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
| Capacity | Common round-up size | Typical use |
|---|---|---|
| 4 Ah | — | Small single-panel systems |
| 7 Ah | Yes | The most common panel battery |
| 12 Ah | Yes | Common mid-size panels |
| 18 Ah | Yes | Larger panels and short NAC loads |
| 26 Ah | Yes | Multi-NAC and addressable systems |
| 33 Ah | Yes | Large systems (35 Ah is a common variant) |
| 40 Ah | Yes | Large systems / extended standby |
| 55 Ah | — | Voice systems and long standby periods |
| 100 Ah | — | Largest common single battery |
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.