How To: Maintain batteries

| December 12, 2008

Most automotive batteries today are advertised as maintenance-free. Of course, this doesn’t mean they need no attention throughout their useful life. Regular inspection and testing are necessary for maximum longevity, but experts differ on how often “regular” is. Some truckers make it part of their normal PM schedules, but manufacturers say twice-a-year visits to the battery box are sufficient. Whatever the case, the effort will pay off in lower replacement costs and less downtime.

Positively factual
Here are a few facts to clear up myths and to highlight some interesting properties of batteries.

  • A concrete floor will not cause a battery to discharge faster than any other type of surface.

  • A fully charged battery freezes at minus 85 degrees F, but a battery with 25 percent of charge freezes at 5 degrees F.
  • A hot battery charges (and overcharges) faster than a cold one.
  • All batteries have normal discharge rates that increase with temperature.
  • The faster a battery is discharged, the less total energy it will deliver. This is known as Peukert’s Law.

The steps below apply to batteries featuring removable inspection caps. Please note: Most automotive batteries are filled with sulfuric acid and release hydrogen vapor. The first is highly corrosive, the second highly explosive. Safety precautions are essential. Never go near a battery with an open flame or lit tobacco. Always protect your eyes with goggles or safety glasses.

  1. INSPECT BATTERIES, CABLES AND CABLE ENDS. Remove the battery box cover and look for obvious signs of trouble: cracked or bloated batteries, large patches of sulfation (the greenish-white or bluish crusts that grow on cable ends), cracked or disfigured clamps, chafed or missing cable insulation, excessive dirt and grease. Using a power washer or a blend of water and baking soda, thoroughly clean the tops of the batteries to make inspection easier and avoid contaminating electrolyte later, when the battery caps are removed.

  2. DISCONNECT AND INSPECT CABLES. If you’re unfamiliar with the cabling, draw a diagram of the cables before disconnecting them from the batteries. Always remove the main ground cable first; otherwise, electrical shocks and sparks will occur if your wrench accidentally touches metal while you’re loosening a positive cable end. Examine all cables for damage and loose ends. If replacements are necessary, use only multi-strand products of pure copper to get the most flexibility and current-carrying capacity.
  3. CLEAN POSTS AND CABLE ENDS. Attach a rotary battery-post cleaning brush to an electric drill and scour the mating surfaces of posts and cable ends. In tight spaces, you may need to remove the batteries from the truck first. Use a high-pressure air hose or hand broom to clear loose debris from the tops of the batteries.
  4. TEST WITH HYDROMETER. With batteries fully charged, remove the vent caps from one and suction enough electrolyte to float the hydrometer’s measuring device. Do not remove the pickup tube from the cell. Note the reading, discharge the sample and repeat on the remaining cells and batteries. The specific gravity (density) readings should be within 50 points on a hydrometer’s scale. Significant differences indicate a weak cell; identically low readings indicate a discharged battery. Consult the manufacturer’s guidelines to determine variances for testing in hot or cold weather.
  5. CHECK ELECTROLYTE LEVEL. Before replacing the vent caps, check the amount of electrolyte in each cell. It should be touching, or near, the bottom of the filler tubes. If it’s low, add only distilled water. Never overfill the cells, because any excess liquid will be purged when the batteries heat up during operation, and that will dilute the electrolyte’s strength.
  6. DETERMINE TEST READINESS. Twelve-volt batteries must register at least 12.6 volts across the terminals to be properly load-tested. Use either a digital voltmeter or your load tester’s internal voltmeter (if it has one) to determine test readiness. If the voltage is sufficiently strong, attach the load tester’s clamps to the battery terminals.
  7. LOAD-TEST BATTERIES. Ratchet up the load to half the battery’s CCA rating, maintain for 15 seconds, then recheck the terminal voltage. The minimum should range from 9.1 volts at 30 degrees to 9.6 volts at 70 degrees. Anything lower would indicate a failing battery. The output of all batteries should not vary more than 2/10 of a volt. Otherwise the extra power of the stronger will be wasted trying to compensate for the weaker. Should a battery fail the test, replace it. If all the batteries in a failed battery’s group are more than a year old, replace them all.
  8. REATTACH CABLES. If the batteries were removed from the truck, place them in the tray and assemble the hold-down hardware. Connect all cables except the main ground, referring to your diagram if necessary. Only after all those clamps are tightened should you attach the final ground. Apply sealant to all the cable ends.


  • Combination wrenches

  • Half-inch socket set
  • Assorted screwdrivers
  • Digital voltmeter
  • Load tester
  • Hydrometer
  • Electric drill
  • Battery post cleaner
  • Battery post sealer
  • Battery charger
  • Putty knife
  • Wire brush
  • Eye protection
  • Distilled water
  • Shop towels

ACDelco:, (800) 223-3526
Exide Technologies:, (800) 782-7848
Interstate Batteries:, (888) 772-3600
Optima Batteries:, (888) 867-8462
East Penn Manufacturing:, (610) 682-4231

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