Steen Sollows writes:
I have recently heard that car chargers damage the battery inside a phone; I am wondering if this is true?
There is a bit of truth and a lot of urban myth in this one. Ideal charging requires two things: (1) a steady source of the correct voltage with appropriate current limitations, and (2) a sensor that knows when the charge is complete. Consider each of these.
Steady voltage: in the past, automobile power was somewhat iffy. Voltage fluctuations were common. But in current automobiles, the system is more stable and reliable. However, you can still get spikes. These can commonly happen when starting the car. The automobile system goes from drawing potentially hundreds of amps from the battery to absorbing a full-on charge from a fired up engine. One should not be surprised if some voltage spikes or fluctuations get through the power take-off point (cigarette lighter for those of you who still do it). Lesser spikes might happen when large demands are made on the system as when the A/C comes on. Are these spikes and fluctuations harmful to a charging electronic device? Probably not, particularly if the device is well-designed. However, I avoid using the power outlets when starting my car on general principles. There is one exception: I have a portable GPS permanently plugged in. It has its own battery, but charges when the ignition is on. When the ignition is turned off, the GPS asks if I want it to shut down, and if I do nothing, it waits 30 seconds and shuts itself off. I have operated it in this manner for several years and had no noticeable battery degradation. Granted, the GPS is not a smartphone, but it is the best experimental evidence I have that car juice is okay for charging a battery.
On the other hand, most quality chargers have some kind of smart chip that can measure the input current and detect when the battery (or cell) is fully charged and vary the voltage accordingly to prevent overcharging and eventual burnout. An obvious example is a charger for the car itself. By that I mean a device you plug into 110 volt AC and use to charge the battery in your car. These devices are quite sophisticated and usually come with an instruction manual that explains the process. The chargers that come with a typical phone simply say, “Plug it in here and wait until the ion shows full” so you really do not know what they are doing.
A problem can arise if you purchase a third-party adapter to go from your car to phone. Since we all like to save money, you might buy a really cheap one that does not have the correct regulation. Again, this might not harm the battery, but I tend to worry about such things, and, for sure, poor regulation will not make it better.
Another aspect to your question is “damage — so what?” If some mild stress occurs each time you plug into the car, how does that translate into decreased useful lifetime for the phone? Assuming you keep a phone for a long time, and assuming its battery can be changed, you will likely have to replace it after a few years regardless of how you charge it. I doubt that using a decent car charging adapter will shorten the life of any reasonable battery by a significant fraction of the normal life expectancy.
All of this assumes, of course, that you are not doing stupid things with the phone like leaving it plugged into a hot, closed car in the summer with the windows closed — maybe laying on the dashboard in the sun. (Leaving your phone visible in an unoccupied car whether plugged in or not qualifies as stupid.) It also assumes you are not unlucky — see video.
So the best I can tell you is to not buy the cheapest adapter, and make sure to unplug it when starting the car. That is what I do.