Fortunately, I did some research so I could document my knowledge of battery care and feeding. You see, without this research, I would have been just as guilty of misleading you as the sales reps have been guilty of misleading their customers. Let me explain a few key battery care and feeding facts. For the sake of this blog, I am only discussing modern LiIon cell phone batteries that have been the standard for almost a decade.
Can you overcharge a battery? NO. My research supported my understanding on this issue. Modern cell phones include circuitry that shuts down the charging process when a battery reaches full charge. This means that charging your phone overnight is perfectly O.K.. The phone will prevent the battery from "overcharging." I have been charging my phones overnight for years. I have never seen a noticeable decline in battery life because of this.
Can a battery overheat? YES. My research, and personal experience, agree on this issue too. A battery can be damaged and lose life expectancy if it overheats. I have had multiple experiences working in the heat of summer where my tablet or cell phone have actually popped up a message to tell me the device was getting too hot to function properly, and then had the device shut down for its own protection. One way to tell if your battery has suffered its own form of heatstroke is to hold the battery and see if there is any bulging in the battery. Repeated exposure to excessive heat will cause the battery components to break down and expand in the battery case causing it to bulge.
Can a battery be overworked? YES. Here is where my understanding of batteries has been wrong for a long time. I assumed that a battery's life expectancy was the same whether it worked 24/7 or just a few hours a day. I thought a battery had X hours of lifetime use no matter how those X hours were used. I was wrong. This is actually where the overcharging myth comes into play. While you can't overcharge the battery, you can overwork it by having it constantly charging and discharging at the same time. If your phone is powered on and checking your email and listening for calls and doing any of a dozen background operations that are normal for a powered up cell phone, it will be using charge from the battery. If it is plugged in at the same time, it will also be recharging at the same time. Too much simultaneous discharging and recharging will cause more than "normal" wear and tear on a battery. This will reduce a battery's life expectancy faster than normal.
Another way to overwork your battery is to run several applications simultaneously for long periods of time. One time I was driving to an appointment several hours away. I had my GPS navigation application running, was streaming a podcast from my phone to my car stereo, and was charging the phone with my car charger, all at the same time. Of course, my phone was also doing its normal duty of listening for phone calls, checking my email, updating my social media notifications with the phone's screen on and bright the whole time. When I stopped for gas along the way, I found out quickly how to tell if your phone is overworked. The phone was hot to the touch. In this case, it was not warm, it was hot. You see, an overworked phone is often an overheated phone. Having this happen once or twice probably won't kill your battery. Doing this frequently certainly will.
An interesting note on battery life expectancy. During my research I discovered that you can actually extend the life expectancy of your battery by keeping the normal charge level below full, or maximum. A full battery has to work harder to maintain that level of charge than a partially charged battery. The less charge a battery has to maintain the longer the life expectancy of the battery. An example would be a smartphone battery that last a day and a half on a full charge. That battery may go a full year before it starts to lose charging capacity. If that same battery was only charged to 3/4 capacity, and only held enough charge to get through a single day, it may be expected to go for as much as two years before it starts to lose charging capacity.
With this new knowledge, I have a number of recommendations.
1. Don't "overcharge" your battery. I know that I said this can't happen, but it is easier to say it this way than to explain the science of overworking the battery by charging it for too long. Restrict charging sessions to less than 24 hours at a time.
2. Keep your battery cool. Don't leave your phone in the sun, or any hot location, for extended periods of time. Heat is your battery's worst enemy.
3. Don't try to keep the battery full at all times. It is better to let the battery run with less charge than more charge. Let the battery run down to 1 bar, or less than 10%, whenever you can.
4. Don't overwork your battery. If you can, try to limit excessive multitasking on your phone. Also be aware that the display is often the largest consumer of battery life. If you don't need the display on, let it go to sleep to preserve battery health and battery life expectancy.
Following these four recommendations will keep your battery healthy, and will keep its life expectancy long and fruitful.
Science geeks will appreciate the following website for more technical information about the science behind proper battery care and feeding.