Water is lost during charging.
Therefore, the best time to water your batteries is always at the end of the charge cycle. Also, as a battery is charged, heating causes the volume of liquid to increase, so there may be a dangerous overflow of electrolyte if it was full when cold. How often you use your batteries will determine the frequency of watering. For example, the weekend fisherman may find he only needs to water the batteries in his boat once a month. A maintenance supervisor for a golf course might need to service the batteries in their golf cars every week.