Dead batteries are slowly injuring your car’s alternator

We all know that pit-in-your-stomach feeling that comes from turning a car key and only hearing a click, but what’s the best course of action when it comes to a dead car battery? Sure, it’s usually possible to just jump it with juice from another power source and be on your way, but the newest video from Jason Fenske of Engineering Explained takes a deep dive into the effects of a dead battery on your car’s alternator.

The question posed is less “can you recharge a battery with the alternator” and more should you do this? To find out, Fenske pops the hood of his Subaru Crosstrek and grabs some data points from the charging system to find out. He measures at startup voltage at the battery, alternator voltage, amperage, and temperature, then he repeats the measurements once per minute for 5 minutes as the car idles. Fenske then takes his Subaru on a 20 minute drive and records more data. The process was completed twice, once with a healthy, charged battery, and again with a dead battery that required a jump to get things moving.

Surprisingly, voltage readings remained very similar in both tests (once the dead one was jumped), but it was the alternator’s temperature and amperage output where the differences became more pronounced. The healthy battery test saw a maximum of 15.25 amps, with readings that dropped off the longer the car ran. The dead battery, however, saw a maximum of 61.8 amps, indicating the alternator was sending a stronger electrical current to the battery in an attempt to recharge it. Temperatures followed this trend, with a maximum of 108.5 °F for the charged unit and 151.2 °F for the dead one.

So what can we gather from these results? Well, it’s apparent by the increased amp output and temperature that the alternator is working much harder when the battery is down on charge. This won’t necessarily spell doom for an alternator right away, but increased heat and wear play a major role in degrading the overall life of the part.

Fenske sums it up best: don’t hesitate to jump-start your car, but if the option is available, the battery should be put on a charger to alleviate stress to the alternator (and possibly extend its lifespan). Heeding this advice might just save you the stress of an alternator replacement in the near future, too.

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