Why Backup Power Is a Core Part of Any Access Control Design
This article explains how battery backup works in commercial access control systems, what happens to electrified door hardware when building power fails, and how to match backup capacity to the devices on your opening schedule. It is written for security integrators, commercial hardware contractors, facility managers in schools and healthcare, and anyone specifying electrified locking hardware who has ever asked: what happens to my doors when the lights go out?
What Is Battery Backup in an Access Control System?
Battery backup for access control is a stored-energy source -- typically one or more sealed lead-acid (SLA) batteries -- connected to the system's power supply. When building AC power is interrupted, the battery automatically takes over, keeping the power supply's DC output active so that locks, readers, controllers, and door position switches continue to function without interruption.
This is different from a general UPS protecting a server. A purpose-built access control power supply with an integrated battery connector manages charge state, monitors battery health, and switches over cleanly under load -- without the voltage spikes that can disrupt solenoid-based locking devices.
Fail-Secure vs. Fail-Safe: The Backup Power Stakes Are Different
Before sizing backup power, you must know how each lock on the schedule behaves without electricity.
- Fail-secure locks (electric strikes, most electrified mortise and cylindrical locksets) remain locked when power is lost. A building with fail-secure hardware and no battery backup has locked doors that cannot be accessed with credentials during an outage -- a serious problem in schools, healthcare, and industrial facilities where staff need to move between zones.
- Fail-safe locks (electromagnetic locks, fail-safe electric strikes) release when power is cut. The lock itself does not need backup power to open, but the access control panel and readers do need it -- otherwise the system cannot log the event, re-lock when power returns gracefully, or continue normal operation.
The practical takeaway: every access control installation benefits from battery backup, regardless of lock type. The reason differs, but the need does not.
How Purpose-Built Power Supplies Handle Battery Backup
Many power supplies designed specifically for access control -- including units in the class of the Securitron AQL series -- include an integrated battery charge circuit and a connector for one or two 12V SLA batteries. Key features to look for:
- Automatic switchover from AC to battery with no output interruption
- Float charging that maintains battery charge without overcharging
- Low-battery alarm output that can trigger an alert at the access control panel
- Fire alarm control panel (FACP) relay input, so the power supply can cut power to fail-safe locks on fire alarm signal
- Power distribution board (PDB) outputs that keep lock circuits and reader/controller circuits separated
A switching power supply alone -- the inexpensive kind used for general electronics -- is not an appropriate primary power source for solenoid-based locking hardware. It can produce voltage spikes that disrupt access control panels and degrade lock solenoids over time. A linear or hybrid power supply is the correct choice for door hardware loads.
Sizing Backup Runtime: The Calculation Most Installers Skip
Runtime depends on two variables: total current draw of all powered devices and battery capacity in amp-hours (Ah).
- Add the current draw (in amps) of every device on the circuit: locks, readers, door position switches, request-to-exit (REX) devices, and any controller powered from the same supply.
- Apply a 1.25 to 1.50 safety margin for inrush and future expansion before selecting the power supply amperage rating.
- A 12V 7Ah SLA battery on a lightly loaded circuit (three to four openings) typically delivers four to eight hours of backup. A 12V 12Ah battery extends that window for larger door counts or higher-draw hardware.
Healthcare facilities and government buildings often require documented runtime minimums -- verify with the AHJ (authority having jurisdiction) and the project's Division 28 specification before finalizing battery sizing.
Centralized vs. Distributed Power Strategy
On multi-floor buildings or large campuses, a single central power supply creates long wire runs and voltage drop problems. Distributed power -- smaller supplies mounted above ceiling tiles or in electrical closets near each cluster of doors -- keeps runs short, simplifies troubleshooting, and limits the blast radius of any single supply failure. Each distributed supply still needs its own battery backup.
Per good practice, power supplies located above accessible ceilings should be reachable by a standard ladder without removing structural elements. Label each supply with its circuit number, battery size, and fire relay connection status.
Access Control Hardware That Complicates the Backup Picture
Some hardware categories introduce additional considerations:
- Electrified exit devices -- products in the Sargent 80 Series and Corbin Russwin ED series class -- may draw higher current on electric latch retraction (ELR) options. Confirm current draw from the device spec sheet before sizing the supply.
- PoE-powered locks -- such as the Sargent IN220 and Corbin Russwin IN220 PoE intelligent locksets -- receive power from the network switch via Power over Ethernet. Backup power for these devices means putting the PoE switch on a UPS, not adding a battery to a traditional power supply. The two strategies are mutually exclusive on the same opening.
- Wireless offline locks -- battery-powered standalone locksets -- carry their own internal batteries and are unaffected by building power outages. However, if the access control server or head-end that manages credentials goes down, offline locks continue operating from their stored access list until the server reconnects.
- Electromagnetic hold-opens -- connected to the FACP -- must release on fire alarm regardless of battery backup status. The power supply's fire relay input handles this, but verify the relay wiring is correct during commissioning.
Code and Compliance Checkpoints
Access control power and backup power touch multiple specification divisions. Coordination between Division 26 (electrical) and Division 28 (electronic safety and security) is required any time electrified hardware is on the schedule. Key checkpoints:
- Dedicated AC circuit for the access control power supply -- shared circuits create nuisance trips and complicate load calculations.
- Battery backup runtime requirements may appear in the project specification, the owner's program, or local AHJ amendments -- do not assume a default.
- Fire-rated openings with electrified hardware must maintain UL listing compliance; verify that the power supply and wiring method do not compromise the door assembly listing.
- In healthcare occupancies, life-safety door hardware may fall under NFPA 99 and NFPA 101 requirements for essential electrical system power.
Practical Guidance for Specifiers and Facility Teams
- Specify a purpose-built access control power supply with integrated battery backup and charge management -- not a generic switching supply with a battery bolted on.
- Size the battery for the worst-case runtime your AHJ or owner requires, then add one battery size for margin.
- Label every power supply enclosure with device count, circuit voltage, battery size, and fire relay status.
- Include a battery replacement schedule in the facility maintenance plan -- SLA batteries typically need replacement every three to five years under normal conditions.
- Test switchover to battery at commissioning and document the runtime with actual load.
Get the Right Power Supply for Your Opening Schedule
DoorwaysPlus.com carries access control power supplies, electrified locksets from Sargent, Corbin Russwin, and Hager, electric strikes, and the supporting hardware needed for a complete opening. Whether you are building out a new school security upgrade, specifying a healthcare corridor, or replacing a failed power supply in an industrial facility, our team can help you match the right supply and backup capacity to your door count and device mix.
Browse access control hardware and power supply options at DoorwaysPlus.com, or contact us with your schedule for a quote.