The issue is usually with the battery, not the charger. You can test the charger with another cell to see if it works.
Use a voltmeter to test each cell’s voltage capacity. Charge your battery fully before measuring its voltage. Using a 6-volt cell, the total capacity reading should be 6.2-6.3V. If it reads 4V, it needs to be repaired or replaced if there are any leaks or damages.