If you have a bad alternator, chances are you’d never recognize the problem in the winter when it’s cold. But the problem will show up pretty fast once the weather warms up and electrical loads increase with air conditioner use. Unless you know what to look for, a dying alternator can be hard to spot in the winter.

Cold batteries don’t charge well because of their chemistry. As truck electrical system expert Bruce Purkey will tell you, “It’s like trying to push energy into a cinder block.”

A typical flooded-cell battery at zero degrees will only take about 2 amps per hour. You might have a 150-amp alternator, but the whole battery pack will take only 8 amps (4 batteries, 2 amps/hour each) Everything may look right, with the volt meter right up there where it’s supposed to be at 14 volts trying to push something into the batteries, but the batteries are losing charge faster than the alternator can make it up, explains Larry Rambeaux, a sales application engineer at Purkeys.  

“Doing a jump start for a dead battery isn’t going to solve the problem,” he says. “It will get the truck going again, but unless it’s run for a long time, it’ll be just as dead as before the next time you try to start it.”

Unfortunately, the same thing might happen in summer when drivers start using air conditioning — except now it might actually be an alternator problem. You might miss it entirely because the system appears to be working properly.

“In winter, people get a false sense of security because they see the volt meter is right up where it’s supposed to be,” Rambeaux says. 

“They can easily miss an alternator with a bad diode that has lost performance, but because the loads are so small, everything looks fine.

“Summertime is when you’re likely to have an alternator change for no reason,” he adds. 

“If the batteries are low, the alternator will put out everything it can, and the batteries will take everything they can get, but the voltage will be down at 12.5 volts rather than 14 — and it could be really hot to the touch. That leads most people to assume the alternator is bad. That heat is actually just a byproduct of putting out that much energy trying to top up the four batteries.”

For example, if one diode fails on a 135-amp alternator, it will lose about one-third of its output. 

“A 90-amp alternator is probably fine in the winter, but in summer when all the extra loads come on and the batteries are more receptive to taking a charge, the alternator cannot keep up, and suddenly you have a road call,” says Rambeaux.

Can Your Truck Batteries and Alternators Handle the Heat?

Another factor that affects alternator performance and battery life is all the other electrical demand. 

When manufacturers build a truck, they put in an average battery that will service the cab and chassis requirements with a little bit extra.

“Typically, OEMs design their C&Cs with batteries and alternators to operate the factory chassis items," says Joe Puff, vice president of Truck Technology and Maintenance at NationaLease.

"What OEMs don’t know is whether or not a customer plans to add additional electrical demands, such as a liftgate, an in-cab refrigerator, an inverter, or other hotel loads."

And in the summer when truck drivers start using air conditioning, that can become obvious.

“These added power demands can stress a basic chassis battery system," Puff says. "When spec’ing batteries and alternators, we need to compare the power draw placed on batteries with the alternator output and drive times, so you have ample time to recharge the batteries. 

"Things like ambient temperature, short drive times, reduced idling, and liftgate usage, all need to be considered when spec’ing the right battery and alternator," he says.

"You could be setting yourself up for frequent battery failures if you settle for the basic factory-spec electrical system.”

If you run trucks through the winter with liftgates and hotel loads and you have weak or inadequate batteries, or an alternator that’s either not up to the task or not at 100% performance, summertime is when the problems will show up.

Keep the Truck Batteries Charged

The key to extended battery life is keeping the battery above its recommended state of charge, Puff says.

"For example, a flooded acid battery starts to degrade quickly when it is discharged to around 50% of its capacity. So, keeping the battery charged is the key to long battery life."

There are many solutions to consider, he notes, including the number and type of batteries, the alternator size, engine auto start with low voltage, progressive power shutoffs, solar panels, overnight chargers, and so on.

Battery Maintenance is More Than a Visual Check

If your definition of a battery inspection means opening the battery box cover to see if the batteries are still there, you may be surprised by how much you’re overlooking.

If you’re unfamiliar with proper battery, charging, starting and cable inspection and maintenance, check out the Technology & Maintenance Council’s Recommended Practices 129A and 132B. 

Rambeaux suggests using an electronic tester rather than a carbon pile tester, because they can do a much more thorough job and provide greater insights into the condition of the electrical system.

“You have to start with testing the batteries to make sure they are at a minimum state of charge," he explains. "If the batteries are at 100% SOC, the alternator isn’t doing anything. You have to test with a good state of charge to make sure the alternator is regulating properly, and you have to test the system when it’s under load, so the alternator is working.”

Using a carbon pile tester is complicated and time-consuming, and it relies on the technician to perform the test properly. Rambeaux says the better (and significantly more expensive) high-quality electronic testers can run all the required tests more or less automatically and consistently, which produces more accurate and documentable results.   

Considering the whole of the electrical system — alternator, starter, voltage regulators, cables and the batteries — there are a lot of moving parts, and each can affect the performance of the other components. Rambeaux says hundreds of alternators, starters and batteries are taken out of service needlessly, simply because they weren’t tested properly.

Before the heat of the summer hits in full is an ideal time to turn over a new leaf on electrical maintenance, and all the money you save on non-warrantable replacements will more than pay for the proper tools and the training to get this vital inspection right.