Clark forklift with error code -15 - Clark Forklift Horn Button Switch , Cat Etc

The description for code 15 is battery voltage to low. The code will flash when the battery volts are less than 1.95 volts per cell when the key is turned ON. Check the battery voltage with volt meter at battery were positive and negative connect. If the battery is 36 volt it should be no less than 35.1 volts, if the battery is 48 volt it should be no less than 46.8 volts. If battery voltage is low check each cell indavidually to locate a bad cell, each cell should have around 1.95 volts, if a couple are dramatically lower that may be your problem. If the battery checks out check your cables and connections (this seems likely because you say its after you charge battery, when you plug and unplug your battery in you wiggle the cables around witch could cause a poor connection.) Check over your cables carefully as corosion can grow inside of them especially near the battery and connector. Hopefully this helps, looking forward to positive feedback thanks.