check the batteries, make sure they have enough fluid in them. If the fluid level is below the cell plates inside add distilled water till they cover up the plates (do not fill to the top of the cell cap opening).
If the charger is constantly running it could be that the batteries are not getting to the maximum voltage to trigger the charger to go into finish mode and shut down. After adding fluid/water run the charge cycle again and see if the charger will go into finish mode and shut off.
If adding distilled water does not fix the problem and the charger continues to run and not shut down you will need to test the batteries to see if they are damaged, they may need to be replaced. Each battery should read at least battery volts, providing these have a set of 6volt batteries they should read 6 to 7.5v per battery depending on the charge level, each cell should read 1.2 to 1.5v or higher depending on the charge level. if you read lower than this then there may be bad cells.
If the batteries pass the tests then i would look into the charger by connecting it to some known good batteries and see if it operates correctly, if it does then replace the bad batteries in the machine. If it does not then look to getting the charger repaired or replaced.