Basic Battery Backup Time Calculation
Battery backup time calculation helps to calculate how long a battery system can supply power to a connected load.
The calculation becomes more accurate as we include real world factors such as
• Depth of Discharge (DoD),
• Efficiency and
• State of Health (SoH).
Battery backup systems are essential for maintaining the uninterrupted power supply (UPS) during outages.
Understanding how to calculate backup time helps in planning energy requirements and system capacity.
Basic Calculation
The fundamental formula for battery backup hours is simple.
Formula
Battery Capacity (Ah) X Battery Voltage (V) / Load Power (W)
Example
Battery Capacity = 150 Ah
Battery Voltage = 48 V
Load = 1200 W
(150 × 48) / 1200 = 6 hours
This basic calculation assumes for ideal conditions with a constant discharge rates and no efficiency losses.
Depth of Discharge (DoD)
Real-world conditions require considering depth of discharge which represents the percentage of total battery capacity that can be safely used.
Most of the batteries should not be fully discharged to maintain longevity.
Depth of Discharge represents the usable portion of battery capacity.
Formula
Capacity x Voltage x DoD / Load
Example
(150 × 48 × 0.8) / 1200 = 4.8 hours
At 80% DoD which is the effective backup time reduces to 4.8 hours providing a more realistic estimate while preserving battery lifespan.
Efficiency Factors
Charge & discharge efficiency accounts for energy losses during conversion and power delivery processes typically around 90%.
Efficiency = 90% (0.9)
Formula
Capacity x Voltage x DoD x Efficiency / Load
Example
(150 × 48 × 0.8 × 0.9) / 1200 = 4.32 hours
State of Health (SoH)
The most accurate calculation includes State of Health (SoH) which reflects battery aging and degradation typically 95%.
SoH = 95% (0.95)
Formula
Capacity x Voltage x DoD x Efficiency x SoH / Load
Example
(150 × 48 × 0.8 × 0.9 × 0.95) / 1200 = 4.10 hours
This calculation provides a realistic backup duration expectations accounting for all practical operational factors.
Understanding these calculations ensures a proper system design and reliable power backup planning for essential applications.
While the basic calculation gives an ideal backup time a real world battery performance depends heavily on DoD, efficiency and battery health.
For accurate system design especially in UPS, solar and substation DC systems always consider these factors to avoid overestimating backup duration.
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