This guide covers the essentials of batteries for emergency lights—why they’re needed, the types available, how to maintain them, and the key differences (lead-acid vs nickel-cadmium). We also include safe replacement steps and a look at new developments affecting battery backup technology. It’s written in plain English for facility managers, safety officers, and building owners. Start with our battery backup emergency lights and battery backup exit signs collections for code-ready, 90-minute operation.
Last updated: August 2025
Quick Picks
Battery Types at a Glance
- Sealed Lead-Acid (SLA): Low cost and rugged in hot/cold; heavier and upright-only.
- Nickel-Cadmium (NiCd): Compact, long life, any orientation; higher cost and needs periodic full discharge.
- Nickel-Metal Hydride (NiMH): Cadmium-free alternative with good capacity; charger sensitivity varies.
- Lithium-Ion: High energy density and low maintenance; premium pricing, special chargers.
Compliance Snapshot
- 90-Minute Rule: Backup must run lights for at least 1.5 hours (UL 924).
- OSHA/NFPA Tests: Monthly 30-sec and annual 90-min tests; log results.
- Service Life: Replace batteries typically every 3–5 years or when runtime fails.
- Match Specs: Replace with the same voltage and chemistry the unit was designed for.
Fast Tips
- Use the Test Button: Verify transfer to battery regularly.
- Prevent Memory: The annual 90-min test conditions NiCd capacity.
- Avoid Deep Discharge: Don’t store SLA fully drained; replace if it happens.
- Recycle: Recycle SLA and NiCd—don’t landfill.
Introduction
All emergency lights and internally illuminated exit signs rely on a rechargeable battery to power lamps when building electricity fails. An internal charging circuit keeps the battery topped up so the fixture is always ready. This guide explains the two most common chemistries—sealed lead-acid and nickel-cadmium—their trade-offs, voltage choices, and how to maintain and replace them safely. You’ll also see emerging battery trends influencing product selection in 2025.
Battery Types in Emergency Lights
Two kinds of rechargeable batteries are most common in emergency lighting units:
Sealed Lead-Acid (SLA) Batteries
Sealed lead-acid batteries have powered emergency lights for decades. Like automotive batteries, they use lead alloy plates and an acid electrolyte to produce electrical energy. Modern SLA units are sealed “maintenance-free” designs. They’re valued for low cost and dependable performance across a wide temperature range, though they’re bulkier and heavier than other options and should remain upright.
Nickel-Cadmium (NiCd) Batteries
NiCd batteries are compact “dry” rechargeable cells with robust cycle life. They mount in any orientation and handle extreme temperatures well. Downsides include higher initial cost and a capacity “memory” if never fully discharged—one reason the annual 90-minute test remains important. In many slim thermoplastic fixtures, NiCd packs are preferred for their size and longevity.
Lead-Acid vs NiCd: Pros & Cons
No single battery is perfect—each brings trade-offs. Here’s a quick side-by-side:
| Battery Type | Advantages | Disadvantages |
|---|---|---|
| Lead-Acid | Lowest cost; reliable hot/cold performance; proven design | Larger/heavier; upright install; susceptible to deep-discharge damage |
| Nickel-Cadmium | Smaller/lighter; any orientation; longer service life; durable in extremes | Higher cost; needs periodic full discharge; initial full charge matters |
Important: Always replace with the same chemistry and voltage the unit was designed for. The charger on the circuit board is tuned to that battery type; swapping chemistries without redesigning electronics risks failure.
Battery Voltage Options
Emergency lighting batteries commonly range from 6 VDC to 24 VDC. Higher voltages can support brighter lamps, more heads, and longer wire runs to remotes with less voltage drop (e.g., 24V systems). Many standard two-head units use 6V. When replacing, match both voltage and chemistry. Too high a voltage can burn out lamps; too low yields dim output—and mismatched chargers won’t properly replenish the battery.
Battery Backup in Exit Signs
Exit signs use similar backup batteries, though their power draw is typically lower. Most modern LED exit signs pair efficient diodes with compact NiCd or NiMH packs to meet the 90-minute runtime. Combination sign-and-lamp units often use a small SLA to feed the flood heads. Battery-free alternatives (photoluminescent and self-luminous tritium) exist for special cases, but standard electric exit signs with battery backup remain the most universal, code-friendly solution for consistent visibility.
Maintenance & Testing
When to Replace
Perform a 30-second push-button test monthly and a 90-minute full test annually. Replace the battery if lamps fail to light on command or can’t sustain the full test duration. Track results: dimming late in the annual test is an early warning. Typical service life is 3–5 years, depending on environment and usage.
Replacing Emergency Light Batteries – Step by Step
- Power Down & Open: Switch off the circuit/breaker if hardwired. Remove the cover (Phillips screw on most units; specialty bits on vandal-proof/hazardous models).
- Locate the Battery: Follow leads from the charger board to the battery (SLA “brick” or shrink-wrapped NiCd/NiMH pack). Disconnect the positive/negative leads.
- Identify Specs: Read the label for voltage, capacity (Ah), chemistry, and model/part number. Note orientation requirements (SLA upright).
- Source a Match: Obtain a like-for-like replacement. Using the wrong voltage or chemistry can damage the unit or fail to charge.
- Install & Secure: Connect red to “+”, black to “–”, seat the battery, and secure with bracket/strap so it won’t move.
- Test & Close: Restore power and press the test button to confirm emergency operation. Recycle the old battery per local rules.
Battery Terminal Types
- Faston Tabs (SLA): Flat blades accept slip-on spade connectors.
- “L” Terminals (SLA): L-shaped posts with bolt hole; less common on small units.
- Plug Connectors (NiCd/NiMH): Two-pin or snap/bullet connectors for quick pack swaps.
A common emergency light battery: 6V 4.5Ah sealed lead-acid.
Opening a unit (left) and the internal layout (right).
Labels often specify the exact replacement type.
Faston tab terminals on SLA (left) and a compact NiCd pack (right).
2025 Trends & Innovations in Emergency Light Batteries
The industry continues to standardize around battery-backup emergency lights as the most universal, code-accepted path to reliable egress illumination. While battery-free options exist for niche scenarios, battery-backed fixtures deliver consistent brightness regardless of ambient light and work seamlessly with NFPA/OSHA testing routines and digital maintenance logs. In mixed-use facilities, they also integrate cleanly with centralized monitoring, letting teams spot a failing pack and replace it before an inspection—minimizing disruptions and risk.
At the component level, we’re seeing gradual adoption of lithium-ion in select models for higher energy density, longer life, and faster recharge—useful in high-duty-cycle environments. NiMH is gaining ground as a cadmium-free alternative in smaller signs. The biggest movement, though, is smarter self-testing/diagnostics: fixtures automatically run the monthly/annual checks, surface faults via LEDs or dashboards, and reduce manual labor while improving compliance. Net result: battery-backup emergency lights remain the go-to standard, with clearer ROI thanks to longer runtimes, simpler upkeep, and better data on fleet health.
Frequently Asked Questions
How long do emergency light batteries last?
Typically 3–5 years. Heat, discharge frequency, and storage conditions affect lifespan. If lights dim or fail before 90 minutes during the annual test, plan a replacement.
Which is better—NiCd or lead-acid?
It depends on the fixture and use case. Lead-acid is economical and handles higher loads; NiCd is lighter, long-lived, and flexible to mount. Use the chemistry your unit was designed for.
Can I upgrade an old unit to lithium?
Not by swapping batteries. Lithium requires purpose-built charging/protection. Choose a fixture designed for lithium if you need those benefits.
Do I need an electrician to replace a battery?
Usually no. It’s a straightforward maintenance task: power down, disconnect old, connect matching replacement, test, and recycle the old battery.
Are exit sign batteries the same as emergency light batteries?
Purpose is the same, but packs differ. Exit signs often use small NiCd/NiMH; high-output emergency lights (and combos) frequently use SLA.