A little more than a century ago, Cadillac introduced the electric starter for cars. It removed a chore, the hand crank, that held back market growth. The starter is noted in history, but few noticed the revolution in electrical systems it brought about.
Batteries were needed to power starters. Then, generators (alternators today) were needed to recharge the batteries. Previously, magnetos provided current to spark plugs. Lead-acid chemical batteries replaced magnetos. For the rest of the century, changing from positive to negative ground circuits and upgrading from 6- 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 are still just premium-priced variations on the electrochemical lead-acid system. They are suitable for concrete producers, as long as they are willing to pay the higher price.
The new batteries have greater deep cycle properties and higher energy density. But they are heavier and more costly than traditional flooded-cell, lead-acid technology.
Where does the electricity come 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 it is off, batteries become necessary.
A fully charged battery remains inert until a circuit connects it to a device. Trucks use lead-acid batteries. High-purity, lead-alloy, and lead-oxide plates are immersed in a bath of diluted sulfuric acid.
When the battery is disconnected (the circuit is open), nothing happens. 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.
Running down batteries
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 is 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. That is the discharge/recharge cycle.
Starting batteries, also called SLI (starter, lights and ignition) batteries, have thinner plates but more of them. Some have lead foam. They provide greater surface area, so higher amounts of current can be delivered. SLIs run less time between discharges than deep cycle batteries with their thicker plates.
Except for cranking amperage (CA and CCA), batteries' properties are measured at 80 degrees F. CA, cranking amps, are measured at water's freezing point, which is 32 degrees F. CCA, cold cranking amps, are measured at 0 degrees F.
At 80 degrees F, battery output and amperage needed to crank a big diesel are assumed to be 100%. Oil thickens when cold, so more current is needed. At 32 degrees F, 65% more power is needed for starting. At zero, it becomes 150% more, two and a half times the current needed at 80 degrees F.
Engineers recommend a total of at least 1800 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 large trucks come with four batteries in the 650-675 CCA range, or three in the 800-900 CCA range. Some operators use two 1000-1100 CCA starting batteries and two or more deep cycle batteries for 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, such as ultracapacitors and high-CCA batteries, may allow fewer starting batteries, leaving room and weight for additional deep cycle batteries. Deep cycle batteries are becoming a viable alternative to generator auxiliary power units that power accessories without idling.
Capacitors, sometimes called super capacitors or ultracapacitors, 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.
Lead-acid batteries are fully discharged when they give up about 40% of their electrical energy. They then take hours to recharge, a process slowed by cold temperatures. Capacitors can be discharged more than 75%, and can recharge in minutes rather than hours.
However, they do have their own issues and concerns. For an in-depth review of capacitors, see “Recommended Practice RP162” from the Electrical Study Group of the Technology and Maintenance Council. Telephone 703-838-1763.
Paul Abelson is a former director of the Technology and Maintenance Council of the American Trucking Associations and is on the board of the Truck Writers of North America. E-mail firstname.lastname@example.org
Maintaining your Batteries
Dirt and corrosion on batteries can create a circuit, letting batteries slowly discharge. Corroded terminals also drain batteries. To prevent this, brush disassembled battery posts and terminals with a baking soda paste, then flush thoroughly. Most batteries last no more than three to four years, but with proper care, some AGM batteries can last twice that long.
Temperature extremes shorten battery life. Too much heat can cook the water in the electrolyte out of a battery. Be careful to avoid overcharging. Smart chargers use computer chips to measure state-of-charge and cut back amperage to avoid overheating.