On Leaving Batteries Plugged In: Safety

So for lifetime, I’ve made my point. Everything in that piece assumes that the BMS and battery are well designed and well matched.

An astute reader writes in wanting to make sure I clarify that this issue around safety for this. This is more complicated.

A Rough Sketch

First off, if any system is not plugged into the wall, is it ‘safer’? This is not a simple question.

Reasons to Unplug For Safety

In the “yes” category, if the system is not plugged into the wall, there is no way for the cell to get more energy, and if the cell has a limited amount of energy it is safer than if it had an unlimited amount of energy for a given size. (See safety here). If one has reason to believe the battery management system (BMS) is flawed/faulty, then unplugging the battery is a good idea. Getting a new phone is a better idea.

This said, a battery that is unplugged still contains energy, and a fair amount of it. And the odds of the BMS being the problem as opposed to the battery are, well, low.

Note in the chart above, just because the battery is being operated in a safe way does not mean it is not degrading. Modern BMS in cell phones and laptops push into the yellow reason to provide more run time. Modern BMS for capital intensive yet cost sensitive applications (cars, grids), tend to operate in the green region, but there are plenty of examples where they push into the yellow.

There is something of a debate in the battery community about the role of a BMS. Some believe that it is to prevent the battery from going into the red. Other believe it is to do that as well as minimize time spent in the yellow. All agree that if a BMS doesn’t keep you out of the red, it is not a good BMS.

So: assuming the BMS is behaving, the battery will not receive more energy than it should. We can generally assume that the BMS behaves because far, far, far more phones would catch fire if it misbehaved.

Reasons to Plug For Safety

Given that the BMS prevents the phone from receiving too much charge, please note that there is a region of the plot that is red on both sides. This means that if a battery is too discharged it can become a safety issue. This seems counterintuitive, but it’s pretty insidious when it happens. Long story short, the materials break down when the battery is overdischarged in a way that makes recharging of the battery really dangerous. Let me know if you care to learn more about that and we can talk more.

Most internal BMS’s work very hard to make sure that the phone turns off well before this point is reached but it is important to recognize that too little energy is bad as well.

A challenge with all current BMS’s is this: as the battery ages, it can be harder and harder to detect the point at which the cell goes into overdischarge vs. the amount of capacity left. Generally the BMS compensates by giving up accuracy, that is, it will display ~10% then suddenly ~1% before immediately shutting down.

In my experience, staying above 50% SoC prevents these states from being reached, and the easiest way to stay above 50% SoC is to err on the side of plugging in vs. floating.

Temperature is the Real Challenge

For any battery, large battery packs in particular (e.g. EVs), thermal management is critical. Heating a battery in any state to too high a temperature is bad news, but operating a battery that is too cold is bad news as well. That nice green window above assumes ~25˚C. Go 15˚C hotter or colder and that window shrinks significantly.

Thermal management requires energy, and leaving the pack plugged in is the best way to provide this energy rather than having the battery shoulder it. This doesn’t really matter for phones, but it is critical for packs > 1kWh. See the Chevy Volt/Bolt BMS for a prime example.

For safety operation of any battery operated device, please stay well within the specified temperature rating of that device. It’s quite important. An unplugged phone on a hot windshield is far more dangerous than a phone plugged in at 25˚C.

Treat Any Battery With Respect

So any battery can catch fire if mistreated. If operating correctly the BMS will try its hardest to stop you from mistreating it. The BMS generally works better when it has power, and plugging it in is the best way to insure it has power.

And so I reiterate my point from before: if we are unplugging our phones to make them last longer or to make them safer, then we are not demanding enough from the BMS nor the phone manufacturer. For both safety and longevity this is something that should be done for us.