Deep Cycle Batteries and Discharge

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How depth of discharge affects the life of your batteriesA battery is called "deep-cycle" when it can withstand adverse cycling or draining. A car battery dies quickly if the headlights are left on a couple of times but a deep-cycle battery can endure a greater cycling level.

A battery that is discharged repeatedly and completely (100% depth) will last only 300 cycles whereas the same battery that is cycled to 25% discharge depth gives 1,500 cycles. This means that a larger battery bank that's discharged to a shallower depth offers longer life and greater capacity – although it costs more. A good maximum discharge depth level is 50%; normal levels are between 20% and 30%.

A battery system connected to the grid usually waits at full capacity until the next grid failure. If power outages are infrequent there is no cycling concern. But the reverse also holds; deep discharges may be a significant concern if there are frequent grid interruptions. If a long power outage happens, your batteries may have their capacity depleted, especially if you're inexperienced in their management. In such cases, battery-metering equipment should be seriously considered.