By Paul Abelson
When Cadillac introduced the electric starter a century ago, it removed a chore — hand cranking — that had held back the automobile's market growth. The starter is noted in history, but few noticed the revolution in electrical systems it brought.
Batteries were needed to power starters, then generators — what we now call alternators — were needed to recharge batteries. Lead-acid chemical batteries replaced magnetos, which had previously provided current to spark plugs. For the rest of the 20th century, changing from positive to negative ground circuits and upgrading from 6-volt to 12-volt systems were the only advancements.
Today, we have gel-cells and absorbed glass mat (AGM) batteries. They offer great advantages, but they're still just premium-priced variations on the electrochemical lead-acid system. They also have greater deep-cycle properties and higher energy density, but are heavier and more costly.
Where electricity comes from
When the engine turns an alternator, rotational energy becomes electricity. After passing through a voltage-regulator, the electricity flows to devices that do useful work. The alternator works when the engine runs, but when the engine's off batteries become necessary.
A fully charged battery remains inert until a circuit connects it to a device. Trucks use flooded-cell lead-acid batteries: high-purity lead alloy and lead oxide plates are immersed in a bath of diluted sulfuric acid.
When the battery's disconnected, nothing happens because the circuit is open. When the circuit to the starter is closed, large amounts of current flow to the starter motor. Like a generator in reverse, the starter uses electricity to turn the engine until it starts running on its own.
The chemical reaction combines lead from the plates with sulfur from the acid to form lead sulfate. That reaction releases electrons to do what we want done. Losing sulfur weakens the acid, lowering the battery's ability to release electricity. That's how batteries run down.
When the alternator converts mechanical energy to electrical energy, it sends electrons back to the battery for charging. It changes lead sulfate back into pure lead and sends the sulfur back into the acid solution. This the process referred to as the discharge/recharge cycle.
Starting batteries — also called SLI batteries for “starter, lights, and ignition” — have thinner plates but more of them. Some have lead foam. They provide more current because they have more surface area, but don't run as long between discharges as thicker-plated deep-cycle batteries.
Having ample amps
Except for cranking amperage, batteries' properties are measured at 80 degrees F. Cranking amps (CA) are measured at water's freeze point of 32 degrees. Cold cranking amps (CCA) are measured at 0 degrees F.
At 80 degrees, battery output and amperage needed to crank a big diesel engine are assumed to be at 100%. Because oil thickens when cold, more current is needed. At 32 degrees , 65% more power is needed for starting; at zero, 150% more.
Engineers recommend a total of at least 1,800 CCA be available. That can be achieved with two 900 CCA batteries, but that leaves little reserve capacity for lights and blowers and no margin for error.
Today, most big trucks come with four batteries in the 650-to-675-CCA range or three in the 800-to-900-CCA range. Some use two 1,000-to-1,100- CCA starting batteries and two or more deep-cycle batteries to provide reserve capacity for lights, heaters, and fans.
Batteries versus capacitors
Most truck batteries tend to be optimized for starting engines, but this may soon change. New technologies like ultracapacitors and high-CCA batteries may allow fewer starting batteries, leaving room (and weight capacity) for additional deep-cycle batteries that are becoming a viable alternative to auxiliary generator units used to power accessories without idling.
Sometimes called supercapacitors or ultracapacitors, capacitors store electricity differently than batteries. Instead of using chemical reactions to store and discharge current, they accumulate electrons on one of two facing plates. Just one ultracapacitor can rapidly discharge the high amount of current needed to crank a diesel.
Capacitors can be discharged more than 75% and recharge in minutes. Lead-acid batteries, on the other hand, are fully discharged when they give up about 40% of their electrical energy. They then take hours to recharge, a process that's slowed even further in cold temperatures.
Capacitors do have their own issues and concerns, however. For an in-depth review, see Recommended Practice RP162 from the Electrical Study Group of the American Trucking Associations' Technology and Maintenance Council (TMC). For more information, call 703-838-1763 or visit www.truckline.com.
— Paul Abelson (email@example.com) is a former director of the Technology and Maintenance Council (TMC) of the American Trucking Associations, a board member of Truck Writers of North America, and active in the Society of Automotive Engineers.