How to Add Distilled Water to a Marine Battery

How to Add Distilled Water to a Marine Battery

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Marine batteries store and deliver 12V (volts) DC (direct current) power for the boat's systems. United States Coast Guard regulations dictate that the boat's VHF (very high frequency) radio and running lights must be powered by the boat's batteries. Marine batteries must be properly maintained in order to preserve maximum voltage output and storage capacity. Distilled water must always be used to top off the cells of the battery to prevent sedimentation in the cells as the water evaporates out of the vent, leaving mineral deposits behind in the battery.

Items you will need

  • Clean rags

  • Safety glasses

  • Small funnel

  • Distilled water

Step 1

Clean the top of the battery with a rag. Remove any water, electrolyte solution and grit/debris from the battery before removing the caps to prevent the introduction of foreign material to the battery cells.

Step 2

Put on the safety glasses. Rotate the battery caps in a counter-clockwise direction, then remove the caps from the cells.

Step 3

Insert the small funnel into each battery cell. Fill each cell with distilled water until the cell is full up to the top of the fill hole. Do not overfill the cell as that will cause electrolyte solution and battery capacity to be lost.

Step 4

Replace the battery caps and tighten them down, by hand, in a clockwise direction. Remove any fluid present on top of the battery with a clean rag to prevent what is known as "static discharge" across the top of the battery due to moisture connecting the positive and negative posts.


  • Use caution when removing the caps. A plugged battery vent can cause pressure to build up in the battery and may cause a spray of battery acid (electrolyte) to be released as the cap is removed. Battery acid can burn skin and mucus membranes and can cause severe injury to eyes.


  • Charge the battery and allow it to rest if you plan to check the specific gravity (SpG) of the battery after adding water. The charging process will mix the new water into the old electrolyte and give an accurate SpG reading. Checking the SpG without charging first will give a false low reading as the water will be at the top of the cell and will register as being low when checked with a hydrometer.
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