Discharge and recharge rates should be considered when selecting and sizing a battery bank. Battery manufacturers publish multiple discharge rates for each battery model, which range from 100 hrs to 1 hr. These are often referenced for various type of applications. The most common in Renewable Energy applications is the 20 hr rate as this closely matches a 1 day period. The rating, (ex C/20) refers to a controlled load (Amps) which can be placed on the battery for a period of time before the battery voltage reaches 1.75 VPC (volts per cell). A high amp draw may be run for a shorter period of time and vice versa.
As an example, a 400 AH battery can support a controlled 20 Amp draw for 20 hours (C/20). Alternatively, the same battery can support a controlled 34 Amp draw over a 10-hour period (C/10), meaning it supplies 340 AH capacity for that period of time. Batteries that are discharged should be recharged as promptly as possible.
An off-grid PV system should be designed to provide a charge current that is capable of recharging the batteries quickly, efficiently and within the window of time when the system is generating peak power (peak sun). The charge current should be within 10-20% of the 20 Hr AH rate (C/20) rate of the battery bank, or the C/4, C/5, or C/6 rate of the battery. Using the Absorption charge time calculation (charge current of 10% of the C/20 rate of the battery bank will take approximately 4.2 hours, plus the Bulk phase (usually 1-2 hours) to bring the bank from 50% to 100% SOC. This is an ideal scenario as a lower charge current will often increase the required charge time beyond the available sun hours causing deficit cycling and/or the use of supplemental charging such as a generator. Often customers who need to supplement charge with a generator do not run it long enough to allow the batteries to reach 100% SOC on a regular basis, causing sulfation problems and capacity loss which then needs to be addressed through corrective equalization.
In some situations, after an energy audit has been completed and the system is sized and installed, the customers will inadvertently add additional and/or unexpected loads. As system size is calculated with the desired depth of discharge and charge requirement based on expected usage, this increases both the storage needs and subsequent charge source to bring the battery bank to a full charge. It will often cause problems as a result of deficit cycling and over-discharge which will shorten cycle life.
To avoid this, the customer should be made aware of the importance of properly sizing the system to meet their specific needs. Discuss any expected short/long-term changes in usage and set the expectation that storage and charging needs may increase to meet their changing requirements.