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In a stand-alone PV system with direct coupling to the user (without inverter), the battery voltage determines the distribution voltage. As now many DC appliances can be found as well in 24V as in 12V, this choice should be made according to system and/or appliance power, as well as the extension of the planned distribution grid to minimize the ohmic wiring losses.
This choice should be done from the early planning of an installation, since the existing appliance voltage usually cannot be changed, and voltage translators will be 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: little 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 the other ones, it will become discharged earlier. As all the cells are connected in series, i.e. with the same current,it may encounter deep discharges, or even reverse polarization (the forced current may reverse the polarity, like for the Hot-spot in a PV array). This will still damage further this bad cell. In the same way, during charging, as the capacity is lowered, the overcharging conditions will produce Gassing before the other cells, leading to loss of electrolyte.
Therefore in high voltage battery packs, in absence of a careful maintenance or compensating strategies, the risk of failure is strongly increased.
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