Big Rig Basics

John Baxter | May 01, 2010

Test for the Deep Cycle

Keep an eye on battery-cell water levels to keep voltage high longer

By John Baxter


Tim Ruth, president of Warehouse Battery Outlet, says truck batteries traditionally offered high CCA, or “cold cranking amps,” because starting the diesel was the primary load. But now that truckers shut down and run various accessories all night, deep cycling, where much of the battery’s stored power is taken out every night and replaced each day, is the battery’s biggest challenge.

Traditional batteries jam a large number of thin plates into each of the 6 cells that make up a 12-volt truck battery. This gives high amps for starting, but when the battery’s charge is pulled down a lot each night, the thin plates warp and bend and soon will short out. Putting on a larger alternator only makes matters worse.

Most trucks today need deep-cycle batteries with a lot of reserve capacity, which is rated at a low amperage draw typical of what cab accessories need when running. A battery with 200 hours of reserve capacity has fewer, thicker plates. It offers a little less CCA, but will last much longer with overnight shutdowns.

Ruth also suggests that when you operate in a no-idle situation to check water level frequently, as the water in the cells will evaporate quickly. Following find steps to do just that.


1. Technician Duke Smith first disconnected the hold-downs and removed the Peterbilt’s battery box cover. If doing this for the first time, draw a diagram of the cable connections before proceeding.





2. He next removed all the negative terminal attaching nuts and disconnected all the negative cables to prevent a short circuit throughout the process.





3. All the battery hold-down nuts had to be loosened and the hold-down bracket had to be removed for access to the cell covers and to inspect the hold-down system.





4. One bad hold-down long bolt broke during disassembly. This was replaced during assembly, because secure mounting means less vibration and longer battery life.





5. Smith removed all the cell covers for testing with a hydrometer by drawing the electrolyte fluid into the instrument.






6. A healthy cell causes all the balls to float near the top of the electrolyte.







7. Smith found a bad cell, indicated by all the balls floating near the bottom of the hydrometer. Ruth said you can also check condition by measuring voltage with the engine off. A battery is dead at 12.1 volts, and fully charged at 12.6 volts.





8. The battery with a defective cell was replaced. It’s normally best to replace all the batteries when this happens to make sure they all do the same amount of work.






9. Clean all cable connections with a powered wire brush.







10. Also clean all the battery posts, even on a new battery, to get good connections.






11. After replacing all the caps and the hold-down mounting bracket, tighten the bracket mounting nuts securely. Then install and gently tighten each positive cable and post connector. Note how Smith holds the wrench to prevent damaging torque.




12. Finally, install all the negative post connections, then coat each connection with di-electric grease or spray protectant.






13. Reinstall the battery box cover and install its hold-downs securely.






14. To bring a discharged battery to its full 12.6 volt charge, charge it at a low rate for 18-24 hours, not just 2 or 3 hours, says Ruth.





We thank the staff of Warehouse Battery Outlet, Quakertown, Pa., for their help with this article.

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