On Leaving Batteries Plugged In: Lifetime

An astute reader asked me to clarify this for safety, which you can find here. This piece focuses on lifetime. Remember: a badly designed or defective battery should never be used at all. This piece assumes that your cell phone is not the 1 in 1,000,000 that has safety flaws.

Recently I answered this question for the verge:

“Should Consumers Leave Their Phones Plugged In All The Time?”

I answered that one should. I gave a fairly lengthy answer that was edited down for the sake of brevity but I’m getting some frothy frothiness from battery nerds, so I will take a bit more space to elaborate on why I said this.

A Bit More Elaboration

Short of leaving a battery almost discharged, everything damages it given 2017-era technology. This means keeping it charged, or watching it move between 30% and 80%. Even when your phone is plugged in, the battery isn’t necessarily charging. The phone knows when to stop. The challenge is that it can stop at a point where the battery lasts longer, or when the phone will give you more talk time. Because consumers typically want the latter, the management system errs toward the latter. Ultimately, in my experience, the enhanced degradation caused by “topping off” is outweighed by the the satisfactory life of a phone (~2 to 3 years) plus the convenience of having a useful phone away from an outlet/power bank.

Quite A Bit More Elaboration

I fully agree with Venkat that the practice and literature indicates holding a battery at 100% state of charge for a battery, in 2017, will speed its degradation. So why would I answer this way?

We are willing to store electric energy in a watch battery for 2 or more years because we value the energy highly for its convenience and small size. Energy for the utility grid is inherently valued at its sale price, a much smaller number.

If The Verge had asked me about this for EV’s or Grid Storage, the answer would be different. But it was for cell phones/portable electronics, where the entire point of the device is one of convenience.

I find it horribly inconvenient to manually manage my phone SOC/SOH. Computers should, ought, and to a large degree already do this for me. They choose on-tap talk time (high SoC) over calendar life (better SoH), and for this application I generally agree with built in BMS strategy.

Could it be better? Of course it can. And here’s where plugging it in, IMHO, helps even more. Every phone manufacturer is, or ought to be, logging critical data about about how batteries fade (and shameless plug, they should use Voltaiq to analyze the results) on a per phone basis. With each generation of cell phone, life gets better and knowledge increases. It’s my wild speculation that manually overriding the built in controls makes it harder for the manufacturers to aggregate the data. Not impossible, just harder.

But what if your battery dies before you’re ready to get a new phone? Fair point. When all is said and done, the battery won’t last forever, so we can just hope it lasts long enough. If it doesn’t last long enough: change it.

It’s generally pretty straightforward to change a battery in a phone or have someone do it for you for order $20. An extra $20 for a few years of convenience? Sign me up.

So, I plug my phone in as much as I can, because

Yes, a cell phone is an investment, but relative to the incremental cost of a new battery, there are much, much more important things to worry about in the world.

UPDATE: Another good reason: if you have any background activity on, the phone will pull down the battery pretty actively even if the screen is off.

the unfortunate tetrahedron

making batteries better one compromise at a time


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Dan Steingart

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Associate Professional Do Nothing Ne’er Do Well

the unfortunate tetrahedron

making batteries better one compromise at a time