You are here:

Physics/Cell phone battery question


I have a question regarding optimum preservation for a cell phone battery.

On many sites I hear do not leave it in the charger all night because:

“The sweet spot for lithium-ion batteries is to keep them charged between 50 and 80 percent. This allows for the charged ions to continue to work and protect the life of your battery,” says Shane. “Charging your device in short spurts throughout the day will give these ions just enough energy to keep them going.”

But I also here many claiming:

Now, going back to your original question, fortunately all smart phones these days are programmed to stop charging the battery once the battery is fully charged, and from that point on, the phone runs solely off of the charger's electricity. So, there's no risk of it overcharging the battery when you leave it plugged in overnight. In fact, when we’re at work, we usually leave my phones plugged in at our desks. When the battery is fully charged, the battery isn’t in use at all, which should extend its lifespan.

The "the battery isn’t in use at all, which should extend its lifespan.
" and "keep them charged between 50 and 80 percent" seem like contradictory statements.

So if I am at my desk should I leave my phone plugged in leaving the battery untaxed or use the battery until 50% then charge it repeat. I would think the former not the latter but .....

Hello Daniel,

I'm afraid I can't add much to what you have found. My reference source on that subject would have to be online. And I recognize that there was a lot of contradiction in what you found online. One thing to add to what you wrote about the smart phone charging system. The charging system of most significant devices, like a tablet for example, will have a smart charger.

Sorry that I don't have more for you on that,


All Answers

Answers by Expert:

Ask Experts


Steve Johnson


I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.


I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

©2017 All rights reserved.