local AHJ requirements)
Battery Capacity (Ah) = Total Battery Capacity a battery can provide in Amp/Hr
Step 1: Calculate the Total Power Consumption of the system by adding up the power consumption of all the
devices which will be connected to the BBU (BDA/MU, Annunciator Panel, FOUs, etc…). Refer to the device
datasheets for Power Consumption values.
Step 2: Calculate the Total Current the system will draw in Amps based on the Total Power Consumption and
48VDC system operating voltage.
Step 3: Calculate the required Battery Capacity to meet the Backup Runtime requirements.
Step 4: Choose the correct Battery option based on the calculated Battery Capacity.
Step 5: Calculate the true Battery Backup Runtime based on the chosen battery.
Example 1: We are using a 2W Class A 700MHz BDA/MU connected to 2 x FOUs (8 fiber Ports Each) and a
V1 AP, all of which will be powered from a single Comba V3 BBU. The local jurisdiction has a requirement to
provide a minimum of 12 hours of Backup Runtime with a Safety Margin of 20%. Comba devices use a 48VDC
operating voltage. We must first look at each device datasheet to determine the power consumption.
Power Consumption – 2W Class A 700MHz BDA/MU: 90 Watts
Power Consumption – 8-Port FOU: 20W
Power Consumption – AP: 3W
Total Power Consumption = BDA/MU Power Consumption + 2 x FOU Power Consumption + AP Power
Consumption
Total Power Consumption = 90 + 20 + 20 + 3 = 133 Watts
Current = Total Power Consumption / Voltage = 133 Watts / 48VDC = 2.77 A
Safety Factory of 20% = 1 + % / 100 = 1 + 20/100 = 1.2
Battery Capacity Required = Current x Battery Runtime x Safety Factor = 2.77 A x 12 hr x 1.2 = 39.89 Ah
Now you will choose the battery that has at least 39.89Ah of Battery Capacity. In this case, you would
choose the Comba 60Ah battery option, as the 30Ah battery does not have enough capacity to meet
the requirement.
Backup Runtime (hr) = Battery Capacity / Total Current / Safety Factor = 60 Ah / 2.77 A / 1.2 = 18.05 hr
Note: If there is not a requirement to include a Safety Factor, then simply remove it from the calculations.
Battery Capacity Required = I x Battery Runtime = 2.77 A x 12 hr = 33.24 Ah
Backup Runtime (hr) = Battery Capacity / Total Current = 60 Ah / 2.77 A = 21.66 hr
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