Inside a lithium cell

A single lithium-ion or lithium-polymer cell — an 18650, a pouch on the end of a JST plug, the flat thing out of an old phone — is chemically the same part in every package. It is full at 4.2 V, empty at about 3.0 V, and called 3.7 V because that is roughly where it spends its life.

Those three numbers are the entire safe operating window.

The curve, and why a voltmeter lies

Drag the slider. Between 90 % and 20 % of charge — which is most of the energy in the cell — the terminal voltage moves by about 400 mV. Seventy per cent of the charge, in less than half a volt, on a curve that is nearly flat.

That is why the "battery percentage" in most hobby projects is wrong. A 4.2 V cell does not fit on a 3.3 V ADC, so it is read through a 2:1 divider — which means a 12-bit converter resolves about 1.6 mV of cell voltage. That sounds like plenty until you notice 1.6 mV is about 0.3 % of charge in the flat region, and that the whole curve moves bodily downwards under load and again when it is cold. Read 3.80 V during a Wi-Fi burst and the cell might be at 60 % or at 85 %.

Phones solve this with a coulomb counter, which measures the charge going in and out and integrates it. Projects solve it by reading the voltage when the radio is off, smoothing it heavily, and showing four bars instead of a percentage.

Four ways to ruin one

Three of these four leave you holding a cell that still works. That is the whole problem with lithium: the damage accumulates invisibly, capacity quietly drops, internal resistance quietly rises, and the failure — when it comes — is not a dead project. It is a cell venting, on a desk, indoors.

Charging past 4.2 V. Lithium plates out as metal on the anode instead of slotting into the graphite. The plating is permanent and it grows.

Draining below about 2.5 V. The copper current collector starts dissolving into the electrolyte and re-plating wherever it likes, including across the separator. A cell left flat in a drawer for a month is usually finished, and recharging one that has been deeply discharged is the one operation where a sensible charger refuses.

Charging below 0 °C. The invisible one. Lithium cannot enter the graphite fast enough at low temperature, so it plates on the surface as metal spikes instead. Discharging in the cold is fine — you just get less. Charging is not.

Shorting it. An unprotected 18650 will deliver tens of amps into a screwdriver without complaint. That is a soldering iron.

C rates, in one paragraph

Currents for a cell are quoted as multiples of its capacity per hour. A 2500 mAh cell at 1C is 2.5 A; at 0.5C it is 1.25 A. Most single cells want 0.5C to 1C to charge and will discharge at 1C to 2C happily. A high-drain 18650 marked 20A is a different chemistry with less capacity, not a better version of the same thing.

What comes next

Every chip in the rest of this chapter exists to hold the cell inside 3.0 V to 4.2 V without you having to remember to. The next page is about the small board that does it as a last resort; the two after that are about the chips that do it as a matter of course.