Measure the voltage at the battery terminals with an accurate voltmeter. If the measured value is below 10 V, the charging current will be very low and additional charging time will be required.
Place the charger in a high place. Some chargers have a reverse polarity protection circuit that prevents charging if the battery terminals are connected incorrectly. A completely discharged battery does not have enough voltage to activate this circuit, even if the wires are connected correctly. This will result in the battery not charging. Therefore, follow the charger manufacturer's special instructions to ensure that the charger turns on and starts charging a low-voltage battery.
The chargers are equipped with voltage and current regulators. The time required to charge the battery depending on different voltage values are given below.
Battery charging time depending on the initial battery voltage
If the charge received by the battery cannot be measured after the charging time has elapsed, the battery should be replaced.
If the charge received is measured during charging, the battery is good and charging should be completed in the normal manner.
[Based on website data www.mazbook.ru]