Door Access During a Power Cut: Plan the 12V Supply and Backup Battery

The card reader is lit, but the door will not respond. Or the lock stays powered while the network controller has gone down. A battery can be healthy while another essential part of the access path is unavailable.

Define what the door must do when building power fails, when the battery runs low and when emergency release is activated. Those are separate events. The building's approved access and escape arrangements determine the design.

Identify Every Device That Needs Power

Start with the lock's exact voltage and normal, peak and holding current. Then add the reader, controller, exit devices and any other equipment supplied from the same circuit. A device marked 12V still needs a compatible voltage range, polarity and sufficient current.

Map dependencies outside the lock box. Some systems can make access decisions locally; others rely on a controller or network path for particular functions. A backed-up lock supply does not automatically keep the network switch, gateway or management computer working.

The diagram is a planning view, rather than a terminal wiring instruction. Have a qualified Specialist confirm the supply connections, protective devices and emergency interfaces for the actual installation.

Read Continuous Output and Battery Ratings Separately

The Tough 12V battery-backed AC-to-DC power supply box combines a low-voltage supply, charging circuit and lockable enclosure. Its listed ratings describe different operating functions:

Do not add 1A and 0.8A and describe the result as available load current. Likewise, the 5A battery-mode figure does not make this box suitable for a continuous 5A installation. Confirm the complete power and charging budget, including peaks and headroom, before selecting it.

A Worked Load Example: One Door, Three Loads

Consider an assumed 12V installation with a lock drawing 0.50A, a reader drawing 0.15A and a controller drawing 0.10A. The combined steady load is 0.75A, or about 9W. These are example inputs; use the actual part-number specifications and measured operating conditions for a real door.

The arithmetic puts the steady load below a 1A output rating. It does not yet prove the supply is adequate: check lock operation, peak demand, losses, the supply's charging arrangement and suitable design margin. A second similar door would bring the example steady load to 1.50A and would already exceed a 1A continuous rating.

If the target is four hours at 0.75A, the load alone requires 3Ah before allowances. That is a useful starting requirement to discuss with the installer. It is not evidence that a particular battery will meet the four-hour target under every condition.

Why 7Ah Does Not Mean Seven Hours

The Tough 12V 7Ah sealed lead-acid battery is rated at the 20-hour discharge rate. For a 7Ah battery, that nominal test rate corresponds to 0.35A. Discharging faster can reduce the capacity available before the relevant cut-off voltage.

A simple calculation of 7Ah ÷ 0.75A gives about 9.3 hours. Treat this as nameplate arithmetic, not a promised runtime. Battery age, temperature, state of charge, discharge rate, wiring losses and the system's cut-off all affect the actual result. The appropriate discharge data and an installed-system test are stronger evidence.

Record the required backup time and an acceptable replacement threshold. Test with the complete intended load under the site's approved procedure, and log the observed duration and battery condition. A battery that powers a reader briefly may still fall short of the required outage period.

Keep Emergency Release Independent of the Convenience Plan

Normal entry, power-failure behaviour and emergency escape serve different purposes. Do not assume that adding a battery should keep every lock energised, or that every access-controlled door must respond identically to a loss of supply.

The applicable fire-safety rules and approved building design determine the required release arrangements. Have the responsible building or access-control Specialist verify them before changing the supply or backup. Backup power must not defeat an emergency release, required fire-alarm interface or safe escape path.

During an approved test, verify the actual door response and release devices as well as the power indicators. Keep people informed and the required access arrangements available. Do not bridge release contacts or bypass protection to make a test appear successful.

Specify the Complete System and Test It Before Handover

A useful requirement lists the lock, reader and controller part numbers; their voltage and current needs; the number of doors; the required backup period; and which network equipment must remain available. Add the installed battery's date and recent test record when replacing an existing unit.

Choose a supply with the correct continuous rating and operating margin, then a compatible battery arrangement capable of meeting the defined duration. Finish with a documented test of normal entry, outage operation, low-battery behaviour and the approved release functions.

If the access controller depends on a network switch, complete the network UPS runtime plan alongside the 12V battery plan. Protecting one link while overlooking another leaves the door system unpredictable.

Frequently Asked Questions