How to Know if Your Semi Truck Batteries Are Going Bad

How To Know if Your Semi Truck Batteries Are Going Bad

A dead battery or batteries on the road costs money, and more importantly, time. Waiting for roadside service to get to you can cost you several hours on your drive time, as well as somewhere between $300 and $600 per incident, according to FleetPride. This figure doesn't even take into account detention fees, missed delivery windows, or the downstream scheduling chaos following a single unplanned stop.

Knowing how to spot dying batteries before this moment hits is the difference between a quick five-minute pre-check and a three-hour roadside wait. Modern trucks compound the risk further. Today's ELD units, telematics systems, and cab electronics draw power continuously, even when the truck is off. A battery that is on its way out might be able to power these things no problem, but then not have enough juice to start your engine in the morning.

In this guide, we will go over common symptoms that will allow us to detect a weak or dying battery, along with how to fix them.

Sluggish Start and the 10.5V Danger Zone

Sluggish cranking is the clearest sign of batteries going out. If your truck engine takes longer to turn over than usual, especially after being turned off overnight, you likely have a battery or batteries going out. 

A slow, labored crank often means internal damage. Inside a lead-acid battery, repeated deep discharges cause the material on the plates to harden and sulfate. This sulfate creates excessive resistance to electrical current. The batteries will struggle to output the necessary amperage to start a diesel engine. Even a healthy 12.6 volt reading while the truck is off can be misleading. 

To read the battery voltage, use a standard multimeter like one here:

 

Batteries discharged to around 10.5V are considered fully discharged, and repeated discharges below this level will reduce its lifespan. According to Truck Expert, a single deep discharge below 10.5 volts can reduce battery life by 15-30%. Most times, this sort of internal damage isn't immediately realized. This is why understanding what a healthy voltage should be is so important. 

With just a multimeter and a couple of minutes of your time, you can monitor your battery health and catch a battery trending toward failure before any problems get the chance to arise. Good batteries will read between 12.6 and 12.8 volts when the truck is turned off. Anything below 12.4V warrants attention.

Voltage alone is only a factor, however. Batteries can give you a good reading and still fail under load. This is why it is important to consider each component of a charging system, not just the batteries.

Often times, a bad battery or batteries can carry the same symptoms as a bad alternator. Knowing how to spot the difference is a valuable skill that any driver can obtain. 

A weak or dead battery fails to hold a charge. A failing alternator fails to deliver one. Understanding the distinction can save you time and money. While many will swap the batteries out when the engine takes a while to turn over, you will inspect the alternator and fix it right the first time. 

Timing can tell you a lot about the underlying problem. A truck that has sat overnight or been off for a while and struggles to crank is often an issue with the battery or batteries. By contrast, a truck that is running and begins to display issues often means a bad alternator. Headlights that dim noticeably while the truck is idling usually mean the alternator is bad. Sometimes your lights will even flicker as the alternator begins to go out.  To measure alternator output, place the red multimeter lead on the cable that goes to the battery. This cable is often covered by red insulation. It's best to place the multimeter lead on the nut where it connects to the alternator. Place the black lead on an alternator mounting bolt or ground until you get a reading on your multimeter. A good alternator should be output 13.5-14.5V.

Parasitic drains add another layer of complexity. Modern trucks have many electrical components that slowly drain the batteries even when the truck is off - roughly 20-80 mA. A parasitic draw occurs when components draw even more power than normal, batteries die quicker, and it's easy to mistake this for a bad alternator/batteries.

To help narrow down the issue, use a multimeter and a load test. 
First, check the voltage after the truck has been off. A voltage of 12.4 or lower signals a weak battery. 
Next, check the voltage while the truck is running. Anything below 13.5 suggests the alternator isn't doing its job correctly.
Finally, load test the battery. Voltage that goes below 9.5 volts indicates a bad battery.

While this is not a shop-quality diagnostic, it should give you some valuable information. 

Visual Cues: Corrosion and Swelling

A swollen or bloated battery case is a safety hazard that needs to be addressed right away. Bloating occurs when internal gases inside the battery build up faster than the battery vent can release them. This is often the case when dealing with overcharging and a malfunctional alternator. A bulging battery case can crack and leak sulfuric acid. It can also rupture. Press on the side of one of the batteries. If it flexes, no test is necessary. Replace the battery immediately. If you smell something akin to rotten eggs, replace the battery. Sulfuric acid gases are leaking and the battery is no good. 

Corrosion on a battery can be caused by many different things and is trickier to evaluate. White or bluish powder on or near the battery terminals are common on older batteries and can usually be managed with a little bit of baking soda and a brush. Corrosion that has began to occur on a cable or terminal is typical for older batteries. It is common to replace your batteries after 3 years. 

Temperature affects battery health as well. According to FleetPride, a battery sitting at 0 degrees Fahrenheit can lose up to 60% of its cranking strength. 

Impact of Age and Frequent Jump-Starts

While heavy-duty truck batteries can last 3 or more years, it is important to take into account the battery use history. If your truck is equipped with an APU, your batteries are working harder and may need to be changed sooner than the recommended 3 years. An inverter will also put strain on your batteries. If you live in an area with brutal winters, you will likely also run into battery issues sooner. After the recommended 3 year period has passed, you will need to monitor your battery health more often. 

Look at the history of jump-starts. One jump-start signals that the batteries have been pushed past a certain point. If the batteries have been jumped frequently, that threshold has been crossed many times. Each time, the cells in the batteries are stressed further. One single jump-start can often be the beginning of the end for your batteries. 

False recovery is another pattern worth understanding. You might think that charging the batteries with a battery charger for a few hours will do the trick. A quick charge can bring the voltage back to normal levels. However, this is not always a permanent fix. The batteries may appear fine on the surface, but once a load is put on them, their health can deteriorate more quickly than before. According to the Heavy Duty Journal, a sluggish engine crank after a full charge indicates diminished battery capacity and imminent failure. 

Road vibrations are another factor in the health of a battery. Constant highway vibration will damage the internal plate structure for your batteries. Over time, small fractures in your batteries can accelerate capacity loss. Combined, battery age, history, and environmental pressure help paint a complete picture.

What You Need to Know

Catching battery failure early can be the difference between a planned shop visit and a costly roadside emergency. Before a bad battery ruins your full day's haul, here are four signals that require immediate attention.

. Sluggish engine cranking even after a full charge. If the engine turns over slowly even after you've had a battery charger charging it for hours, then the cells in the battery are probably degraded beyond repair. You will likely need to replace a battery or more.

. Visible bloating or excessive terminal corrosion. Physical damage showing on the case or a large amount of white or blue powder build up around the posts signals excessive charging and internal breakdown. Don't drive with a battery that looks to be compromised.

. Electrical flickering or dimming lights, especially at idle or low RPM. Gauges that waver and lights that dim when the engine idles suggest that the batteries are struggling to hold voltage under minimal load. This is a sign of internal resistance. However, this can also be a sign of a weak alternator.

. Frequent jump starts or voltage drops below 10.5V. If you find yourself jumping the batteries off more than once a month, one or more of the batteries have likely failed. A load test pushing a battery below 9.5V is almost always a battery problem, not the alternator failing.

Navigating Your Maintenance Schedule

A well-planned route means nothing if your truck isn't reliable. Here is how to attain better reliability each day.

Pre-trip battery checks should be done each time you're doing your pre-trip inspections. When checking fluid levels and tire health, take a minute to observe your battery box. Do you see any corrosion or bulging on the sides of any of the batteries? Are all the nuts in place and tight against each post? If you have a multimeter, measure the voltage by touching the black lead to the negative terminal and the red lead to the positive terminal. Make sure if your unit has multiple batteries that you disconnect the cables that connect the batteries together so that you get a reading for each individual battery. If this is too much of a hassle, turn the key and look at the dash. Many units often show the voltage of the combined batteries. If there is an issue, a battery icon will sometimes illuminate on the dash. If the voltage reads below 12.4V, figure out which battery is below the required voltage. Charge the battery(s) and consider replacing one or more if the problem persists.