Maintenance Q&A
Failing batteries

By Paul Abelson, senior technical editor

¬†Q. I know this has been a particularly severe winter, but I don’t think my batteries should have gone dead as much as they did. I needed six jump starts this winter. My 2006 Peterbilt came with four 625 CCA batteries, but the last time I replaced them, I decided to save some weight. I bought three 900 CCA batteries to get more amps (2,700 vs. 2,500) and eliminate one battery. Is this why they kept dying? What should I do about it? The batteries are only two years old.

A. Many things can contribute to short battery life, both directly and indirectly. Switching from four to three batteries may be an indirect cause, but there are many things to check first. As you mentioned, it was a severe winter with record, or near record, snows in many areas. Some communities even ran out of salt and snow- and ice-fighting chemicals in February or early March. Many substituted sand for salt, adding an abrasion factor. This record use of chemicals may have taken its toll on your wiring.

The first thing to check is wire condition on the tractor and, if you pull your own, the trailer. Unless connectors and wire harnesses are completely sealed, fine spray from treated roads will find any opening in the wires or connectors. It will wick into the strands and corrode the wires under the insulation. Corrosion increases electrical resistance, lowering wire’s current-carrying capability. Eventually, more and more battery power will be needed to do the work done easily by the alternator alone in warmer weather or with undamaged wire.

Check for internal corrosion on battery cables at the alternator and on connectors and cables at the batteries. Your Pete’s batteries are under the cab step where they are bombarded with spray from your steer tires. They are also hung outside the frame rails rather than being nestled between them, as recommended by TMC. There, they are subject to more vibration and shock loads. This becomes more important as the batteries age, especially since you changed battery configuration.

All flooded lead-acid batteries are made with lead and lead sulfate plates submerged in a sulfuric acid solution. The plates are separated with inert plastic grid material. This keeps the plates from touching each other and creating a short circuit within each battery. The plates are in sets, with each set capable of producing a little over 2 volts when a circuit is completed. In series, six groups, called cells, produce the nominal 12 volts each battery is rated at.

Below each group of cells is a well. As the plates function, they are subject to road shock and vibration. Material flakes off from the plates and collects in the wells to keep it from shorting out the cells. If the wells fill, they short the battery.

All Group 31 batteries, the size most commonly used on Class 8 trucks, must be the same size to assure they are interchangeable. In order to get more amperage into a battery, more lead (larger plates) is needed. With fixed battery dimensions, room for the plates is gained by making shallow wells. Higher-quality batteries put additional protection around the plates, but wells still fill eventually. That’s why the switch to higher CCAs batteries may have affected battery life indirectly.

Another factor may have been increased idling. Today’s alternators put out maximum current with the engine at or near 1,000 rpm and above. Idling in traffic or in snow may drop rpm to as low as 600 rpm. Your alternator may turn only 3,000 rpm or less, not enough to deliver sufficient charging current. Your battery, not your alternator, will be providing current to run your heater and defroster fans, your headlights and all your accessories. When you get going again, you may not fully replenish the batteries.

In order to fully charge batteries, the alternator must push out current at about 14.2 volts. At low rpm, voltage is reduced. Regulators limit only high voltage. Slipping fan belts, caused by belt wear or worn automatic belt tensioners, can affect charging.

Here are a few things to check before making any major decisions about batteries. Make sure wiring is intact. Check connectors and ground terminals for corrosion. Be sure no wicking of corrosive salts has occurred. Check the charging system and the components that drive it. Include drive belts, tensioners and cables. Have your alternator output checked.

Check your batteries with a carbon pile load tester, which will indicate whether any cells have shorted out. Merely checking a battery’s specific gravity will tell you about the concentration of sulfuric acid electrolyte, but not about the battery itself.

When temperatures fall, oil thickens and more energy is needed to crank your engine. But as temperatures rise, battery output is greatly reduced. At 0 degrees, the temperature at which cold cranking amps are measured, available battery power is just 81 percent of what it was at 80 degrees while cranking power required to start is two and a half times (250 percent) what is needed at 80 degrees.

Absorbed glass mat batteries were developed to combat vibration and shock. They have a sealed construction and do not need bottom reservoirs since there is no internal battery debris. Electrolyte is absorbed into fiberglass separators. AGMs cost more but last far longer and have more power. They do, however, require special chargers to avoid internal damage. Depending on how long you plan to keep your Pete, you may want to consider them. LL

DO YOU HAVE A maintenance question?
Send your question to Paul Abelson, senior technical editor, in care of Land Line Magazine, PO Box 1000, Grain Valley, MO 64029; email them to or fax questions to 630-983-7678. Please mark your message Attention: Maintenance Q&A.

Although we won’t be able to publish an answer to all questions in Land Line, we will answer as many as possible.