Why the last twenty per cent of a charge takes so long

This looks at battery charging curve from the practical end — what holds up once conditions stop being ideal.

Two phases with different rules

A lithium cell is charged first at a constant current, during which its voltage rises steadily and charge accumulates quickly. When the cell reaches its maximum safe voltage the charger holds that voltage constant and the current it accepts falls away steadily.

The second phase can take as long as the first while adding far less charge, which is the entire explanation for the perceived slowdown. The transition point is typically somewhere around seventy to eighty per cent, which is why advertised times stop there.

Why the current has to fall

Charging moves lithium ions into the negative electrode, and near full there is progressively less room to accept them. Forcing current in anyway causes metallic lithium to deposit on the electrode surface instead of intercalating, which is permanent and dangerous.

In practice, those deposits reduce capacity and can eventually form structures that pierce the separator, which is the failure mode behind the most serious battery incidents. The tapering current is the charger avoiding this, not the manufacturer being cautious for the sake of it.

Temperature moves the whole curve

Charging speed is limited more aggressively as the cell warms, because heat and high voltage together accelerate degradation. Charging a cold cell is worse still: below roughly freezing, lithium plating occurs even at modest currents, so devices refuse or heavily limit charging when cold. This is why a phone left in a car overnight charges slowly or not at all until it warms up.

In practice, a device charging slowly in a hot room is protecting itself and the behaviour is correct.

Reading fast-charge claims

Times quoted as a percentage in a number of minutes almost always describe the constant-current phase from empty. The full charge takes considerably longer, and the difference between the marketing figure and the full time is often large. Very high charging powers are achieved by splitting the pack into two cells charged in parallel, which halves the current each cell sees.

That is a genuine engineering solution rather than a marketing trick, and it costs internal volume.

Trickle charging and staying at full

Once full, a charger stops and lets the cell rest, topping up occasionally rather than holding it at maximum voltage continuously. Time spent at high state of charge is itself a stressor, independent of charging current.

The short version: overnight charging optimisation features hold the cell partway and finish shortly before you usually wake, which targets this directly. They rely on a predictable routine and behave poorly when your schedule is irregular.

This is the general case; a specific device may behave differently by design.

What this means for habits

Charging in the middle of the range is fastest per minute and gentlest on the cell, so short frequent top-ups are better than deep cycles. A charge limit of around eighty per cent captures nearly all the practical capacity while avoiding the slow, stressful phase entirely. Charging to full occasionally is fine and helps the percentage estimate stay accurate.

In practice, charging fast when you need to and slowly when you do not is a better rule than always doing either.

The takeaway

The fast part is the first three quarters. The rest is chemistry refusing to be hurried.

Understanding the failure mode tells you more than the feature list does.