Cycling of a Battery - Deep versus Shallow

Batteries are designed to absorb and give up electricity by a reversible electrochemical reaction.

A cycle in the battery world occurs when you discharge a battery and then charge the battery back again to the same level. The battery is designed to absorb and give up electricity by a reversible electrochemical reaction. How deep a battery is discharged is termed depth of discharge. A shallow cycle occurs when the top 20% or less of the battery's power is discharged and then recharged. Some batteries, like automotive starting batteries, are designed for this type of cycling only. The plates of active material are thin with large overall surface area. This design can give up lots of power in a very short time.

The second type of cycle is a deep cycle where up to 80% of the battery capacity is discharged and recharged. Batteries designed for deep cycling are built with thicker plates of active material which have less overall surface area.

Because of the lessened availability of surface area for chemical reaction, these batteries yield just as much power relative to their size, but do so over a longer period of time. This type of battery design is preferred for a PV system because discharging a battery to a deeper level is normal during extended cloudy weather.

The depth of cycling has a good deal to do with determining a battery's useful life. Even batteries designed for deep cycling are "used up" faster as the depth of discharge is increased. It is common practice for a system to be designed with deep cycle batteries even though the daily or average discharging amounts to a relatively shallow depth of discharge. Shallow cycle your deep cycle battery for the most cycles.

Temperature Effects

The speed of the chemical reaction occurring in a lead-acid battery is determined by temperature. The colder the temperature the slower the reaction. The warmer the temperature the faster the reaction and the more quickly the charge can be drawn from the battery.

The optimum operating temperature for a lead-acid battery is around 77 degrees Fahrenheit. You may have experienced this effect when starting a car on a cold morning; the engine just doesn't turn over as quickly. Warm that same battery and you will see a major improvement.

For this reason we like to see batteries placed indoors or in a heated and ventilated space to maintain them between 55 and 80 degrees. If we do install them in a unheated space, battery capacity must be increased to compensate for this de-rating. High temperatures can drastically shorten the life of the battery and should be avoided.

Next - Battery Voltage and Charge

Return to Batteries

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