Battery-Storage System Design
The battery capacity necessary, the battery voltage and the suitable battery type are decisive factors when selecting the battery.
Battery capacity
The starting point when selecting a battery is the necessary battery capacity. The necessary battery capacity depends primarily on the following factors:
Bridging time
The bridging time is the time period, in days, for which the off-grid system can supply the loads exclusively from the battery. For off-grid systems with generators, a bridging time of 1 day should be expected.
Energy consumption per year (EAnno)
The energy consumption in the off-grid system to be expected per year is dependent on the loads installed and their energy requirements ( > Estimating the Electrical Loads).
Average battery efficiency during electric discharge (ηBatt)
For off-grid systems, the average battery efficiency when discharging is approximately 0.9.
The battery capacity is normally specified in kWh or Ah.
Calculation:
Observe the usable battery capacity
In order to achieve the longest possible battery service life, only the usable range of the battery should be used for charging and discharging. The battery capacity calculated here is based on this usable range. With lead-acid batteries, a usable range of approximately 50% of the nominal capacity is typical, with that of lithium-ion batteries being approximately 80% (see battery manufacturer documentation). If the battery manufacturer specifies a depth of discharge of 100%, SMA recommends for safety reasons that a depth of discharge of only 90% be expected.
Observe standard sizes
Batteries are not available in all sizes. Battery manufacturers offer standard sizes. Selecting the next highest standard size is to be recommended. Observe the usable battery capacity when selecting.
Not the minimum battery size
Minimum battery capacity in terms of lead-acid batteries: 100 Ah per installed kWp of a PV system
Minimum battery capacity in terms of lithium-ion batteries: 50 Ah per installed kWp of a PV system
Lithium-ion batteries in off-grid systems
In order to meet the requirements of off-grid applications, the Sunny Island has a high overload capacity. To be able to use this overload capacity, the output powers and output currents must be taken into consideration when selecting the battery. Clarify whether the battery can supply these currents. Note the list of approved batteries (see Technical Information) and the data sheets as well as the minimum configuration list of the manufacturer, if applicable.
Nominal battery voltage
All Sunny Island devices use batteries with a nominal voltage of 48°V.
Battery type
The Sunny Island is compatible with the following battery types:
Lead-acid batteries
The battery room must be ventilated in accordance with the battery manufacturer specifications and with the locally applicable standards and directives (see battery manufacturer documentation).
Lead-acid batteries of the designs VRLA and FLA are used with Sunny Island devices.
Lithium-ion battery
If connecting a lithium-ion battery, the following must be observed:
The battery must comply with the locally applicable standards and directives and must be intrinsically safe.
The lithium-ion battery must be approved for use with the Sunny Island. The list of lithium-ion batteries approved for the Sunny Island is updated regularly (see the technical information “List of Approved Batteries” at www.SMA‑Solar.com).
If none of the lithium-ion batteries approved for use with the Sunny Island can be used, lead-acid batteries must be used.
DC fuse
The external DC fuse protects the battery connection lines of the battery inverter. Furthermore, the DC fuse enables DC-side disconnection of the battery inverter.
A DC fuse must always be installed between the battery and the Sunny Island. SMA Solar Technology AG recommends the use of a DC fuse specially aligned for use with the Sunny Island from enwitec electronic GmbH & Co.KG.
The fuse links in the DC fuse must also be aligned for the Sunny Island (see Sunny Island battery inverter installation manual).