Fire Alarm Panel Batteries: Sizing, Replacement, and the 24-Volt Load-Test Myth
June 24, 2026
Battery questions are some of the most common ones we see come through Codebook Carl — replacement intervals, load testing, and especially the perennial "is it a deficiency if the batteries drop below 24 volts?" Here's what the Canadian standards actually say, and where the common assumptions go wrong.
Two standards, two jobs
Fire alarm battery requirements live in two different documents, and confusing them is the root of most arguments on site:
- CAN/ULC-S527 (the control unit standard) sets the design requirements — what type of battery, and how much capacity the system must carry.
- CAN/ULC-S536 (inspection and testing) sets the maintenance requirements — when to replace batteries and how to test them during the annual inspection.
If you're sizing a system, you're in S527. If you're inspecting an existing one, you're in S536.
How batteries must be sized (S527)
S527 requires that batteries used for emergency power be rechargeable storage type (Clause 4.3.1) — not primary cells. On capacity, the standard gives two scenarios (Clause 4.3.6):
- No engine-driven generator: the batteries must carry 24 hours of standby (supervisory) load, then the rated alarm load on top of that.
- With an engine-driven generator backing the system: 4 hours of standby, plus the alarm load.
The alarm-load portion has its own duration requirement (Clause 4.3.7) — 2 hours, 1 hour, 30 minutes, or 5 minutes — and which one applies is driven by the building code for that occupancy, not by the standard alone. This is why "how big do my batteries need to be?" doesn't have a single answer: it depends on whether there's a generator and what the building code demands for the alarm period.
When batteries must be replaced (S536)
This is the one that surprises people. Under S536, Clause 9.4, the replacement trigger is the manufacturer's date code or documented service-life interval. Only when there's no manufacturer documentation of service life does the standard impose a hard ceiling: replace within 4 years.
So the widely repeated "batteries every 4 years" rule is the fallback, not a universal mandate. If the manufacturer specifies a shorter or longer documented life, that governs. The four-year clock is what applies when you can't establish anything better.
The load test — and the 24-volt myth
Here's where the most persistent misconception lives. Many techs believe a fire alarm battery "fails" if it drops below 24 volts under load. The standard contains no such threshold.
S536's load-test method (Annex C1.2, which is informative guidance) walks through the procedure:
- Disconnect the batteries from the system.
- Apply a load resistor across the batteries for 5 minutes.
- Record the endpoint voltage.
- Remove the resistor, reconnect the batteries.
- Note the charging current.
Read step 3 carefully: the standard tells you to record the endpoint voltage — it does not publish a pass/fail number. There is no "24 volts or it's a deficiency" line anywhere in S536.
So what is the criterion? It comes from two places the standard defers to:
- The battery manufacturer's specifications for end-of-discharge voltage, and
- The control unit's listed minimum operating voltage — the point below which the panel can no longer guarantee it will perform.
A 24-volt nominal system is built from two 12-volt batteries in series, but the panel's listed minimum operating voltage is well below 24 V — that's the whole point of standby capacity. A battery reading slightly under 24 V under load isn't automatically a failure; a battery that can't hold above the panel's rated minimum for the required duration is. The number that matters is the manufacturer's and the panel's, not a round "24."
What this means in practice
- Sizing a new system? Check whether there's a generator (24 h vs 4 h standby) and confirm the alarm-load duration the building code requires. That's S527.
- Doing the annual? Replace on the manufacturer's date code; fall back to the 4-year ceiling only when there's no documentation. Run the load test, record the endpoint voltage, and compare it against the manufacturer and panel specs — not a memorized 24-volt rule. That's S536.
- Arguing on site about a "24-volt deficiency"? The standard doesn't set that threshold. Point to the panel's listed minimum operating voltage instead.
Every requirement above is drawn from the actual text of CAN/ULC-S527 and CAN/ULC-S536. If you've got a specific situation — a particular panel, a generator-backed system, an unusual occupancy — ask Codebook Carl and you'll get the governing clause with an exact citation you can verify.