State of charge is the amount of charge that can still be drawn from a battery, given as a percent of its nominal capacity, where 100% means full and 0% means empty. For example, if 25 Ah is taken from a 100 Ah battery, its state of charge is 75%. Inverters such as the Sunny Island work out state of charge by counting ampere-hours in and out of the battery, then correct it using full-charge detection and battery voltage. After a boost, full or equalization charge, the calculated value is reset, and an estimated error value shows how accurate the current reading is. Lithium-ion batteries that have their own battery management system send their state of charge to the inverter, and the BMS uses it to adjust charge limits. If the battery is not brought to full charge from time to time, cells can drift apart and the displayed value can stop matching the real state of charge, so periodic calibration matters. Example conservation ranges are 60% to 65% for lead-acid and 25% to 30% for lithium-ion, and backup reserve and depth of discharge settings are also expressed against state of charge. A charge controller regulates battery state of charge in most PV systems with battery storage.
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related_toc1 drafted · lead SoC estimation methods
SOC is found by ampere-hour counting with full-charge detection, plus recalibration from battery voltage.
sec=1.2 h=0872223561b6sec=h327 h=2e9d5ed84e19sec=h1 h=a8ba0e902184c2 drafted · lead PV charge controller
Most PV systems with battery storage need a charge controller to regulate battery state of charge.
fig=65 h=0335ef930464c3 verified · fact Float charge
Float charge ends when SOC drops below 70% of available charging capacity; reaching 100% by float charge takes several days.
sec=h17 h=668e52e11175sec=h640 h=1238d4052c9bverified by: review-workflowverified on: 2026-10-24
c4 drafted · lead Battery charging stages
SOC is reset to 90%, 95% or 100% after boost, full or equalization charge respectively, under default settings.
sec=h327 h=2e9d5ed84e19sec=2.2 h=e012c72d98e3c5 drafted · lead Lead-acid battery
With lead-acid batteries the Sunny Island needs several cycles to calculate SOC correctly and shows 50% after commissioning.
c6 drafted · lead Lithium-ion battery
A lithium-ion BMS reports SOC to the inverter and adjusts charge limits using SOC.
sec=4.1 h=71239ba9a2d6sec=h1 h=a8ba0e902184c7 drafted · lead Backup reserve
sec=h21 h=58f7fdd29624tbl=131.r5 h=eb5e7bb9c3fcreview note: support check: partial - Backup reserve percentages and a state-of-charge maintenance range appear, but no explicit relation between SOC and backup reserve is stated.
c8 drafted · lead Depth of discharge
sec=h21 h=58f7fdd29624review note: support check: partial - Depth of discharge is shown with percentage figures, but SOC itself and its relation are not explicitly stated.
related_to c6related_to c5related_to c1related_to c5related_to c1explains c1related_to c1