How to Test a Deep Cycle Battery

Electric golf carts use deep cycle batteries.

A deep cycle battery may look like a regular engine-starting battery on the outside, but it is designed to perform in a much different environment. A starting battery is only used for a few seconds at a time to get the engine running, then is kept fully charged by the alternator to ready it for its next use. By contrast, a deep cycle battery can be used continually until it is fully discharged, which would eventually ruin a starting battery. Deep cycle batteries are used in recreational vehicles and boats as a backup for lighting and other accessories when no electric hook-up is available, or as power to electric vehicles, such as golf carts.

Things You'll Need

• Distilled water (optional)
• Baking soda (optional)
• Small brush (optional)
• Battery terminal brush (optional)
• Rag (optional)
• Battery load tester

Instructions

1. Allow the battery to cool and remove the caps on top of the battery. Check the electrolyte levels and add distilled water to bring the levels up to the full mark, if necessary. Replace the caps.

2. Clean the battery terminals if they are corroded with a solution of baking soda and water. Brush the solution onto the corroded areas and allow it to work in for a few minutes. Wipe the top of the battery with a clean rag. Use a battery terminal brush for severe corrosion.

3. Charge the battery fully by plugging in the electrical cord on your camper or boat or by plugging in the battery charger of your golf cart. Unplug the electrical cord or charger when the charge indicator shows that the battery is fully charged.

4. Load test the battery to determine if the battery has a defective cell. Start by removing the negative battery cable with an adjustable wrench and pushing it to the side. Remove the positive cable in the same fashion.

5. Connect the black cable from a battery load tester to the negative battery terminal, then connect the red cable to the positive battery cable. Rock the cable clamps back and forth to get a good connection, if necessary.

6. Press and hold the load switch on the load tester for 10 seconds.

7. Check the readout on the load tester. A reading of 12.45 volts or slightly higher means the battery has no dead cells, and a lower reading may indicate the battery is faulty. Release the load switch.

8. Disconnect the load tester from the battery. Connect the positive battery terminal to the battery first, then the negative terminal.

Tips & Warnings

• It does no good to load test a battery that has low electrolyte levels, has dirty or corroded terminals, or is not fully charged. Make sure these tasks have been completed or a load test may give you false results.

• Always wear safety goggles when working around batteries.

• Never add battery acid to deep cycle batteries. Add only distilled water and only after the battery has been fully charged.

References

• Marine Electronics: Battery FAQ
• Tradervar.com: Battery Load Tester Instructions