Battery Bank Sizing Steps
Step 1: Raw capacity = daily load x autonomy days / DOD. Step 2: Ah at system voltage = raw capacity x 1000 / system voltage. Step 3: Batteries in series = system voltage / individual battery voltage. Step 4: Parallel strings = required Ah / individual battery Ah (rounded up). Total batteries = series x parallel.
Frequently Asked Questions
Battery bank (kWh) = daily energy use x days of autonomy / depth of discharge. For example: 4 kWh/day x 3 days / 0.8 DOD = 15 kWh. At 48V, that's 15,000 Wh / 48V = 312.5 Ah. Always round up to the next standard battery configuration.
Lithium iron phosphate (LiFePO4) batteries are increasingly preferred for solar storage due to their long cycle life (2,000-6,000 cycles), high depth of discharge (80-90%), low weight, and no off-gassing. Lead-acid (flooded or AGM) batteries have lower upfront cost but require more maintenance, have lower DOD (50%), and shorter lifespan.
Battery bank lifespan depends on the chemistry, depth of discharge, temperature, and charge/discharge rates. Lead-acid: 200-500 cycles at 50% DOD = 1-3 years in daily cycling. AGM: 400-700 cycles. LiFePO4: 2,000-6,000 cycles = 5-15+ years. Temperature extremes significantly reduce all battery types' lifespan and capacity.
C-rate describes how fast a battery is charged or discharged relative to its capacity. A 1C rate discharges the full capacity in 1 hour. C/10 (0.1C) discharges at 10% per hour = 10 hours to full discharge. Lead-acid batteries prefer slow charge/discharge rates (C/10 to C/20); lithium handles higher rates.
Wire batteries in series to increase voltage (add voltages, capacity stays the same). Wire in parallel to increase capacity (add Ah, voltage stays the same). For a 48V system with 12V batteries, wire 4 in series. For more capacity, wire multiple series strings in parallel.
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