Steel Architectural Sealed Lead Acid (SLA) Red Letter Dual Circuit Nickel–Metal Hydride (NiMH) Steel
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Steel Exit SignsArchitectural
ArchitecturalDual Circuit
Dual CircuitRed Letter
New York City and Chicago require exit signs to have red lettersSealed Lead Acid (SLA)
SLA batteries are the most reliable and widely used power source. They can recharge much faster than a flooded lead acid battery, and the calcium added to their plates helps reduce water loss. Sealed lead acid batteries can recycle the gases they produce and can prevent damage to the battery when the charge rate is controlled. These batteries must always be kept in upright positions and protected from leakage.Nickel–Metal Hydride (NiMH)
NiMH batteries can have two to three times the capacity of an equivalent size NiCd, and their energy density approaches that of a lithium-ion cell. Nickel–Metal Hydride batteries are good for high current drain applications because of their low internal resistance.
The disadvantage of NiMH batteries is their high rate of self-discharge. NiMH batteries typically lose 4% of their charge per day of storage. LSD NiMH batteries significantly lower self-discharge, but at the cost of lowering capacity by about 20%.