Marine Battery Maintenance

Properly Maintain Lead-Acid Batteries for Long Service and Safety

Aug 16, 2007 Alan Sorum

Most vessels use lead-acid storage batteries for starting engines and operation of onboard electrical equipment. Proper maintenance will lengthen their service life.

Lead-acid Batteries used on boats present several hazards to owners interested performing their own maintenance. Lead is a well known toxic heavy metal. It shouldn't be a danger unless the battery case is broken. The electrolyte used in the battery composed of sulfuric acid and water. Sulfuric acid will severely burn eyes and skin. Goggles and acid resistant gloves should be worn while servicing the fluid levels of conventional lead-acid batteries. Charging a battery produces explosive hydrogen gas. Avoid introducing sources of ignition near the battery and provide good ventilation. Wash hands after working around batteries.

Visual Inspection - Marine battery maintenance begins with a visual inspection:

  1. Look at the exterior of the battery. Look for cracks in the case, dirt or corrosion on terminals, and leaking electrolyte. Batteries with any external damage must be replaced and recycled
  2. Check battery cables and connections, looking for any damaged components. Cables need to be clean, insulation intact and not be frayed. Connections need to be clean and tight
  3. Tighten battery connections to manufacturer's standards

Open Circuit and Specific Gravity Testing - External visual inspection doesn't tell the whole story of battery condition. Testing battery open circuit voltage and specific gravity of its electrolyte give a more accurate picture of the battery's health. The electrolyte can only be checked on flooded electrolyte batteries.

Specific Gravity Test - Specific gravity describes the density of a fluid as compared to water. A number higher than one means the liquid is denser than water. A hydrometer purchased from a parts store is used to test specific gravity of the electrolyte.

  1. Do not add water prior to testing
  2. Fill and empty the hydrometer with electrolyte from the cell to be tested two or three times before pulling out a sample
  3. There is a scaled float inside the hydrometer. Fill the tube with electrolyte high enough to freely float the scaled float
  4. Record the value from the scale and record it. Return the electrolyte to its cell. Repeat the process for each battery cell
  5. Replace vent caps for each cell and ensure the battery case is clean
  6. Correct scale readings for temperature. A chart will be printed on the float or come with the hydrometer
  7. Reading between cells should be similar and be at or above 1.277 ± .007
  8. If readings are below 1.277 ± .007, fully charge the battery and retest it

A battery with a specific gravity below 1.277 ± .007 may be old and near the end of its useful life. Other contributing factors can be excessive discharge, loss of electrolyte, or a bad battery cell

Open Circuit Voltage Test - Accurate testing depends on the battery being left idle with no charging or discharging for at least 6 hours with 24 hours being preferable.

  1. Disconnect the battery cables
  2. Measure voltage using an accurate DC voltmeter
  3. Compare the value with a state of charge chart

Abbreviated Charge Chart

  • 100% 12.73 volts
  • 80% 12.50 volts
  • 60% 12.24 volts
  • 40% 11.96 volts
  • 20% 11.66 volts

Electrolyte Levels for Flooded Batteries – On batteries that allow access to the electrolyte, the fluid should cover the interior lead plates. Water should be added after fully charging a battery. Check electrolyte levels at least once a month. There is a fill well visible under the vent cap. Fill to within an 1/8 inch of the bottom of the well. Avoid overfilling the battery. Try to use distilled water for servicing a battery. Ensure vent caps are tightened after checking fluid levels. Never add acid to a battery.

Cleaning - Keeping batteries clean helps with making external inspections. Cracks and leaks will be more visible. Wear acid resistant gloves and goggles while cleaning a battery.

  1. Check that vent caps are secure
  2. Clean the top of the battery with a cloth and a mixture of baking soda and water
  3. Rinse off the battery with fresh water
  4. Clean terminal connections and battery post with a cleaning tool sold for this purpose
  5. Place a thin coat of petroleum jelly on terminal and connectors
  6. Keep the battery location clean and dry

The copyright of the article Marine Battery Maintenance in Boating & Sailing is owned by Alan Sorum. Permission to republish Marine Battery Maintenance in print or online must be granted by the author in writing.
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Comments

Dec 3, 2008 4:25 PM
Guest :
Hydrochloric acid?? I thought that flooded lead-acid batteries contained about 37% Sulfuric acid
Dec 6, 2008 9:21 PM
Alan Sorum :
Good catch. It is sulfuric acid. Thanks.
2 Comments