Why Does My Phone Security Camera's Battery Drop Even Though It Is Plugged In? Weak Chargers, Thin Cables, and What 'Charging Slowly' Really Measures

You set up an old Android phone as a security camera. It is plugged in. The lock screen says "Charging". You come back the next evening and the battery is at 61%. Two days later the phone is dead, and so is the recording.

Short answer: "Plugged in" is not the same as "gaining charge". A phone that records video with the screen off, keeps Wi-Fi up and maybe streams, can use power faster than a weak charger or a thin cable can deliver it. The battery then covers the difference, slowly, until it runs out. Android's "Charging slowly" label does not catch every case, because it is based on what the charger says it can supply, not on whether the battery is actually going up. The reliable check is a one-hour test at full workload, described below. The usual fix is a better charger and a shorter, thicker cable, plugged into a wall socket.

This guide is for anyone running a phone camera 24/7: a shop counter, a storeroom, a gate, a baby's room. It needs no special tools.

Why can a phone lose battery while it is charging?

Think of the battery as a water tank. The charger fills it from the top. The phone's work (camera, video encoding, Wi-Fi, mobile data) drains it from the bottom.

• If the charger fills faster than the phone drains, the level goes up.
• If they are equal, the level stays flat.
• If the phone drains faster, the level goes down, even with the cable connected.

A phone used normally is mostly idle with the screen off, so even a weak charger wins. A camera phone is different: it is never idle. It is working all day and all night. That is why a charger that "always worked fine" for overnight charging can lose the race when the same phone becomes a camera.

What does "Charging slowly" actually mean on Android?

Many people trust the lock-screen label. It is worth knowing exactly where it comes from.

In the Android Open Source Project (AOSP), the label is decided in a SettingsLib class called BatteryStatus. The function getChargingSpeed() reads two numbers that the phone's charging hardware reports about the charger — the maximum current and the maximum voltage — multiplies them, and compares the result with two thresholds (AOSP commit 28c6ad8):

if (maxChargingMicroWatt <= 0) {
    return CHARGING_UNKNOWN;
} else if (maxChargingMicroWatt
        < context.getResources().getInteger(R.integer.config_chargingSlowlyThreshold)) {
    return CHARGING_SLOWLY;
} else if (maxChargingMicroWatt
        > context.getResources().getInteger(R.integer.config_chargingFastThreshold)) {
    return CHARGING_FAST;
} else {
    return CHARGING_REGULAR;
}

The default slow threshold in AOSP is 5,000,000 microwatts, which the source comment describes as 1 A at 5 V — that is, 5 watts. The "fast" threshold is 7,500,000 microwatts, "1.5A @ 5V". If the charger reports no voltage, Android assumes 5 V (AOSP commit "Charging speed based on voltage"). Google has since moved to a stricter 20 W bar for the "fast" label (Android Authority), but the 5 W "slow" line is the one that matters here. The unit tests in the same commit make the line very concrete:

Three things follow from this, and all three matter for a camera phone:

1. The label is about the charger's maximum, not the phone's usage. A 5 W charger shows plain "Charging". It says nothing about whether 5 W is enough for camera + encoder + Wi-Fi.
2. It does not measure the cable. A thin or damaged cable loses voltage along the way (see below). The label can look normal while less power arrives.
3. Phone makers can change the thresholds and the wording. These are configurable resources. Samsung, Xiaomi and others may show different text. Treat the label as a hint, never as proof.

So "Charging slowly" is a useful warning when you see it. But not seeing it does not mean everything is fine.

The one-hour test that gives you the real answer

This is the test to run before you trust any camera-phone setup. It takes one hour and a pen.

1. Set up the phone exactly as it will run: camera recording with the screen off, Wi-Fi on, and live view or YouTube Live on if you use them.
2. Use the charger and cable you will actually use, in the socket you will actually use.
3. Note the battery percentage and the time.
4. Leave it alone for 60 minutes. Do not unlock it to check — turning the screen on adds load.
5. Note the battery percentage again.

How to read the result:

• Went up: the charger is winning. Good.
• Stayed the same (at 100% or a battery-protection limit like 80–85%): also fine; the phone is holding its level on charger power.
• Went down: the charger is losing. At that rate, work out how many hours the battery will last. If it dropped 4% in the hour, a full battery gives you roughly 25 hours before the phone dies. That is not a 24/7 camera.

If the room gets much hotter in the afternoon, repeat the test then. Heat makes charging slower on most phones, so a hot shelf can turn a "just enough" charger into a losing one. For more on heat and long-term battery wear, see Can You Leave an Old Android Phone Recording 24/7?

Why the cable matters as much as the charger

USB charging at the basic level uses 5 volts. Every cable has some electrical resistance in its wires, and the current has to go there and back. By Ohm's law, the voltage lost in the cable is current × resistance.

A simple illustration (example numbers, not a measurement of any specific cable): if a long, thin cable has 0.4 ohm of total resistance and carries 1 amp, it loses 0.4 V. The charger sends 5.0 V; the phone sees about 4.6 V. Many phones respond to a sagging voltage by drawing less current, so less power arrives — and the lock screen may still say "Charging".

What helps:

• Short and thick beats long and thin. A 3-metre cable to reach a high shelf is the most common cause of this problem. Move the charger closer (an extension board near the camera) instead of using a longer USB cable.
• Replace cables that are bent at the plug, loose in the port, or cheap freebies. Damage often sits inside the plug where you cannot see it.
• Clean the phone's port. Lint packed into the USB port stops the plug seating fully. Use a wooden toothpick gently, with the phone switched off.

Which charger should power a 24/7 camera phone?

Simple rules that work almost everywhere:

• Use a wall charger, not a computer USB port or a TV's USB port. A standard USB 2.0 port on a computer is specified for 500 mA. At 5 V that is 2.5 W — half of Android's 5 W "slow" line. It may keep an idle phone alive, but not a phone recording video all day.
• Use the phone's original charger, or one from a known brand, rated at least 2 A (10 W). A rating above what the phone needs is fine; the phone only takes what it can use. A charger that is too weak is the problem, not one that is too strong.
• Avoid wireless charging pads for this job. Wireless charging produces extra heat, and a camera phone already runs warm.
• Avoid the cheapest unbranded chargers. A fake charger may not deliver the current printed on its label, and it is the part most likely to fail when the mains voltage jumps.

In India: check the charger's R-number with the BIS CARE app

If you are in India, chargers sold legally carry a Registration (CRS) mark with an R-number printed on the label. The Bureau of Indian Standards' own BIS CARE app (free on Google Play, package com.bis.bisapp) has a feature called "Verify R-No under CRS": you enter the eight-digit R-number and it shows the registered manufacturer's details (BIS blog: BIS CARE app). The same app lets you lodge a complaint for "Misuse of Registration (CRS) Mark".

If the R-number does not return a registration, or the brand in the app does not match the brand on the charger, do not use that charger for a camera that must run day and night (India TV News guide). It takes two minutes and costs nothing.

In other countries, look for the local safety mark your market uses, and buy from a shop you can go back to.

For developers: reading the real number in code

If you build a camera or monitoring app, the lock-screen label is not something you can rely on either. The public API gives you something better: the battery's actual current.

BatteryManager.BATTERY_PROPERTY_CURRENT_NOW returns the instantaneous battery current in microamperes. The documentation's sign convention is that positive values mean net current entering the battery from a charge source and negative values mean the battery is discharging; if the device does not provide the value, you get Integer.MIN_VALUE (BatteryManager reference).

val bm = context.getSystemService(BatteryManager::class.java)
val microAmps = bm.getIntProperty(BatteryManager.BATTERY_PROPERTY_CURRENT_NOW)

val pluggedIntent = context.registerReceiver(null, IntentFilter(Intent.ACTION_BATTERY_CHANGED))
val plugged = pluggedIntent?.getIntExtra(BatteryManager.EXTRA_PLUGGED, 0) ?: 0

if (microAmps != Int.MIN_VALUE && plugged != 0 && microAmps < 0) {
    // Plugged in, but the battery is still draining: the charger is losing.
    // Worth surfacing to the user instead of a silent "Charging" state.
}

Treat it as a hint, not a verdict: an instantaneous reading moves from second to second, and you should confirm the sign and unit on the actual devices you support. Average it over a few minutes, and cross-check against the change in battery percentage over an hour before you show a warning. The percentage test above remains the ground truth.

How this fits with Background Camera RemoteStream

Background Camera RemoteStream turns an Android phone into a camera that records with the screen off and keeps the video on the phone — no cloud account. It can also show a live view in a browser on your local network through its built-in web server, and stream to YouTube Live; per the Play listing, browser live view and YouTube Live are Pro features, so check the listing for the current free vs Pro details.

Screen-off recording already removes the biggest power user on any phone — the display. But no app can make a weak charger stronger. Run the one-hour test once with your real setup, and you will know whether your camera can actually run 24/7, or whether it is slowly running out.

Related reading:

• How to Check an Old Phone's Battery Before Using It as a Security Camera — the battery is your backup in a power cut, so check its health first.
• What Happens to a Phone Security Camera During a Power Cut or an Internet Outage?
• What Does It Actually Cost to Run an Old Phone as a Security Camera for a Year?

Quick answers

My phone says "Charging" but the battery goes down. Is the phone broken?
Usually not. The charger or cable cannot keep up with the camera's power use. Try a stronger wall charger and a short cable, then repeat the one-hour test.

Does "Charging slowly" mean my charger is fake?
No. In AOSP it means the charger reported less than about 5 W. Many genuine old chargers are 5 W or less. It only means the charger is weak for heavy use.

Can I power the camera phone from a laptop or TV USB port?
Not reliably. A standard USB 2.0 port is specified for 500 mA, which is 2.5 W at 5 V. Use a wall charger.

Will a bigger charger damage the phone?
A good charger with a higher rating is fine; the phone draws only what it can use. The risk comes from fake or damaged chargers, not from a higher genuine rating.

Where do I get the app?
Google Play · superfunicular.com

Sources: AOSP SettingsLib BatteryStatus commit 28c6ad8 · AOSP commit 0c859ae (slow threshold 5,000,000 µW, "1A @ 5V") · Android Authority on the Android 15 fast-charging threshold · Android BatteryManager reference · BIS CARE app (Bureau of Indian Standards) · India TV News: verify a charger with BIS CARE