Why Do Diesel Trucks Have Two Batteries?

If you’ve ever popped the hood of a diesel truck and noticed not one, but two batteries crammed into the engine bay, you’ve likely pondered why diesel engines need the extra juice. After all, nearly every diesel pickup that rolls off the assembly line features a dual-battery charging system. 

While gasoline-powered trucks can get by with a single 12-volt battery, diesel trucks have far more demanding electrical components that require the added juice of a secondary battery. Factors like the high demands of starting, heating, and powering the high-compression diesel engine make dual batteries not just a convenience feature, but a necessity. 

In this article, we’ll break down the main reasons diesel trucks run dual batteries, how the two-battery charging system works, the advantages the system offers, and our highest-rated batteries for diesel applications. Stay tuned to learn everything there is to know about diesel charging systems. 

Diesel engines require significantly more amperage to start than gas engines due to their unique, compression-based ignition systems. Whereas gasoline engines use spark plugs to ignite the air-fuel mixture in the cylinders, diesels rely solely on cylinder pressure and heat generated during compression. 

To crank the high-compression engine over, the starter on a diesel engine has to produce far more torque than a comparable gasoline engine’s starter. That extra effort and energy need more electrical current, which a single automotive battery typically isn't capable of producing. 

To ease starting, diesel engines are fitted with two batteries. With two batteries wired in parallel, diesel charging systems deliver far more amperage to the starter to ensure the sustained current needed to spin the engine over. 

On cold mornings, diesels may need a little help getting the airflow into the combustion chamber hot enough to fire. Depending on the engine brand, this heating is handled by two primary heating technologies. 

On Cummins inline six-cylinder diesels, found in HD Ram applications, the engine features a grid heater. This heater, mounted inline between the cylinder head and intake horn, features a grid of wires that heat rapidly when energized. Intake air then passes through the grid, where it warms significantly before reaching the combustion chamber. 

On V8 diesel engines, like Ford’s Power Stroke, GM’s Duramax, and the all-new 2026+ Cummins inline six-cylinder engines, glow plugs are used. These heaters, similar to spark plugs, protrude into the individual cylinders. When charged with current, they warm the combustion chambers directly, making cold-weather starts easier. 

These heating elements draw a tremendous amount of electrical current—often for several seconds before the engine fires. Dual batteries ensure there’s plenty of power for these heaters to operate while still supplying enough juice to the starter and other electrical components.

Most diesel trucks feature two batteries wired in parallel, not in series, which keeps the system at 12 volts while doubling the available cold cranking amps (CCA) and system capacity. Wiring the batteries in parallel offers several advantages, including a higher current output, longer reserve capacity, reduced strain per battery, and more stable voltage while cranking. 

Even though diesels use two batteries, they’re connected via a crossover cable and serve as a single, high-amperage power source. 

In most trucks, neither battery is considered the “main” battery. When wired in parallel, both batteries share all electrical loads and supply power equally—they charge, discharge, and function together. 

While one battery may be “primary” for specific functions due to its proximity to certain components—for example, the driver’s side battery typically sends more juice directly to the starter since it’s closest—both serve the same role. 

In addition to a more reliable start, dual batteries serve several benefits in diesel trucks. 

In cold temperatures, diesels can be notoriously difficult to start—especially with aging grid heaters or glow plugs. A diesel’s two-battery charging system supplies the amperage necessary to fire heater elements and send power to the starter. 

Tow packages, aftermarket lights, snow plows, and winches all draw significant electrical current. Dual batteries help to keep voltages stable under load and reduce wear and tear on the charging system. 

So long as both batteries are correctly matched and maintained, having two batteries helps share the charging system's workload, reduce strain, and extend service life. 

Unlike old-school, fully mechanical diesel engines that contributed to the technology's reputation for reliability, modern diesels employ computers, sensors, and high-pressure, electronically controlled fuel systems to run efficiently. 

Diesel trucks need high cold-cranking amps, deep-cycle durability, and strong reserve capacity. Check out these three rugged battery options that provide all these qualities and more, making them excellent options for diesel applications. 

The Odyssey Extreme Series is built for trucks that demand maximum starting power and vibration resistance—like a diesel. These AGM batteries deliver impressive long-term durability and strong performance, making them ideal for the demands of a diesel application. 

Features: 

• Extremely high CCA for fast diesel starts

• Pure lead plates for enhanced longevity

• Superior vibration resistance—ideal for heavy-duty trucks

• Deep-cycle capability for accessory-heavy setups

• Long service life with strong reserve capacity

The Optima YellowTop is a dual-purpose AGM battery that’s become exceptionally popular in the automotive aftermarket world. With its unique spiral-cell design, the YellowTop offers excellent cycling capabilities and strong starting power. 

Features: 

• Reliable starting power with deep-cycle performance

• Highly resistant to vibration and shock

• Fast recharge rate

• Ideal for trucks with winches, plows, lighting, or audio upgrades

• Maintenance-free AGM design

If you’re looking for reliable performance without spending a fortune on two new batteries, check out the Performance Series from ODYSSEY. Though a step down from the aforementioned Extreme series, the Performance series still offers strong cranking power and exceptional value.

 Features: 

• High CCA output for diesel starters

• Strong reserve capacity for accessories

• Durable AGM construction

• Great balance of price, performance, and longevity

Like single-battery charging systems, dual-battery systems last longer when properly maintained. Follow these tips to keep your diesel’s batteries operating optimally for the long haul: 

• Replace batteries as a pair to prevent the new battery from prematurely wearing out

• Clean terminals regularly to avoid voltage drops

• Use a battery terminal protector to help prevent corrosion

• Test charging voltage to ensure alternator health

• Use a battery tender (trickle charger) if the truck sits for long periods of time

• Watch for slow cranking, which is often the first sign of a weak battery

• Monitor batteries for leaks or swelling

• Only replace batteries with those that match or exceed factory specifications

Jump-starting a diesel truck is virtually the same as jump-starting a gasoline-powered truck, with a few additional steps. Here’s a general breakdown of the process: 

Step 1: Locate the battery that’s closest to the starter. Typically, this battery will have a positive lead running directly to the starter. 

Step 2: Attach one end of the jumper cable’s positive (red) lead to the battery’s positive terminal. 

Step 3: Attach the other end of the red lead to the donor battery's positive terminal. 

Step 4: Attach one end of the negative (black) lead to the donor vehicle’s negative battery terminal. 

Step 5: Attach the other negative lead to a grounding point on the truck’s engine block or chassis. 

Step 6: Let the donor vehicle idle for several minutes to charge the battery. 

Step 7: Start the diesel truck. 

Step 8: Remove the cables in reverse order.