Sizing a VRLA Battery for Runtime and Cycle Life

Undersizing a sealed VRLA battery is the most common cause of premature failure. A block that is worked too hard delivers less than its rated capacity and ages fast. Sizing well means matching capacity to the real load, the required runtime and the way the battery will actually be discharged.

Start with the load and the runtime

First pin down the load. Express it in watts, or in amps at the battery voltage, and note the backup time you need — say 30 minutes or 8 hours. The rough capacity is load current × runtime in hours, giving amp-hours (Ah); or use watt-hours (Wh) and divide by system voltage. That figure is only a starting point, because a battery’s printed Ah rating assumes a slow, gentle discharge.

• Derate for discharge rate. Ratings are usually quoted at the 20-hour rate (C/20). Pull the same battery flat in one hour and you may see barely half the rated Ah — the Peukert effect. Size from the discharge-curve data for your actual rate, not the headline number.
• Mind depth of discharge (DoD). Cycle life falls sharply with deeper discharge. A VRLA taken to 50% DoD lasts many times longer than one run to 80%. For cycling duty, oversize so each cycle stays shallow.
• Correct for temperature. Capacity is rated at 20–25°C. Cold cuts usable capacity; sustained heat roughly halves service life for every 10°C above rating.
• Add ageing headroom. End-of-life is defined at 80% of initial capacity, so design to the load with the battery already aged — typically 20% spare.

Stack these factors and the block you buy is comfortably larger than the naive Ah sum — which is exactly the point. A lightly loaded battery runs cooler, holds capacity and reaches its full cycle life.

Not sure which capacity fits your standby or cycling load? Browse our VRLA battery range or ask us to size it with you.