Why your phone charges slowly in the car

Charging is a chain

Four things have to agree before current flows fast: the power source, the charger, the cable and the phone. Each one advertises a maximum, and the actual rate is the lowest of the four. That is why buying a bigger charger often changes nothing at all.

The cable, which is the most common cause

USB-C cables look identical and are not. Without an E-marker chip — a small identifying component in the connector — a USB-C cable is limited to 3 A. At 20 volts that is 60 watts, and that ceiling applies no matter what the charger can supply or what the phone can accept.

Nothing tells you. There is no warning, no indicator and no visible difference. A cable that came free with something, or one bought on price, is very likely capped, and it will sit in your car quietly costing you half your charging speed for years.

Cables also fail progressively rather than suddenly. The strain point just behind the connector flexes every time you plug in, and a partly-broken conductor still charges — just slowly, and sometimes only at certain angles. If wiggling the cable changes the charging indicator, the cable is finished.

Anker publishes 140W, a 35,000-bend lifespan and a 175 lb pull tolerance on the 765, which is the kind of figure worth having in a car where a cable gets yanked. We compared the options in best USB-C cables for a car.

The port you plugged into

Cars have had USB ports for years, and many of them were designed to read music from a flash drive rather than to charge a modern phone. Those ports are frequently limited to well under what a phone will accept, and on some cars the port in the console and the port in the armrest are not even the same.

A 12V socket charger bypasses all of that. The socket itself is a much higher-current circuit — typically fused at 10 to 15 A, which is 120 to 180W of headroom — and a decent charger converts that properly.

Heat, which the phone will not tell you about

Phones reduce charging current as they warm, and they do it silently. There is no “charging slowly because I am hot” message — the percentage simply climbs more slowly than it should.

In a car the heat sources compound: sun through glass, a screen at full brightness for daylight visibility, a processor running navigation, and — if you use one — a wireless charging pad adding its own losses. A phone in a windshield mount in July is in close to the worst thermal environment it ever experiences.

Moving the phone into vent airflow is the single most effective fix, which is one of several arguments for a vent position covered in vent vs dashboard vs windshield. Switching from wireless to a cable on a long drive is the second.

Protocol mismatch, especially on Android

Fast charging is negotiated. The phone and charger agree on a voltage and current using a protocol, and if they do not share one, the phone falls back to a slow default.

The version of this that catches people out is PPS. Several Samsung phones reach their advertised speed only over Programmable Power Supply, an extension of USB Power Delivery — and a charger can be perfectly good, high-wattage and PD-capable while not supporting PPS at all. The phone charges; it just never goes fast.

Anker publishes PD and PPS explicitly for the 335, and UGREEN publishes PPS for its 130W unit. If you carry a Samsung, that line on the spec sheet is the one that matters. We went through this properly in best car chargers for Android.

Two devices sharing one charger

A “67W” charger is very often 67 watts total, split dynamically between its ports. Plug in a second device and your phone's share drops, sometimes by half, and sometimes the split is not even — some chargers drop the first port to a much lower ceiling the moment a second is occupied.

Per-port figures are the ones to read, and they are the ones makers are least keen to headline. Anker's 335 and 535 share a marketing figure and differ entirely on per-port behavior, which we set out in Anker 335 vs 535. The general arithmetic is in how many watts do you need.

The unglamorous one: lint

Phone charging ports collect pocket lint, and repeated plugging compacts it into a solid pad at the bottom of the port. The connector then cannot seat fully, which produces intermittent charging, slow charging, or a cable that only works at one particular angle.

1. Power the phone off completely
2. Use a wooden or plastic toothpick, never metal, and never anything conductive
3. Scrape gently along the sides and lift the debris out rather than pushing it deeper
4. Do not use compressed air, which drives it further in

It is remarkable how often this is the entire problem, and how rarely anyone checks it before buying a new charger.