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Back to: Advanced Physical Security Integration (APSI)

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Lesson 1.4: Power Budgets & Battery Backup

Module: 1 – Foundations of Physical Security Architecture

Prerequisites: Lesson 1.3 (Ohm’s Law)

Estimated Time: 45–60 Minutes

1. Learning Objectives

By the end of this lesson, you will be able to:

  • Calculate the total load (Amps) for a system to select the correct Power Supply Unit (PSU).
  • Apply the “25% Headroom” rule to prevent PSU failure.
  • Determine the required Battery Capacity (Ah) to keep a system running during a blackout.
  • Distinguish between High Voltage (110/220V) and Low Voltage (12/24V) and know when you need a licensed electrician.

2. Sizing the Power Supply Unit (PSU)

A common mistake is buying a power supply that barely covers the load. This leads to overheating and premature failure.

The Process:

  1. Inventory: List every device connected to the PSU.
  2. Sum the Current: Add up the Max Current Draw (Amps) for each device.
  3. Apply Headroom: Multiply the total by 1.25 (25% safety margin).
  4. Select PSU: Round up to the nearest standard PSU size (e.g., 2A, 4A, 6A, 10A).

Example Calculation:

You are powering a 4-door access control panel.

  • Controllers (x1): 0.5 Amps
  • Card Readers (x4): 0.1 Amps each = 0.4 Amps
  • Maglocks (x4): 0.5 Amps each = 2.0 Amps
  • Motion Sensors (x4): 0.05 Amps each = 0.2 Amps

Total Load: $0.5 + 0.4 + 2.0 + 0.2 = 3.1 Amps

Safety Margin:

3.1 Amps x 1.25 = 3.875 Amps

Selection:

You cannot use a 3A supply. You need a 4A or 6A Power Supply.

3. Battery Backup (Standby Power)

Security systems must work when the building loses power. We use lead-acid batteries (similar to car batteries) measured in Amp-Hours (Ah).

What is an Amp-Hour?

One “Amp-Hour” means the battery can provide 1 Amp of current for 1 Hour before dying.

  • Or 0.5 Amps for 2 Hours.
  • Or 2 Amps for 0.5 Hours.

The Formula:

$$Battery Capacity (Ah) = Total Load (Amps) \times Desired Standby Time (Hours)

Scenario:

Using the 4-door system above (Load = 3.1 Amps).

The Fire Marshal requires the system to stay alive for 4 Hours during an outage.

Capacity = 3.1 Amps x 4 Hours = 12.4 Ah

Selection:

Standard battery sizes are 7Ah, 12Ah, 18Ah, and 40Ah.

A single 12Ah battery is slightly too small (12.4 needed). You should use two 7Ah batteries in parallel (Total 14Ah) or one 18Ah battery.

4. Wiring Batteries: Series vs. Parallel

Sometimes you need more voltage (24V), and sometimes you need more capacity (Ah).

  • Series Wiring (Voltage Adder):
    • Connect Positive (+) to Negative (-).
    • Result: Voltage doubles, Amps stay the same.
    • Use Case: Creating a 24V system using two 12V batteries.
  • Parallel Wiring (Capacity Adder):
    • Connect Positive (+) to Positive (+) and Negative (-) to Negative (-).
    • Result: Voltage stays the same, Amps (Capacity) doubles.
    • Use Case: Extending runtime from 4 hours to 8 hours.

5. High Voltage vs. Low Voltage

Integrators generally live in the “Low Voltage” world, but you must respect the boundary.

  • Low Voltage (< 50V):
    • Includes: 12VDC, 24VAC, PoE (48V).
    • Safety: Generally safe to touch (won’t kill you, might tingle).
    • Regulation: In most jurisdictions, you do not need a full electrical license, but you must follow the National Electrical Code (NEC) Article 725 (Class 2 Circuits).
  • High Voltage (> 110V):
    • Includes: Wall outlets (120V/220V), lighting circuits.
    • Safety: Lethal. Do not touch.
    • Rule: Never run high voltage and low voltage in the same conduit. The high voltage creates electromagnetic interference (induction) that disrupts data signals and can fry sensitive electronics.