How to optimize medium duty truck electrical systems for maximum performance and life

A heavy duty truck has many similarities to its smaller brother, the medium duty truck, but there are some areas where the medium duty truck can be quite different. One of those areas is the way the truck is driven.

While a heavy duty truck may spend significant amounts of time running long distances at highway speeds, a medium duty truck may spend most of its time making short trips around town at much lower speeds. It may make many stops, idle very little (if at all) while stopped, have a large number of electrical loads (such as electric liftgates, interior and marker lights) operating while stopped and only be driven a short distance before the next stop.

This type of use can stress the medium duty truck electrical system and ultimately lead to increased electrical failures and road calls.

What can be done to enhance the electrical system to resist the effects of this type of use? Making sure the main components in the medium duty truck electrical system are the best for its application.

Let’s take a closer look at each of these components



Medium duty trucks are often equipped with as few as two group 31 batteries.  Compared to a heavy duty truck, which often have four, a medium duty truck has half as many.

The main purpose of the battery pack is to crank the engine. Therefore, sufficient cranking amperes (CCA) need to be considered when selecting a battery.

Since the battery pack also provides power to any electrical load operated while the engine is off, the depth of discharge on a two battery pack will be greater than the depth of discharge on a three or four battery pack.

Depth of discharge is the percentage of battery capacity that has been discharged, expressed as a percentage of maximum capacity. Greater depths of discharge will shorten battery life.

Having more than two batteries in the battery pack of a medium duty truck can reduce the depth of discharge and increase battery pack life.

A second factor that impacts battery life is the frequency at which the battery is discharged or “cycled.” More cycles will decrease battery life. Using batteries that resist cycling damage, such as high cycle flooded cell or AGM (Absorbent Glass Mat) batteries, can increase battery pack life.

Also, make sure you know what the warranty coverage of the OEM battery pack. The longer the warranty coverage, the more “comfortable” the battery manufacture is that the battery is correct for the application.



When selecting a charging system for a medium duty truck application, larger performance alternators are worth considering. Although selecting a unit that can provide a large percentage of its rated output at lower engine rpm is important for charging the battery pack and operating electrical loads, a charging system that provides great output at high engine rpm isn’t of much value if the truck is never operated at those levels.

Other recommendations for charging systems include using a remote voltage sense system. This will help maintain battery pack state of charge and make sure the output wiring of the charging system is properly sized to handle increased charging system performance.

Having a high performance alternator is of little value if the wiring can’t deliver that increased performance to the rest of the truck. TMC (Technology & Maintenance Council) Recommended Practice (RP) 129, Heavy Duty Vehicle Cranking and Charging Troubleshooting: 12-Volt Systems, recommends no more than a 0.5V drop at rate output for the output circuit of a medium and heavy duty truck.



Because medium duty trucks are used in applications with more stops, they also will be making more starts. This means more wear on moving parts in the cranking motor, such as bushing, bearings, brushes and commutators.

Learning which cranking motors have the greatest cycle lives is useful for selecting the best unit. Like the charging system having a cranking circuit (battery cables) with low resistance will help the truck start in all conditions.

TMC RP 129 recommends no more than 0.5V drop at 500 amps in the cranking circuit of a medium and heavy duty truck.



A fleet best practice is to make an operational analysis of the electrical system. Purkeys Fleet Electric, by way of example, can provide a recording device that is small and easy to install. It can “ride a long” for up to 30 days capturing electrical data while the vehicle is being operated as it is normally is operated.

This data can be downloaded and analyzed to determine if the electrical system is correct for the application. If not, the necessary recommendations to improve the electrical system can be offered.

Bruce Purkey is the chief creative engineer at Purkeys Fleet Electric ( For more than 20 years, the company has delivered innovative, cost-effective solutions to the electrical needs of commercial fleets – both large and small. In addition to identifying electrical problems, the company educates and train staff to prevent negative situations.