By Paul Abelson, senior technical editor
Q: I have a battery problem. In March 2010 I got a brand new Kenworth T660 with a Cummins engine that’s legal to idle. Everything was fine until November when I sat out a snowstorm in Montana. I idled to keep warm. Even though it burns almost a gallon an hour, I figure it will take a lot of idling to equal the price of an APU.
The first day, everything was fine. But the second day, I shut off the truck to get a shower and have breakfast. An hour later, the truck wouldn’t start. It was jump started. The Kenworth dealer in town said everything checked out OK. The truck ran fine for about a month or so; then the same thing happened again. Not even a snowstorm, just cold. The truck wouldn’t crank. Again to a (different) Kenworth dealer. This time, I insisted they replace the batteries, which they did under warranty.
Same again in March. I knew then to look beyond the obvious. I had them check the alternator, belts and wiring. Nothing. The batteries all tested good. This is getting expensive and my dealer said no more batteries. Can you help?
A: You are right to look beyond the immediate and obvious. The problem involves the batteries, but they are a symptom, not a cause. Before we look for causes of your battery failures, let’s examine some basics.
Batteries rely on chemistry to store and release energy. A sulfuric acid solution reacts with lead and led oxide plates. As long as switches are off, nothing happens. With switches on, chemical reactions occur. The acid reacts and creates lead sulfate on the plates and releases electrons. The more sulfate, the less oxide and pure lead remains, reducing the ability to generate current.
During charging, electricity reverses the chemical process, sulfur from the sulfate recombines to strengthen the weakened sulfuric acid and oxygen recombines on the positive lead plates. These chemical processes are temperature sensitive. The colder it is, the less available power the batteries have and the longer it will take to recharge the batteries.
Battery properties, except for cranking amps (CA) at 32 degrees and cold cranking amps (CCA) at 0 degrees, are measured at 80 degrees. Without any discharge, a battery charged to 100 percent at 80 degrees will have only 83 percent power at 32 degrees, 61 percent at zero and just 45 percent at 20 below. Meanwhile, starting demands from engines increase as temperature drops. At 32 degrees, 65 percent more energy is needed to crank an engine than at 80 degrees.
But the batteries are out in the cold, not under hood as in a car. Their ability to recharge was affected. When your engine idled, it probably ran at 600 to 800 rpm. Alternators put out maximum current at about 5,000 alternator rpm, or about 1,000 engine rpm or more. The output curve of an alternator climbs steeply, then levels off at 5,000, so at lower rpm current output was low.
With the batteries reduced ability to take a charge and the demands of your truck’s hotel loads (heater fans, fridge, cooking equipment, computer and entertainment system) you probably weren’t generating enough current to replace what you used. Even with the engine on, you ran down your batteries.
Your charging system works, but a low to medium idle isn’t fast enough to generate the current your batteries need for a cold weather 34-hour turnaround.
If you maintain a high idle, 1,000 rpm or more, fuel consumption will suffer but your battery problems may be solved. In your operating environment, I would re-examine the operating economics of an APU. LL
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