How should a battery set and battery charger be sized for a DC supply system?
The battery capacity shall be based on design load. The design load equals to 125% of the DC load demand including 15% spare for ancillary loads and 10% considered for battery aging. If not determined in project design specifications, the selected battery should usually supply power for approximately 48 hours. Therefore
C50 = 125% of DC demand load (A) × Autonomous hours (usually 48 h for stand-alone systems)
C50 is the 50 hours rate capacity of the battery which can be found in documents provided by battery manufacturer. Generally Cx is the battery capacity in Ampere hours (Ah), specified for an appropriate discharge rate (A). Note that batteries have different capacities for different discharge rates.
Sometimes there are determined specifications in projects which must be followed in battery selection. The requirements below are example of project requirements.
Batteries supplied as part of battery charger system shall be either sealed lead acid battery or lithium ion battery. Standard (non-sealed) lead acid batteries will generate hazardous substances (i.e. hydrogen) during operation and shall not be used. Batteries shall be sized to supply the base station load for a minimum period of 48 hours to and end voltage of 87.5% of the normal voltage.
Battery charging rate and voltages should be manufacturer’s recommendations. Battery chargers should be typically sized at the 10h rate (AS/NZS 4509.2). That means the charger is sized so the re-charge time from maximum depth of discharge and no load on the system does not exceed 10h. Note that this is just for charger sizing and does not imply that the battery can be fully charged in 10 hours. The following equation can be used to estimate the required size.
Ibc = 0.1 × C10 + Is
- Ibc: Maximum charge current of the battery charger in Amps.
- C10: 10h rate capacity of the battery. The battery bank is selected according to the previous section based on C50 supply capacity. Please consider that C10 does NOT always equal to 5×C50. C10 should be found from manufacturer documents. If documents are not available, use C10 = 5×C50 estimation.
- Is: the current feeding the field consumers by battery charger while charging the battery bank; if required by project specifications, otherwise to be neglected.
The project requirements may have less conservative concerns for charger sizing. The requirements below have been used in one of projects.
Battery chargers shall be capable of supplying maximum station load as well as charging the attached batteries to 85% of capacity within 16 hours after a full battery discharge. Battery chargers shall supply an output voltage not exceeding 25.8 VDC in a 24 VDC system. This output shall have less than 2% ripple. Battery voltage sensing for all battery chargers shall be directly at the battery banks and not at a remote termination.