Solar Battery Cable Sizing
Battery interconnect cables carry very high current at very low voltage — a combination that demands genuinely thick conductors.
Battery interconnect cables — running between battery modules, and from the battery bank to the inverter-charger — are often visibly, dramatically thicker than any other cable in a solar-battery system. There's a clear reason: they carry very high current at very low voltage.
Power equals voltage times current. Delivering a meaningful amount of power (several kW) at a low battery voltage (12V, 24V, or 48V) requires proportionally much higher current than delivering the same power at, say, 230V AC — and higher current directly demands a larger conductor cross-section for both ampacity and voltage drop reasons.
A percentage voltage drop on a 48V battery circuit represents a much smaller absolute voltage margin than the same percentage on a 230V circuit — 3% of 48V is under 1.5V, a genuinely tight number to hit on any meaningful cable length. This is why battery interconnect voltage drop limits are typically held very tight, often below the limits applied to general AC circuits.
Never assume a "reasonable-looking" cable size for battery interconnects based on general intuition from AC wiring experience — run the actual current and voltage-drop numbers explicitly, since the combination of high current and low voltage produces results that surprise people used to AC-side sizing.
Engr. Jason Morales — Founder, SolarEnergyPH & OhmWorks