Blinded By The Lights

Tony Bartels upgraded to a 130-amp alternator to keep the lights bright on his 1994 Peterbilt.

Many owner-operators like to add lights. But, as Bruce Purkey of Purkey’s Fleet Electric puts it, “The truck maker typically does not overspec the alternator.” This means that extra lighting often requires modifying the electrical system. Here’s why:

  • If you don’t generate as much power as you’re using while you drive, plus enough reserve for battery charging, the batteries will run down.
  • If you use more power than the electrical system is designed to supply while you’re parked, the batteries will run down.
  • Even if your alternator and batteries are big enough, if your charging circuit wiring isn’t, the batteries will run down.
  • “A typical truck will have an alternator that puts out in the range of 130 amps,” Purkey says. “Most fleet trucks will draw under 60 amps.” This supplies the basic equipment, though most trucks end up pulling over 80 amps by the time the driver adds what he wants, Purkey says. Still, this leaves just 50 to 55 amps (with a 135-amp alternator) to charge the batteries. The problem comes when you add a lot of lighting that draws a substantial amount of power and puts a big dent in this reserve.

    Adding lights doesn’t necessarily mean you have to do major surgery on your electrical system. If you control power use carefully, the present system might handle the job, especially if you run over the road, allowing plenty of time for the batteries to get recharged.

    Kent Bode, aftermarket group manager at Grote industries, recommends careful planning for an auxiliary light system. “The plan should include the location, number, and amperage draw of each light. The amperage draw can generally be obtained from the printed catalog and literature from the lamp manufacturer.

    “Add up the amp draw for each lamp to determine the total electrical load that the system will draw. The goal of the system should be to minimize the electrical load and still provide the appearance that the driver wants. A great way to minimize this load is to use LED (light emitting diode) lights.”

    Purkey estimates that LEDs reduce power consumption by 90 percent. For example, an incandescent lamp that draws 0.5 amp could be replaced by an LED that draws 0.05 amp. Furthermore, says Jay Gillam of Peterson Manufacturing, “You get 10 times the life expectancy for, at most, four times the price or as little as two and a half times the price.”

    Bode gives another example: “An incandescent lamp might draw .66 amps while the LED version of the same lamp may draw only 0.06 amps. Add 40 lights to a tractor and you need 26.4 amps for incandescents and only 2.4 amps for LEDs.”

    George Brodbeck, a Truck-Lite regional manager, says one of his company’s incandescent lights draws 0.3 amps, while a comparable replacement LED draws 0.07, about a fourth of the amps.

    “In an incandescent light, a tungsten filament gets hot and emits light,” Brodbeck explains. “An LED is an electronic chip made of silicon and exotic metals. When energized, the material emits light, but it doesn’t even have to get hot.”

    The other reason to use LEDs is fuel costs. Running a late model truck at 60-65 mph requires about 200 hp. At that speed, with a 200-amp alternator at full power, almost 4 percent of your fuel costs could be going to electricity.

    If you plan to add a lot of lighting, there’s also a danger of damaging your system. Bode says, “It is critical that a vehicle’s power requirements not exceed the capacity of the alternator or batteries that a vehicle is equipped with, as overloading them can result in permanent damage to the electrical system.” When batteries are drawn too low, it shortens their life by damaging the plates. An alternator that is constantly recharging badly drained batteries while also operating the vehicle can fail early due to overwork.

    “The alternator and batteries should be stamped or tagged, noting their amp-carrying capacity,” Bode says. “The actual electrical draw of a vehicle must also be determined prior to developing the auxiliary lighting system. A qualified technician will be able to measure the actual load that is being placed on the electrical system prior to adding lights.”

    Every time you use the starter, the amp-hours used out of the batteries have to be put back in. If just a little less is put back than is taken out on a continuing basis, the batteries soon run down and you will have a no-start. If it takes 125 amps to run your electricals and you have a 135-amp alternator, that leaves only 10 amps for recharging. However, if you shut off lights that take 50 amps, the electricals will then draw only 75 amps, leaving 60 amps available for recharging.

    So even if you have added a modest string of lights, determine whether your alternator can handle the additional load. Fortunately, not all lighting systems take enough power to throw the electrical system out of balance. Ed Noger, of Noger Trucking, who runs refrigerated trailers from New Jersey to New England, has a late-model Peterbilt conventional with about 18 standard-size marker lights and a couple dozen smaller lights, most of them incandescents. The truck has the original 135-amp alternator, and he says he’s never had a charging system problem.

    Consider what happens if you add 30 incandescent marker lights that draw .3 amps. This would add 9 amps to the tractor’s power demand. If you had a reserve of 55 amps, you’d cut that reserve by about 16 percent – a significant hit, but perhaps not a critical one if you run under favorable conditions. Daylight driving without lights, long periods on the road without an engine shutdown, and relatively few cold starts add up to an easy life for the electrical system. But running at night, making short runs and shutting down at the end of each run (meaning frequent starter use), combined with overnight shutdowns in cold temperatures, all greatly add to the burden on the charging system.

    If you added 30 efficient LEDs that require .07 amp, for 2.1 amps total, you’d be okay. But adding a large number of incandescent lamps would make it wise to upgrade the alternator. For example, with 130 amps of charging capacity and 30 amps in lighting added to your electrical system, you should ideally install a 160-amp alternator.

    If in doubt, take the truck to a reputable electrical shop or dealer and have measurements made. Purkey says a technician with an inductive ammeter can do this without disconnecting anything. The technician will get readings with everything on to see how much of the total alternator output remains available for charging. With an accurate description of how you use the truck, the tech can advise you whether to up your alternator’s power.

    Two questions worth asking an alternator supplier are how well the unit performs at low rpm and whether or not the unit can be adjusted.

    Tony Bartels and his wife Robin run a 1994 Peterbilt. After Tony added a large number of lights the OEM alternator lasted only 40,000 miles. Even after having it rebuilt and upgraded, the lights would dim at idle. So he replaced the alternator with a 130-amp, brushless Leece-Neville model which has good output at low rpm and is adjustable.

    Setting that unit to produce 13.5 volts with everything on at 1,000 rpm prevents his lights from dimming when he stops at a traffic light. The unit gave excellent life and was replaced later with a similar Prestolite unit (made by the same manufacturer) that also performs well. Four NAPA truck batteries handle the lights during shutdown without causing starting problems.

    Adding lights requires looking at the entire electrical system, not just at the wiring or the choice between incandescents or LEDs. If you follow the experts’ advice and get accurate measurements of your system, you should be able to add lighting without creating trouble.


    When installing your own lights, experts recommend following guidelines of the lamp maker, using the proper gauge of wire and paying attention to the number and quality of connections.

    “Increased amperage loads require the use of heavier gauge wire,” says Ken Bode of Grote Industries. Also, “The number of connections should be kept to a minimum to prevent voltage loss. Additionally, any connections that are exposed to the elements must be weatherproofed. This can be accomplished by using sealed plugs that are designed to connect directly to the manufacturer’s lamps, or by using heat-shrink tubing or heat-shrink connectors at all junction points.”

    Connection plugs should be well packed with dielectric grease, advises Peterson’s Jay Gillam. “Otherwise, physical damage or corroded connections will give you trouble,” he says. “Where splicing, solder the connection, then cover with a heat-shrink material. Don’t just use insulating tape.”

    Specialist Bruce Purkey points out that for trailer lighting, it’s not smart to rely on the standard 7-way plug to the trailer that also carries the ABS signal. “The J-560 plug is not designed for all that load. Add a separate, fused circuit that’s of sufficient gauge wire both for the load and the distance.”

    Tony Bartels installed an elaborate incandescent lighting system on his 1994 Pete using continuous-rated, golf cart solenoids as relays to switch the power on and off near the lights. “Don’t use starter relays, designed to operate only for short periods,” he recommends. Prefabricated wiring with integral plugs gave him good connections.


    “When parked, it’s feasible to leave the lights on if you are just running a few LED marker lights,” says Mark Assenmacher of Peterson Manufacturing. However, if you insist on showing off a big string of fancy new lights with the truck shut down, one of the best approaches is to upgrade the typical three-battery system to four batteries.

    The best compromise is a dual-purpose battery. This type has both reserve capacity, or the ability to supply low loads for a long time without going dead, and cold cranking amps, which means high amperage for a short time, as when starting up.

    If adding a battery seems like an unsatisfactory solution, consider adding a low-voltage disconnect. Intra Technologies makes an LVD that, when the battery voltage drops to 12.1 volts, begins sounding alarms. If enough loads are not turned off to keep the voltage above that level, after five minutes, the alarm is sounded for 30 seconds and the LVD disconnects the loads. Once voltage rises above 12.8, the loads are reconnected.
    The device costs $150 to $350, depending upon application and features. Harnesses are available for most heavy-duty truck brands, and the device and harness can be ordered from a local distributor or from Intra.

    The ideal setup is having both the extra battery and the LVD. Your lights could stay on for a while, yet you’d be sure not to run even the four-battery set down and have a no-start.


    You can’t upgrade your electrical system by just changing the alternator. Wiring size is critical in making the system work.

    “Flowing electricity is like water in a pipe,” says Bruce Purkey of Purkey’s Fleet Electric. “Unless you have a big enough hose to deliver the water, it won’t work.”

    The voltage drop between the alternator and batteries must be .5 volts or less. When upgrading the alternator, you must either change the wiring between batteries and alternator and between batteries and ground, or add additional wiring to ensure that the voltage drop does not increase. Failure to do this means a drop in voltage that will keep the batteries from charging properly.

    Also, the wire from alternator to batteries must have a fuse installed in it or else you risk a fire, Purkey points out. Were this wire to ground against any metal part of the truck because of frayed insulation or a loose connection, it would create a dead short circuit and draw hundreds of amps more than the wire’s rating. That’s why you need a fuse.

    “Also be careful about belts and pulleys,” Purkey advises. “You may need to redo the drive system, especially if going from a unit that draws three hp to one that draws six or seven.” Severe belt slippage can easily boil the grease out of bearings and ruin your new alternator, so a change in pulley and belt size is in order with a big power jump. With a small increase – say, from 130 to 160 amps – you may be able to get away with just making sure belts stay in good shape and at the proper tension, using a tensioning gauge.

    The Business Manual for Owner-Operators
    Overdrive editors and ATBS present the industry’s best manual for prospective and committed owner-operators. You’ll find exceptional depth on many issues in the Partners in Business book, updated annually.
    Partners in Business Issue Cover