Can a Bad Alternator Ruin a Battery

Alternators and batteries are vital parts of cars, boats, RVs, and other electric systems, and issues with one component may cause additional issues with other components of such systems.

Since the battery charge level in such systems depends on the proper operation of the alternator and its charge controller, people wonder if a bad alternator can ruin a battery.

Published: May 13, 2022.

Short Intro To Alternator Operation

When the main engine runs, the alternator converts mechanical energy into electrical alternate current (AC current) which is then rectified into direct current (DC) via a diode bridge (Graetz Bridge) or some similar device and smoothed enough using capacitors and other components.

Such electric energy is then used to power the car's electric systems and to charge the battery.

Note: modern cars use a somewhat more complicated electronic system, but the principle is the same.

So, what can happen to the alternator that may ruin the battery?

If the alternator becomes too weak for whatever reason, it is unable to keep the battery fully charged, shortening the battery's lifetime - lead-acid batteries last longer when they are constantly charged above 85-90% SoC.

And if the alternator dies completely, the car will operate for some time since the battery will power ALL electric and electronic systems, but after the battery's voltage drop below a certain level, the car will simply stop and the battery will be almost completely discharged - and that is not a good condition for starting lead-acid battery.

Most charge controllers that regulate DC output voltage come with certain safety features that cutoff the alternator from the rest of the electric system if the output voltage crosses a certain threshold, but issues with the alternator's too high voltage may first damage such components and in the cascade damage the battery by charging it with too high voltage - overcharging the battery is also far from being a good condition for any lead-acid battery.

Temperature Compensation

For lead-acid batteries to be properly charged, a temperature compensation must be provided, especially during winter to avoid batteries from being undercharged and during summer to prevent the batteries from being overcharged.

Temperature compensation is done (if done at all) via electronic components that regulate output DC voltage and strictly speaking, they are not part of the alternator, although such components may be built into the alternator.

If You have issues with the battery longevity in your vehicle, consider checking the charge controller (alternator may be just fine), if possible - the newer model may include temperature compensation and other improvements.

Long Story Short: If You wonder if the bad alternator can ruin the battery, the answer is yes, the bad alternator can ruin the car's battery in many ways.

So, when driving, if the "Check Battery", "Check Alternator" or some similar Error/Warning message pops out on your car's dashboard, stay calm and if required, stop the vehicle... Driving the car in such a condition can result in even more damage to the car's electric and other systems...