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Battery Voltage Choice

In a stand-alone PV system with direct coupling to the user (without inverter), the battery voltage determines the distribution voltage. Today, many DC appliances are available in both 24V and 12V. This choice should be based on system and appliance power, as well as the planned distribution grid extension, to minimize ohmic wiring losses.

This choice should be made during the early planning stage of an installation because existing appliance voltages typically cannot be changed, and voltage converters are expensive and not 100% efficient.

The rated distribution values could be chosen according to the following criteria (inverter supposed directly connected on the battery pack):

  • 12V: small systems for lighting and TV:
    • Appliance max power < 300 W,
    • Corresponding current 25 A,
    • Inverter : about < 1 kW.
  • 24V: medium size, household with fridge and little appliances, or wiring extension to more than 10 m:
    • Appliance max power < 1000 W,
    • Corresponding current 42 A,
    • Inverter : about < 5 kW.
  • 48V: special industrial or agricultural use:
    • Appliance max power < 3 kW,
    • Corresponding current 62 A,
    • Inverter : about < 15 kW.

Higher powers require either high DC voltages (special appliances) or AC feeding through inverter.

NB: In a battery pack, when a cell is weaker than others, it will discharge earlier. Because all cells are connected in series with the same current, it may experience deep discharges or even reverse polarization (similar to hot spots in a PV array), which will further damage the weak cell. Similarly, during charging, as capacity decreases, overcharging will cause gassing before other cells, leading to electrolyte loss.

Therefore, in high-voltage battery packs without careful maintenance or compensating strategies, the risk of failure is significantly increased.