Why Do Diesel Trucks Have Two Batteries?

Have you ever popped up the trunk of a diesel truck and noticed that there are two separate batteries under the hood? Given that gasoline-powered vehicles seem to do perfectly fine with a single battery setup, why do diesel trucks need two batteries? 

At first glance, adding an extra battery appears to be a terrible idea. It not only adds several pounds of additional weight to the vehicle, but the cost of the additional battery is also eventually borne by the buyer. You wouldn't be blamed for thinking that adding an extra battery is just one of the many ways in which manufacturers eke more money out of unsuspecting consumers.

As it turns out, there are several interesting reasons for manufacturers to use two batteries in diesel trucks, and nearly all of them boil down to the fact that these vehicles use diesel engines and how differently they work compared to gasoline engines. This has become a standard practice among major truck manufacturers in the U.S. Here are the technical reasons behind manufacturers choosing to equip diesel pickup trucks with two batteries under the hood. 

If you have had extensive experience with gasoline and diesel-powered vehicles, you might have noticed that diesel vehicles are typically harder to start — especially in colder weather conditions. This is not unusual because diesel engines operate at much higher compression ratios than gasoline engines. A direct result of this issue is that diesel engines require a substantial amount of initial energy to heat and bring them to life, especially compared to gasoline engines.

The best way to provide this initial surge of power to start up these diesel engines is to connect two batteries in parallel. Batteries connected in parallel let the car operate at the same voltage (12 volts) as other cars, while doubling the amount of current supplied (measured in amperes or amps). The higher the amperage, the higher the amount of power generated to crank the engine. This higher amperage by connecting two batteries ensures that diesel trucks have ample electrical power to play around with during the startup phase.

Another reason for using dual batteries on diesel trucks is that the engines on these vehicles also use glow plugs. These are heating elements placed inside diesel engines that heat up diesel engines in cold weather to ensure quicker startup. Note that glow plugs are not the same as spark plugs, which are almost exclusively used on gasoline engines. Heating these glow plugs also requires a considerable amount of energy, which is best provided by a dual battery setup.

The most noticeable advantage of a dual battery setup on diesel trucks is that the vehicle always has a secondary power source in the event that one of the batteries fails. The dual battery setup also lets the vehicle power many accessories and auxiliary electrical appliances like refrigerators, lights, winches, and camping equipment. These types of equipment are increasingly common on modern-day trucks, which have long lost their utilitarian tag to become lifestyle vehicles.

On the flip side, the increased complexity of a two-battery system makes repair and maintenance of the electrical system slightly more complex. Diesel engines are already known for being more expensive to maintain, with replacement components also typically priced higher. A dual battery system only adds to this problem. If that wasn't all, an incorrectly configured dual battery setup may also cause additional electrical issues with the truck and, in some cases, even damage components.

Replacing batteries on diesel trucks with dual batteries is also expensive because experts suggest replacing both batteries simultaneously for optimum performance. So, you may have to give up on an old battery that is in excellent working condition just because its companion battery has stopped working. Nevertheless, if you happen to own a diesel truck with a dual battery setup, you now know why that is the case.