The 12V deep-cycle lead-acid batteries is an energy storage product specially designed for off-grid photovoltaic scenarios. Different from shallow-cycle batteries for vehicle starting, it adapts to the frequent charge-discharge mode of photovoltaic systems. During the day, solar panels convert solar energy into electric power to charge the battery; at night or on rainy days, the battery releases DC power, which can be converted into AC power for loads via an inverter. It realizes bidirectional conversion between electric energy and chemical energy through chemical reactions between electrolyte and plates. The charging current should be controlled within 0.2C of the battery capacity to avoid plate damage. Its stable charge-discharge cycle perfectly fits the intermittent power generation of off-grid photovoltaics.