If you manage life‑safety systems, batteries are the backbone of code‑ready egress lighting. This guide explains which batteries power emergency lights, exit signs, and exit‑sign/emergency‑light combos, how to size and select packs (e.g., 4.8V Ni‑Cd exit sign battery, 6V/12V sealed lead‑acid emergency light battery), how to meet the 90‑minute UL 924 runtime, and how to plan testing, recharging, and replacement without over‑spending.
Last updated: October 2025
Quick Overview
All three device families—emergency lights, exit signs, and combos—must deliver 90 minutes of illumination on battery during a power loss. That requirement drives battery chemistry, voltage, capacity, and charger design. Most legacy and current systems use either sealed lead‑acid (SLA) or nickel‑cadmium (Ni‑Cd) packs tailored to low‑wattage LED loads.
- Goal: Meet code runtime without oversizing.
- Approach: Match voltage exactly, choose capacity for your load/runtime, and keep clean test logs.
- Outcome: Fewer nuisance failures, faster inspections, predictable replacement cycles.
Battery Chemistries & Where They’re Used
- Sealed Lead‑Acid (SLA): Common in emergency lights and many combos. Pros: economical, strong standby capacity. Considerations: heavier; prefers moderate temperatures.
- Nickel‑Cadmium (Ni‑Cd): Common in exit signs and compact combos. Pros: wide temperature tolerance, handles frequent tests well. Considerations: regulated recycling for cadmium.
Some newer models use lithium families, but SLA and Ni‑Cd remain the most prevalent in listed equipment and replacement programs. Always follow the fixture’s listing and manual—chemistry swaps are not arbitrary.
Voltage & Capacity by Device Family
| Device Family | Typical Chemistry | Common Voltages | Capacity Range (examples) | Notes |
|---|---|---|---|---|
| Exit Signs | Ni‑Cd | 4.8V (4‑cell stick common) | 600–1000 mAh | Low load, compact packs; ideal where space is tight. |
| Emergency Lights | SLA | 6V, 12V | 3.4–12 Ah (e.g., 6V 4.5Ah; 12V 7Ah) | Great standby runtime and current for lamp heads. |
| Exit/Emergency Combos | Ni‑Cd or SLA | 4.8V, 6V, 12V (model‑dependent) | Ni‑Cd: 700–1000 mAh; SLA: 4–9 Ah | Check nameplate—combos vary widely by design and head wattage. |
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How to Select & Right‑Size a Replacement
- Match the voltage exactly: read the unit nameplate/submittal (e.g., 4.8V Ni‑Cd for signs; 6V/12V SLA for lights/combos).
- Size capacity to the load: use the fixture’s wattage on battery and target 90 minutes. More Ah = longer runtime and longer recharge, but larger size/weight.
- Confirm connector & geometry: plug style, lead length, and pack shape (4‑cell stick, 5‑cell cluster, SLA “brick”) must fit without strain.
- Check environment: hot mechanical rooms and cold vestibules affect runtime and life; Ni‑Cd often tolerates temp swings better.
- Preserve the listing: use like‑for‑like or a manufacturer‑approved equivalent to maintain UL 924 compliance.
Tip: If a unit barely fails the annual 90‑minute test, verify both capacity and actual load (including remote heads) before increasing Ah.
Charging, Testing & Recharge Windows
- Runtime target: continuous 90 minutes on battery under load.
- Recharge window: many chargers require about a day to fully recharge after a full 90‑minute discharge.
- Test cadence: monthly functional (often ~30 seconds) and one annual 90‑minute test; self‑test options automate this and record status codes.
For a deeper, hands‑on review of charging practices for sealed lead‑acid specifically, see the resource in Deep‑Dive Resources below.
Maintenance, Storage & Replacement Planning
- Replacement budget: plan for ~3–5 years depending on chemistry, temperature, and test frequency.
- Storage: keep spares cool and partially charged per chemistry guidance; avoid deep discharge during storage.
- Records: keep test dates, pass/fail, corrective actions, and replacement dates—this speeds AHJ reviews.
- Recycling: use approved channels—Ni‑Cd contains cadmium (regulated); SLA is recyclable through many vendors.
Troubleshooting by Device Type
Exit Signs
- Fails 90‑minute test: Ni‑Cd capacity diminished—replace pack; verify 4.8V spec and connector.
- Indicator LED fault: check AC feed, battery connector, and charger status codes.
- Dim letters on battery: ensure a full recharge cycle completed after last test.
Emergency Lights
- Short runtime: SLA Ah too low for lamp heads; verify total wattage and consider higher‑capacity SLA (same voltage, approved form factor).
- Flicker on transfer: inspect terminals and charger board; look for sulfation signs on aged SLA.
Combos
- One head dimmer: check wiring and head wattage; ensure both heads are within rated battery load.
- Frequent self‑test fails: review ambient temperature and make sure recharge window isn’t being cut short by repeated tests.
Compliance Notes (UL 924 / NFPA 101)
UL 924 listings and the Life Safety Code require continuous illumination of the path of egress during a power loss. Preserve the product listing by matching battery voltage, chemistry, and capacity per the manufacturer, and maintain documented monthly/annual tests. Where available, self‑test features simplify compliance and help catch issues early.
Deep‑Dive Resources (Read Next)
Use these focused guides for step‑by‑step procedures and side‑by‑side comparisons:
FAQ
What batteries are most common across these devices?
Exit signs: typically 4.8V Ni‑Cd stick packs. Emergency lights: usually 6V or 12V SLA “bricks.” Combos: either Ni‑Cd or SLA depending on head wattage and enclosure size.
How long do emergency light batteries last?
Plan for about 3–5 years depending on chemistry, temperature, and how often units are tested on battery.
Can I swap SLA for Ni‑Cd (or vice versa)?
Only if the manufacturer lists and supports it. Chemistry affects the charger profile and product listing; use approved replacements.
How do I choose capacity (Ah or mAh)?
Match voltage first; then size capacity to the unit’s battery‑mode wattage and target runtime. For lights or combos with remote heads, total the head wattage.
What’s the correct way to recharge an SLA after a full test?
Allow a full recharge cycle (often ~24 hours) before running another discharge test. See the Best practices for recharging SLA guide in Deep‑Dive Resources.
Why did my exit sign fail the 90‑minute test?
Likely an aging 4.8V Ni‑Cd or an incomplete recharge after recent tests. Confirm charger status, then replace the pack with the listed spec if capacity has dropped.
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