Battery Types and Characteristics

Lead acid

The workhorse of industrial standby power.

• Vented (flooded) lead acid — free electrolyte, refillable, long life, tolerant of abuse and high temperature, and the state of each cell can be assessed directly by measuring specific gravity. It emits hydrogen during charging and therefore needs a ventilated battery room, spill containment and acid-resistant surfaces.
• Valve regulated lead acid (VRLA) — sealed, with the electrolyte absorbed in a glass mat or gelled. No topping up, minimal gas emission in normal service, and installable in a cabinet. The trade-offs are shorter life, far greater sensitivity to temperature, and the absence of any direct way to measure cell condition — a VRLA battery can fail with no external warning, which is why impedance testing matters so much for them.

Nickel cadmium

More expensive, but extremely robust: very long life, tolerant of deep discharge, high and low temperature, and mechanical abuse, with a gradual and predictable end of life rather than a sudden one. Widely used for switchgear tripping duty and in harsh environments. Its voltage characteristic differs from lead acid, so the charger and the number of cells must be matched to the system.

Lithium ion

High energy density, long cycle life, fast charging and low maintenance, increasingly used in UPS applications. It requires an integral battery management system, and the fire behaviour is different from lead acid and needs specific consideration in the installation design, detection and response arrangements.

The properties that matter

• Capacity — quoted in ampere-hours at a specified discharge rate and end voltage. Capacity is lower at higher discharge rates, so a battery rated at a ten-hour rate delivers substantially less if discharged in one hour. Sizing must use the correct rate for the duty.
• Temperature — the dominant life factor. Lead acid life is roughly halved for every eight to ten degrees Celsius above the design temperature. A battery room without effective temperature control will not achieve its design life, whatever is done to it.
• Cycling — standby batteries are designed for float service with occasional discharges, not for daily cycling.
• Ageing — capacity falls over life, and a battery is normally considered at end of life when capacity falls to eighty per cent of rated, because degradation accelerates beyond that point.