Battery Backup Size Calculator
Calculate the required battery capacity for backup power systems used in telecommunications, radio systems, repeaters, emergency communications, and other DC-powered equipment.
WHAT THIS CALCULATOR DOES
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Calculates required battery capacity
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Supports 12V, 24V and 48V systems
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Accepts load in Watts or Amps
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Uses batteries discharge characteristics
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Accounts for discharge characteristics
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Designed for RF communication systems
Engineering Notes
Actual battery capacity depends on discharge current, temperature, battery age, and manufacturer specifications.
Design Recommendations For a Battery Back-Up System
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Design for at least 50% remaining capacity.
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Account for battery aging.
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Consider low-temperature derating.
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Use manufacturer discharge curves whenever available.
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Oversize batteries for mission-critical systems.
📘 Battery Backup Calculator
Why Battery Capacity Isn't Constant
A 100 Ah battery does not necessarily provide 100 A for one hour, 50 A for two hours, and so on. The relationship between discharge current and available battery capacity is not linear. (see Figure 1).
Battery manufacturers specify a battery's rated capacity at a particular discharge rate, known as the C-rate. The specified C-rate depends on the battery chemistry and its intended application. (see examples in Table 1).
The relationship between C-rate and discharge time is shown in Table 2
Figure 1. SBS C11 battery capacity (green) and the discharge to the indicated cell voltage curves (blue, orange, red).
Battery Type | Application | Standard | C-rate Standard | Discharge Time |
|---|---|---|---|---|
Standard General-Purpose Lead-Acid | Deep-cycle solar, off-grid storage, automotive | IEEE 1013 (Stand-Alone PV) | C/20 (0.05C) | 20 Hours |
Telecom / Standby Lead-Acid | Cell tower backup, data centers, UPS | IEEE 485 (Stationary Applications) | C/8 or C/10 (0.1255C or 0.1C) | 8 to 10 Hours |
Lithium-Ion / LiFePO4 | Portable electronics, EV storage, standard power | Standard commercial specifications | 1C | 1 Hour |
High-Drain Lithium Polymer | Drones, RC craft, high-burst robotics | Specialty high-discharge specifications | 20C to 30C | 2 to 3 Minutes |
Table 1. C-Rate by Battery Type and Application
📘 Battery C-rate to Discharge Time Calculator
C-Rate | Discharge Time (100%) |
|---|---|
30C | 2 Minutes |
10C | 6 Minutes |
1C | 1 Hour |
C/8 or 0.125C | 8 Hours |
C/20 or 0.05C | 20 Hours |
Table 2. C-Rate and Discharge Time Relationship
📘 Battery C-rate / Discharge Time Calculator
📘 Battery Backup Calculation Workflow
Sizing a critical battery backup system requires shifting from standard linear math to real-world electrochemical physics. Below is the precise engineering map showing how the calculator processes your inputs step-by-step.
Step 1: Initial Parameters
Enter system voltage and Power (Watts) or Current (Amperes).
Step 2: Calculate the Target Capacity Pool
Enter:
Required backup time
Efficiency of DC-DC distribution or inverter equipment
Remaining Battery Capacity. Lead-acid batteries have longer service life when deep discharge is avoided. A 50% target can be used as a conservative design value for lead-acid systems.
Operating Temperature (Cold weather decreases battery capacity. At freezing temperatures (-5°C), lead-acid capacity drops to roughly 72% of standard capacity.)
Aging Safety Margin: An additional capacity margin can be applied to account for battery aging over the system's service life.
Peukert exponent (coefficient that corrects change in capacity with change in the load current)
Step 3: Enter Selected Battery Parameters
Battery or Battery Cell are used for the system
Enter Selected Battery Capacity
Enter Selected Battery C-Rate
Output:
Continuous current is the battery load current during a blackout
Peukert demand is the effective battery discharge rate that includes the energy lost to internal heat and stress
Min Bank Target is the minimum system capacity that satisfy the system requirements
Status:
PASSED - the battery exceeds system requirements
MULT-STRING: Multiple parallel battery strings are required to reach the target capacity
TOO SMALL: The chosen battery is insufficient to support the load for target runtime window.
Total Physical Blocks - total number of 12V batteries or 2V cells
Blocks in series - number of blocks connected to achieve the selected system voltage, creating a single string
Parallel strings - number of strings connected in parallel to achieve target capacity
Why isn't battery capacity linear?
Higher discharge currents reduce the available battery capacity due to internal chemical limitations.
Why stop at 50%?
Repeated deep discharges greatly shorten lead-acid battery life.
What is C rate?
The C rate defines how quickly a battery is discharged relative to its rated capacity.
Can I use AGM batteries?
Yes. AGM batteries follow similar discharge principles but always consult the manufacturer's discharge curves.
Frequently Asked Questions
Why doesn't a 100 Ah battery provide 100 A for one hour?
Battery capacity depends on the discharge rate. The manufacturer's rated Ah capacity is specified at a particular discharge rate, and available capacity generally decreases as discharge current increases.
What is the C-rate of a battery?
The C-rate expresses the discharge current relative to the battery's rated capacity. For example, a 100 Ah battery discharged at 5 A is operating at 0.05C (C/20).
Why does the calculator ask for a remaining-capacity target?
The remaining-capacity target establishes how much battery capacity should remain available at the end of the required backup period. A higher target results in a larger required battery bank.
Why does temperature affect battery capacity?
Battery performance changes with temperature. The manufacturer's specifications and discharge curves should be used when temperature conditions differ significantly from the conditions used for the rated capacity.
Why is the calculated battery bank larger than the load calculation alone would suggest?
The calculator can account for factors such as discharge characteristics, remaining-capacity target, environmental conditions, and aging margin. These factors can increase the required battery capacity compared with a simple current × time calculation.
Can I use AGM batteries?
Yes. AGM batteries follow similar discharge principles but consult the manufacturer's discharge curves.