There are several ways to determine if a lead-acid battery is fully charged, including voltage monitoring, changes in charging current, and observing battery status. Different methods can be used depending on the situation, as detailed below:

Voltage is the most crucial method. Lead-acid batteries have rated voltages such as 12V and 24V. The open-circuit voltage after a full charge (after disconnecting the charger and letting it stand for 1-2 hours) has clear standards:

12V lead-acid battery: The open-circuit voltage after a full charge should be 12.7V~12.8V; if in float charging mode (charger not disconnected), the float charge voltage is usually 13.5V~13.8V.

24V lead-acid battery (composed of two 12V batteries in series): The open-circuit voltage after a full charge is 25.4V~25.6V, and the float charge voltage is 27V~27.6V.

If the battery voltage remains below the above values ​​for an extended period, it may not be fully charged; if it exceeds these values ​​significantly (e.g., a 12V battery exceeding 14V and remaining thereafter), it may be overcharged.

Judging by changes in charging current (for compatible smart chargers): The current of a smart three-stage charger changes with the battery level:

In the initial charging phase, the current remains stable (e.g., 20A, 25A);
As the battery voltage approaches its rated value, it enters a constant voltage phase, and the current gradually decreases;
When fully charged and entering the float charging phase, the current drops to a very low value (typically 0.02C~0.05C, 4A~10A for a 200AH battery, eventually potentially dropping to 1~2A).

If the current remains stable at a low value for a prolonged period and does not decrease further, while the voltage remains stable at the float charging voltage, the battery is fully charged.

Judging by Battery Appearance and Condition:

Electrolyte Condition (Applicable only to maintainable lead-acid batteries): After a full charge, the electrolyte will have uniform, fine bubbles (from the electrolysis of water), and the liquid level will remain between the standard graduation marks. If there are only localized bubbles or no bubbles at all, the battery may not be fully charged.

Temperature and Bulging: After a full charge, the battery temperature will rise slightly, but it will not become noticeably hot. If the battery bulges or the casing becomes severely hot, it is likely overcharged, not undercharged.

Judging by Charging Time: Combining battery capacity and charging current, the charging time can be estimated: For example, a 200AH battery charged at 20A will take approximately 10 hours in the constant current phase, plus 2-3 hours in the constant voltage and float charging phases, for a total charging time of about 12-13 hours. If the estimated time is reached and the charger indicator light turns "fully charged" (e.g., green), the battery can be considered fully charged.