SLA Battery Size Chart

The standard sealed lead-acid (SLA / VRLA) battery capacities stocked for fire alarm control panels. Each battery is 12 V; two in series make the 24 V the panel runs on. Size the battery by calculating the required amp-hours and rounding up to the next capacity here. These capacities come from manufacturer catalogs — NFPA 72 sets the required Ah, not the battery size.

Standard 12 V SLA capacities

VRLA / manufacturer
Nominal amp-hour capacities of 12 V SLA batteries used in fire alarm panels, wired in pairs for 24 V. The middle column flags the sizes battery calculators commonly round up to.
CapacityCommon round-up sizeTypical use
4 Ah—Small single-panel systems
7 AhYesThe most common panel battery
12 AhYesCommon mid-size panels
18 AhYesLarger panels and short NAC loads
26 AhYesMulti-NAC and addressable systems
33 AhYesLarge systems (35 Ah is a common variant)
40 AhYesLarge systems / extended standby
55 Ah—Voice systems and long standby periods
100 Ah—Largest common single battery
Terminal type varies by capacity and manufacturer — small batteries use F1/F2 faston tabs, larger ones use nut-and-bolt (insert) terminals. 35 Ah is a common variant of the 33 Ah size. Physical dimensions vary by maker and are not shown.

Sizing and replacing

You never pick a battery size directly — you calculate it. The standby battery calculation gives a required amp-hour figure; you round it up to the next capacity on this list. Rounding down would leave the system short of its required standby-plus-alarm runtime once the battery ages.

Both batteries in a panel must be the same capacity and are replaced as a matched pair — mixing an old battery with a new one drags the pair down to the weaker cell. Batteries are typically replaced on a schedule (commonly every 3–5 years) regardless of apparent condition, because SLA capacity fades well before the battery fails outright.

Common questions

What size battery does a fire alarm panel use?

Fire alarm panels use 12 V sealed lead-acid (SLA/VRLA) batteries wired in pairs to make 24 V. The standard capacities are 4, 7, 12, 18, 26, 33, 40, 55, and 100 Ah; 7 Ah is the most common. You calculate the required amp-hours and install the next standard size at or above it.

Why are there two batteries in a fire alarm panel?

Each SLA battery is 12 V nominal, and a fire alarm system runs on 24 V, so two identical batteries are wired in series (12 V + 12 V = 24 V). Both must be the same capacity and replaced as a matched pair.

How do I know which capacity to buy?

Run the standby battery calculation for your system, then round the result up to the next capacity on this list — never down. The calculation adds 24 hours of standby current to 5 minutes (or 15 for voice) of alarm current and multiplies by the aging factor.

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