Emergency exit signs and lights are powered by one of four rechargeable battery types: lead calcium, nickel-cadmium (Ni-Cad), lithium iron phosphate (LiFePO4), or nickel-metal hydride (NiMH), each suited to different fixtures and operating conditions.
During normal operations, emergency exit signage and lighting receive power from a building’s electrical grid. However, when that power is interrupted, the signage and lighting’s power reserve is activated, and the fixture provides light via a backup power source. An internal rechargeable battery within the lighting fixture often provides this power.
Because these emergency exit signs and lights are a building occupant’s lifeline to safety, they must be reliable. Building codes and life safety laws mandate key requirements for backup power to ensure reliability. Emergency lighting and exit signs must be rechargeable, with a charge time of fewer than 24 hours, and provide enough electrical current to keep the lamps lit for a minimum of 90 minutes when the unit is engaged.
These requirements tie directly to the code standards covered in our post on emergency exit lighting codes, which set the 90-minute minimum runtime referenced above.
There are four main types of batteries used in emergency exit signage and lighting: Lead Calcium batteries, Ni-Cad (Nickel-Cadmium) batteries, Lithium Iron Phosphate batteries, and Nickel-Metal Hydride batteries.
Lead Acid and Lead Calcium Batteries: A Great Choice for Inverters and Hazardous Location Lights & Exit Signs
Lead-acid batteries are the oldest form of rechargeable batteries. The original lead-acid battery contained antimony, calcium, tin, and selenium in the lead electrodes, which were high-maintenance and heavy. They are rarely used today but can still be found in some central battery backup systems.
The new generation of lead batteries is Lead Calcium Batteries. This type of lead-acid battery uses only calcium and lead as electrodes. It is fully sealed, eliminating the need to check water levels and making it maintenance-free. Lead-acid batteries are more efficient than their predecessors. They are corrosion-resistant, perform better in cold weather, have a low self-discharge rate (meaning they will work for an extended period), and have no excessive gassing.
Lead Calcium batteries are ideally suited where lower upfront costs are a consideration. They also store more capacity in amp-hours, and temperature doesn’t significantly affect capacity or self-discharge. Lead-acid batteries are best in terms of reliability and operational capability. They tolerate slow, fast, and overcharging better than other batteries and can withstand long-term inactivity.
Most emergency signage and lighting, including steel emergency lights, wet-location emergency lights, and hazardous-location emergency lights, use lead-acid batteries. Also, if a remote headlamp is needed, lead-calcium batteries can be configured to accommodate the extra voltage required to support the lamp. Because of their reliability, lead-calcium batteries are also used in most inverter units.
Nickel-Cadmium Batteries (Ni-Cd or NiCad), The Choice for Exit Signs and Lighting That Requires Small Batteries
The nickel-cadmium battery (Ni-Cd battery or NiCad battery) is well suited for emergency lighting and signage applications, where its small size and light weight are important considerations. These cylindrical AA-sized batteries offer numerous advantages over traditional sealed lead-acid batteries, which use nickel oxide hydroxide and metallic cadmium as electrodes.
NiCad batteries are often used in indoor applications where thermoplastic emergency lights are recommended. They perform well in rigorous working conditions, such as extreme temperatures, and can be installed in any orientation without considering battery acid. This compact battery also boasts a fast recharge time of as little as 3.5 hours and has an extended lifespan of nearly 15 years. While the initial cost is higher than that of lead-acid batteries, NiCad batteries are lighter and cheaper to ship.
The nickel-cadmium battery is not ideal for wet applications or for hazardous or explosion-proof fixtures, and it does not have enough charge to operate remote lamps when needed.
Lithium Iron Phosphate Batteries, Long Life Span and Minimal Maintenance
The lithium iron phosphate battery is a type of lithium-ion battery that uses lithium iron phosphate as the cathode material and a graphitic carbon electrode with a metallic backing as the anode. Lithium iron phosphate (LiFePO4 or LFP) batteries have a lower energy density and operating voltage than some other battery types. Still, they are well-suited for backup power and are rapidly becoming more widely used.
LiFePO4 batteries offer many benefits compared to lead-acid batteries and other lithium batteries. They are smaller, lighter, and available in various dimensions, capacities, and shapes. Lithium iron phosphate batteries have a life cycle of more than 10 times that of lead-acid batteries and perform well under extreme temperatures. Due to their slow discharge rate, LiFePO4 batteries can be stored for extended periods without active maintenance. They are environmentally friendly, low-toxicity, and won’t overheat or catch fire.
While the initial startup costs for LiFePO4 batteries may be higher than those of other options in the marketplace, their long lifespan and zero maintenance make them cost-effective.
Nickel-Metal Hydride Batteries, High Capacity and Environmentally Friendly
A nickel-metal hydride battery (NiMH or Ni-MH) has a chemical reaction at the positive electrode similar to that of the nickel-cadmium battery. However, the negative electrodes use a hydrogen-absorbing alloy instead of cadmium. NiMH batteries are typically used as a substitute for similarly shaped alkaline batteries because they have a compatible cell voltage.
NiMH batteries are ideally suited for high-current-drain applications. They can have two to three times the capacity of NiCd of the same size, with significantly higher energy density. Nickel metal hydride batteries have a slightly shorter life cycle than lithium batteries, but are more cost-effective.
Nickel Metal Hydride is an environmentally friendly battery. They don’t contain harmful levels of lead, mercury, or nickel that can contaminate drinking water if the battery isn’t disposed of properly. They are also easy to store and transport because they are not subject to regulatory control.
Battery Types at a Glance
| Battery type | Lead Calcium | Ni-Cad | LiFePO4 | NiMH |
|---|---|---|---|---|
| Best for | Inverters, hazardous location and wet location fixtures, remote lamp heads | Small, lightweight thermoplastic indoor fixtures | Long life, low maintenance applications | High-current-drain applications |
| Typical lifespan | Long, tolerates overcharging and inactivity well | Around 15 years | More than 10 times a lead-acid battery’s life cycle | Slightly shorter than lithium, still durable |
| Recharge time | Standard, under 24 hours per code | As fast as 3.5 hours | Under 24 hours per code | Under 24 hours per code |
| Not recommended for | N/A within its typical use cases | Wet locations, hazardous or explosive-proof fixtures, remote lamps | N/A within its typical use cases | N/A within its typical use cases |
| Environmental notes | Sealed, maintenance-free | Contains cadmium | Low toxicity, will not overheat or catch fire | Free of lead, mercury, and cadmium, not subject to regulatory control |
Frequently Asked Questions
How long must an emergency light battery hold a charge?
By code, an emergency exit sign or light’s battery must keep the lamps lit for a minimum of 90 minutes once the backup power source is engaged, and it must fully recharge in under 24 hours.
Which battery type is best for wet or hazardous locations?
Lead-acid batteries are typically used in wet- and hazardous-location emergency lights and exit signs, since Ni-Cad batteries are not recommended for those environments.
What is the most environmentally friendly battery option?
Nickel-metal hydride (NiMH) batteries are the most environmentally friendly option covered here. They do not contain lead, mercury, or cadmium, and they are not subject to the same regulatory control as other battery types.
Can any of these batteries power a remote lamp head?
Lead-calcium batteries can be configured to supply the extra voltage a remote lamp head requires. Ni-Cad batteries, by contrast, do not carry enough charge to operate a remote lamp when needed.
Which battery has the longest life cycle?
Lithium iron phosphate (LiFePO4) batteries offer the longest life cycle of the four types, rated at more than 10 times that of a standard lead-acid battery.
Make Sure Your Emergency Exit Signs and lighting contain the Right Battery for the Job.
Isolite emergency lighting experts ensure that our emergency lighting solutions contain the battery backup sources ideal for each fixture and product. Whether it is our Elite Edge-Lit Master Exit sign with its long-life, high-temperature fused nickel-cadmium battery, our HLX-C corrosion-resistant, wet location rated sign powered by a sealed lead-calcium battery, the fully recessed MIGN2 compact LED emergency light powered by a maintenance-free nickel-metal hydride battery, or our long-life thermally safe high-capacity lithium iron phosphate battery-powered LED Emergency Drivers, Isolite has the emergency lighting to fit your needs.
Learn More about battery packs and inverters.
