Why your phone suddenly charges slower with the same charger

Nothing has changed: the same adapter, the same cable, the same socket. Yesterday the phone filled in an hour and today it wants three. There is no error, no warning, and usually no notification beyond a laconic line about charging slowly. The explanation is almost always the same: fast charging is not a property of the charger, it is an agreement between the charger, the cable and the phone, and when the agreement cannot be reached, both sides fall back to the slowest thing they both understand.

How the negotiation works

A modern USB-C charger starts at a basic, safe level that any device can accept. Before it delivers anything more, the two ends talk. The phone states what voltages and currents it can take; the adapter states what it can supply; they settle on the highest profile they share. Some of this runs over the dedicated communication channel in USB Power Delivery, some over manufacturer-specific schemes layered on top. The result is a contract, and it is renegotiated whenever conditions change.

Several things can break that contract without breaking anything visible:

• The cable. A cable without the right internal wiring cannot carry the negotiation at all, and a cable rated for a lower current will be held to it. Cables degrade — a bent connector or a damaged conductor drops the pair back to the basic profile. This is the most common cause by a wide margin, and the cheapest to test.
• The port. Lint compacted into the phone's port stops the connector seating fully. Partial contact is enough for a trickle and not enough for a negotiated profile.
• The adapter. Multi-port chargers split a shared budget. Plugging a second device into the same adapter can drop the first one to a lower profile instantly.
• Temperature. Both ends monitor heat. A warm battery, a warm room or a phone under load will make the phone ask for less, and the reduction can be dramatic.

Why heat matters more than anything else

Charging is an exothermic process, and lithium cells age faster when they are hot. Every phone therefore has a thermal policy that reduces charge current as the battery warms, and the policy is aggressive by design. A phone charging under a pillow, in a car in the sun, or while the screen is on and a game is running will charge slowly no matter what is plugged into it.

The same policy explains why the last portion of a charge is always slower. Lithium cells are filled in two phases: a constant-current phase that does most of the work, and a constant-voltage phase where current tapers as the cell approaches full. The taper is chemistry, not marketing, which is why a headline charging figure is nearly always quoted to a partial state of charge rather than to full.

Battery protection features

Many phones now learn your routine and deliberately hold the charge below full overnight, completing it shortly before you normally wake. Others cap the charge at a user-set ceiling, or slow the rate when the phone is plugged in for long periods. These are all worth leaving on — they are the single most effective thing you can do for cell longevity — but they do make the charge time look worse. If your phone has started behaving this way after an update, look in the battery settings before you blame the hardware. The wider question of what actually preserves a cell is covered in reading battery health.

Testing it in the right order

Change one thing at a time, and change the cable first. Borrow a cable known to work with the same adapter and the same phone. If the speed returns, the cable was the problem and no further investigation is needed.

If it does not, try the adapter on its own with nothing else plugged into it. Then look into the port with a light — compacted lint looks like a flat grey floor where the connector should seat, and it comes out with a wooden toothpick and patience, with the phone powered off. Then let the phone cool to room temperature, leave the screen off, and try again.

Only when all of that has failed is the battery or the charging circuit a reasonable suspect, and at that point the useful evidence is the charge rate at a low state of charge with a cool battery, because that is when the phone should be asking for the most.

What not to conclude

A slower charge is not proof of a worn battery. A worn cell holds less energy, so it often reaches full sooner than it used to, not later. Slow charging with normal runtime points at the delivery chain. Fast charging with short runtime points at the cell. The two symptoms are almost opposites, and treating them as the same thing is how people end up replacing a healthy battery to fix a frayed cable.