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?

1. The debate between degredation due to high voltage hold vs. degredation due to over-cycling/over-discharge is a hot one. Not going to fan the flames here. The first thing I said to the Verge that was cut was that doing anything with your battery short of of leaving it at 25% SoC will damage it. To hedge a bit, the extent of the damage is less with each incremental improvement in cells. For example, when I was a graduate student taking a cell repeatedly to 4.2 V was certainly an early death for the battery. Now it’s more like 4.3 V. It’s not to say that 4.2 V is as good as 3.9 V. It’s just not as bad as it used to be, and it’s better than 4.3 V. Which is better than 4.4 V. Etc.
2. Beyond this, every cell has a custom designed mix of anode and cathode that changes its current-voltage (IV) performance, and the exact mix and control strategy is, for better or worse, generally kept secret to the phone manufacturer and battery supplier. We simply don’t know, without a complicated tear-down and reverse engineering of the battery and battery management system (BMS), what the actual state of charge when the phone says “100%” is. Some apps give voltage indication, yes, but without knowing the specific formulation and composition of the cathode, we don’t know exactly.
3. So, The BMS is supposed to do this for us. The BMS already stops the phone from continually charging: we when plug our phones in we are not attaching them to an unconstrained 5V source but rather a pretty sophisticated algorithm which tries to balance how long the phone will last today vs. how long the battery will last, period. The BMS is already avoiding true 100% SoC, as going here every time would give you a battery that dies after a few months. So, where is the line?
4. The challenge is, for them, and for us, do we want to have greater talk time on demand (pushing a full charge on demand) or do we want greater calendar life (having our phone batteries last longer).
5. Thus, the phone manufacturer and battery supplier have far more knowledge about the system than I do, and the consumer, so I think the phone manufacturer should be responsible for figuring out that balance between state of charge (SOC) and state of health (SOH). That is, should the phone be optimized for talk time or for cycle/calendar life?
6. In a perfect world, the user would be able to just tell the phone what they want the balance to be. I am an iPhone user, thus far I do not have this option. A cursory search of the Android World indicates there is such third party control. If you are reading this, and have an Android, by all means use these if you want.
7. And the “if you want” is the critical bit. As John Newman said so well:

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

1. I want to have a full charge when I’m away from my desk.
2. It’s the most convenient thing for me to do.
3. In 2017, anything you do to a battery short of never using it will degrade it.
4. I expect battery management systems to improve.
5. I’d rather spend my free time hacking at other things or playing with my kids.

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.