Two failures that look the same

Before anything else, work out which one you have, because the fixes are completely different.

If it is as strong as it ever was, but for less time — the cell has lost capacity. This is normal ageing and it is the usual case.

If the top setting is weaker than it used to be, and it cuts out earlier — the cell is not the whole story. Either the motor or its bearing has stiffened, which draws more current for less output, or the cell's internal resistance has risen enough that it cannot supply the peak current any more.

The second one explains the thing that confuses people most: a device that dies at 40 per cent on the display, then starts working again after ten minutes on the charger. The display is not lying about the charge level. What has happened is that under load, the voltage of a tired cell sags below the cutoff, so the device shuts down while there is still energy in it. Take the load off and the voltage recovers. That is a cell near the end, not a software bug.

Why "drain it completely" is the wrong advice

The instruction to fully discharge before recharging comes from nickel-cadmium cells, which developed a genuine memory effect. Lithium-ion does not have one. What it has instead is a strong dislike of being taken to zero.

Discharging a lithium cell to its minimum does two things. It puts the cell in the state where the electrolyte breaks down fastest, and if it sits there — a toy left flat in a drawer for months — the copper current collector can start to dissolve, which permanently reduces capacity and, in the worst case, creates an internal short.

Practically: charge it when it is low, do not wait for it to die, and do not leave it flat for months.

Why leaving it on the charger is also wrong

The other half of the problem is the opposite behaviour. Many inexpensive devices have a simple charging circuit with no proper end-of-charge management, so once the cell reaches full it either trickles or just sits there at maximum voltage.

A lithium cell held at full charge is the most chemically stressed it ever gets. Capacity loss roughly doubles for every 10°C, and holding at full charge is a separate multiplier on top of that. A device that lives on a charger and only comes off for an hour at a time will lose capacity noticeably faster than one that is charged and then unplugged.

The target range, if you want to be deliberate about it, is roughly 20 to 80 per cent. That is not something you can manage precisely on a toy with four indicator LEDs, but "unplug it when the light says full" is most of the benefit.

The claimed runtime is not a lie, exactly

A "60 minute" claim is measured under specific conditions: brand new cell, lowest setting, room temperature, and sometimes with the motor unloaded.

Current draw is roughly proportional to intensity, so a high setting can be two to three times the draw of a low one. Cold reduces what a lithium cell can deliver, which is why it lasts less on a winter walk than in a warm room. And an ageing cell reduces all of it.

None of this is deception in the legal sense. It is a claim made at the most favourable point of a wide range, and every category does it.

The contacts are the underrated suspect

Before concluding the cell is dying, clean the charging contacts.

Magnetic chargers and the flat pads they sit on are both exposed metal, and both accumulate a film of sweat, skin oils, dust and lubricant residue. That film adds resistance. A charger with a simple voltage-based end-of-charge detection sees the voltage rise and concludes the cell is full, when what actually happened is that the current dropped because the connection got worse.

The result is a device that spends its life at 50 per cent charge, which feels exactly like a failing battery.

Wipe both surfaces with a dry cloth or a little isopropyl alcohol on a cotton bud, then charge. This fixes a genuinely surprising share of "the battery is going" complaints.

Charging from the wrong source

A modern phone charger supplies 2A or more and is fine. What is not fine is a laptop USB port in a low-power state, an old hub, or a car socket that drops out when the engine starts. Those deliver so little current that the device may take many hours to charge, and if it can run while charging, it may draw more than it is receiving, which produces a charge that never completes.

If a device charges fine from a wall adapter and badly from a laptop, the device is fine.

When to stop using it

• It swells. Stop immediately and do not charge it, puncture it or put it in household waste. A swollen lithium pouch is a fire risk, and it needs to go to a household waste recycling centre that takes batteries.
• It dies within minutes of a full charge. The cell has effectively failed.
• It gets hot while charging. Warm is normal. Hot enough to notice through the casing is not.
• Capacity is under about 60 per cent of what it was. It will keep working, badly, for a while, but the failure mode is unpredictable.