What happens to the battery of an old phone plugged in 24/7

Reusing the Android sitting in a drawer is a good idea in every respect but one: that phone will stay plugged in day and night, usually on the nightstand of someone who is asleep. And its battery has several years and hundreds of charge cycles behind it.

That isn't a footnote. It's the part of the idea to examine before you mount anything.

The workload is not what wears the battery

The first assumption most people make is that running a camera with detection all day will cook the phone. We measured that, and it isn't what happens.

With the room empty — which is most of the day — the watching work takes roughly 1% of one core. With a person in frame it rises, but it stays a fraction of what a game or a video call costs. A phone on watch does light, steady work, which is the gentlest regime there is.

Temperatures we measured on a Galaxy S20+ running like that:

• 20-23 °C across a full night of watching, in a cool room
• 29-30 °C watching around the clock in an office, plugged in the whole time

Our warning threshold sits at 45 °C, so in a temperate climate there's plenty of headroom. But that headroom is eaten by the room, not by the app: at 30 °C ambient — an ordinary summer across much of the world — the same phone will run considerably higher, somewhere in the 38 to 42 °C range. That last figure is an expectation, not a measurement: when we have data from a home in a hot climate we'll publish it the way we publish everything else.

What does wear it: being full, permanently

One number from our measurements says more than the temperature does: the test phone lives at 100% charge, status "full", permanently. It never drops.

A lithium battery ages faster the longer it sits near 100%, and faster still if it's also warm. Both together are exactly the situation of a phone used as a camera. Put differently: normal phone use, dropping to 20% and climbing back, is kinder than leaving it plugged in and untouched.

When an aged cell degrades, one of its known failure modes is producing gas and swelling. It's a physical problem and it's visible: the phone stops being flat.

How to check it, in thirty seconds

Before you leave the phone in place, set it face up on a flat table and look at it from the side, at eye level.

• If it rocks when you push one corner, the back is bulging.
• If a gap appears between screen and frame, the battery is pushing from inside.
• If the back cover is separating on its own.
• If one edge of the screen sits higher than the other.

Any of those signs means the same thing: that phone doesn't go on anyone's nightstand. Stop charging it, don't squeeze it to test, never puncture it, and don't put it in household waste — take it to an electronics recycling point.

If it's healthy today, the check doesn't end there. It's worth setting it on the table again every couple of months. A phone nobody ever picks up is a phone nobody ever looks at.

The three settings that actually change something

• Turn on the manufacturer's charge limit. Samsung calls it "Protect battery" and caps charging at 85%. Several brands have an equivalent under another name, in the battery menu. It's the only measure that addresses the cause: if it never reaches 100%, it never spends months there.
• Let it breathe. Hard, ventilated surface: no pillow, no bedsheet, no drawer, nothing stacked on top. Take off a thick case if it has one. Away from direct sun and from a heater.
• Decent cable and charger. The original, or a known brand. A cheap charger that runs hot adds heat exactly where you don't want it.

And one more, the right answer that almost nobody follows: the best phone for this is one with a healthy or freshly replaced battery. Replacing the battery in an old model costs a fraction of a camera and removes the whole problem.

When the drawer phone simply isn't suitable

Some cases are a plain no, and it's better said early:

• It's already swollen or deformed, or the back doesn't close flush.
• It shuts down on its own with charge left, or gets hot just from charging.
• It has a replacement battery of dubious origin.
• It will sit in a hot, unventilated room with no charge limit available.

In any of those, a slightly newer phone from another relative's drawer beats insisting on that one.

What we do about it

Velia turns an Android you already own into a care aid whose AI runs on the phone and provides best-effort alerts. That means we are proposing continuous plugged-in use, so battery condition, heat, airflow and periodic checks are part of a responsible setup.

We ask for it at the end of setup, right when you're about to leave the phone in position: look at the battery, rest it on a flat table, and turn on the charge limit if your brand has one. It blocks nothing — it's a warning, and there's a record that it was shown.

And we keep watching. The phone on duty reads its own battery temperature every minute, and the app shows it, along with how many minutes it has been hot. If a phone has spent hours above the threshold, that gets said rather than quietly passed over.

Since we're talking about nightstands, here's something we say just as plainly: at night, with the lights off, the app sees nothing. We measured it, we published it, and the app says so while it's happening.