Li-ion batteries in flashlights: how they work and how to care for them

Almost every flashlight today runs on a lithium-ion cell. It is the best compromise between capacity, weight and price we have so far – but it is also chemistry with its own rules. Know them and the cell lasts years. Do not, and two winters later you will notice by the halved runtime.

A Li-ion cell has no single voltage but a range. Fully charged it sits at 4.2 V, nominally it is counted as 3.7 V, and as it discharges it falls towards 3 V, where the protection circuit cuts it off. That fall is why a light loses output towards the end – the driver can no longer draw enough current for turbo and steps itself down.

There is one more thing worth knowing. Capacity in mAh alone does not tell you how much energy is in a cell. Multiply it by voltage and you get watt-hours – the unit in which anything can be compared.

A cell's designation is not a model name but its dimensions in millimetres. An 18650 is 18 mm across and 65 mm long, a 21700 measures 21 × 70 mm, a 26650 is 26 × 65 mm. More volume means more energy – and a bigger, heavier light.

• 18650 – the most widespread format. The FL5 at 3,350 mAh runs on it, as do the FL6 and FL7 at 2,000 mAh and the FL4 car light.
• 21700 – a third more energy in the same class. Used by the HL1 headlamp at 4,000 mAh and the FL2 at 3,000 mAh.
• 26650 – the fattest of the three; in the FL3 it gives 3,500 mAh.

The format is worth remembering on lights with a removable cell. The FL1 takes four of them, the FL2's cell is explicitly replaceable – with those you can buy a spare and swap it in the field. The HL2, by contrast, has a built-in cell, the price of its 66 × 25 × 31 mm module; but it charges in an hour and a half.

• Heat. The fastest route to lost capacity. A car cabin in summer sun easily reaches sixty degrees, and a Li-ion cell degrades an order of magnitude faster at that temperature than at room temperature. A light in the glovebox is fine; one on the dashboard is not.
• Deep discharge. The protection circuit cuts the cell off, but if you then leave it discharged for six months, the voltage keeps falling through self-discharge. Below a certain point no charger will bring it back.
• Storing it fully charged. A fully charged cell ages faster than a half-empty one. When storing a light for the season, aim for around 40–60 % and somewhere cool.

The first point counts double for a light that sits in a car all year – we covered it separately in the article on what a car flashlight needs to do.

• Run the lowest level that does the job. The difference is brutal: the FL7 gives 2,050 lumens on turbo for 3 hours, but up to 42 hours at lower levels. The HL1 does 2,000 lm on Max for four hours, and 200 lm in Eco for up to 230.
• In frost carry the light in an inside pocket. A cold cell delivers less current and the light steps itself down; once warm, the output returns.
• Charge before the cell empties completely. There is no need to run it down until the light shuts itself off.
• Keep the port under the flap clean and dry. The flap only seals when closed – a quoted IP rating always assumes it is shut.

The watt-hour figure has one more use, because air transport rules are based on it. Airline policies start from a 100 Wh limit, below which you need no approval – and all our lights are far under it; even the most powerful FL1 has 32.6 Wh across all four cells.

Spare cells always belong in cabin baggage, never checked luggage, with the contacts taped or in a case. Check the specific conditions with your carrier before travelling, because they differ.

A Li-ion cell's life is counted in cycles, and one cycle is not one visit to the charger – it is the total consumption of one full capacity. Discharge to half and top up and you have used half a cycle. Two such halves make one cycle.

End of life is not the moment the cell stops working but when it falls to eighty percent of its original capacity. Until then the light works entirely normally, only the runtime shortens – which is why most people notice it not gradually but suddenly, once, out in the field.

In practice this means that frequent short top-ups do not harm a cell, rather the opposite. Charging from eighty to a hundred as you go is gentler than regularly running it to shutdown.

Cells come in two versions. A protected one carries a small board watching over overvoltage, deep discharge and short circuits – you can tell it by being two to three millimetres longer. An unprotected one has none and relies on the light's own electronics.

When buying a spare cell, stick to what the light came with. And watch the length: a protected cell may not fit a tight tube, while an unprotected one in the wrong light loses the only safeguard it has.

It equally makes no sense to buy cells with suspiciously high capacity. For the 18650 format today's realistic maximum is around 3,500 mAh; anything above that on the wrapper is usually just printed.

Most of our lights charge directly over USB-C, which is the more convenient option – nothing to take apart. The time then depends on capacity: the HL2 at one thousand mAh is done in about an hour and a half, the FL7 at two thousand in three hours, the FL3 in five, and the FL1 with four cells takes around nine.

The FL2 takes the other route and comes with a separate cell charger. It looks like a step backwards, but it has an advantage: you charge the cell outside the light, so with a second cell you keep going without a pause – and its IP68 rating does not have to rely on a port flap.

Charge at room temperature. Let a cell you have just brought in from the cold warm up first; charging a chilled Li-ion harms it more than the frost itself.

• The light steps down from its highest mode after a few minutes, even fully charged and not hot.
• The battery indicator drops in jumps – from full to half within moments.
• Runtime is roughly half what the light delivered when new.
• The cell is swollen or something has leaked from it. There is nothing to consider here: stop using such a cell immediately and take it for recycling, not to the bin.

To sum up, choose the cell according to how you want to use the light. A replaceable 18650 or 21700 makes sense wherever you are far from a socket for long – take a second cell and you keep going. A built-in battery with USB-C is more convenient for daily carry. And whichever you have, three rules apply to both: do not leave it in heat, do not leave it discharged, and do not store it fully charged. The rest of flashlight care we wrote up in a separate article.