18650 power: capacity, BMS and safety

The 18650 is the most widely used lithium-ion "can" on the planet: it is what laptop batteries, power banks, cordless drills and EV packs are built from. For our purposes it is the perfect building block for off-grid power: a handful of 18650s will happily keep an MMDVM hotspot running in the field or serve as a spare bank for a radio. But Li-ion does not forgive carelessness — so this is not only a conversation about capacity, it is first and foremost about safety. Read all the way down to the red-bordered callout; it is not there for decoration.

What is an 18650, anyway

The name is just a size: a cylinder 18 mm in diameter and 65 mm long (the trailing zero means "cylindrical shape"). Inside is lithium-ion chemistry. The basic figures of a single cell that you must memorize:

• Nominal voltage — 3.6–3.7 V (average over the discharge).
• Full charge — 4.2 V. Above that is dangerous.
• Lower discharge threshold — usually 2.5–3.0 V. Below that the cell degrades, and may even die.

In other words, the working "window" of a single cell is roughly 3.0 to 4.2 volts. All power design revolves around never stepping outside that window.

Real capacity: where sellers lie

Capacity is measured in milliamp-hours (mAh). As of 2025–2026 the honest ceiling of a single 18650 is around 3500 mAh, with top-tier branded cells reaching ~3600 mAh. The typical spread on the market:

• 1500–2000 — weak/old cells;
• 2100–2600 — standard;
• 2700–3200 — high capacity;
• 3300–3500 — the practical maximum.

BMS / protection board — we never build without one

A bare cell does not monitor itself in any way. Between the cells and the load you put a BMS (Battery Management System), or at least a simple protection board (PCM/PCB). Its job is to watch voltage and current and instantly break the circuit with MOSFETs if something goes wrong:

• Overcharge protection — cuts charging when it goes past ~4.25 V. Overcharging Li-ion leads straight to fire.
• Over-discharge protection — disconnects the load at the lower threshold (about 2.5 V) so the cell does not "drive down to zero".
• Short-circuit protection and overcurrent — breaks the circuit on a short, when current floods into heating up.
• Overheat monitoring — on the more serious boards.

A BMS is chosen by the number of cells "in height" (in series): 1S — one cell (3.7 V), 2S — two (7.4 V), 3S — three (11.1 V), and so on. Parallel cells (several cells "in width" for capacity) do not change the S number — that is simply more mAh at the same voltage step.

Balancing and matching cells

A pack of several cells uses only identical, matched cells: one model, similar capacity, similar internal resistance, ideally the same "age". You must not mix old and new, different brands, or different capacities: during charge/discharge they drift apart in voltage, one cell gets overloaded, and the whole pack degrades faster. For multi-section packs use a balancing BMS (with a balancer) — it pulls the cells toward each other so they age evenly.

Charging: the CC/CV profile

Li-ion is charged with the CC/CV algorithm — "constant current / constant voltage":

1. CC (constant current) — the charger holds a fixed current (say, 1 A) while the cell voltage rises smoothly.
2. CV (constant voltage) — once 4.2 V is reached, the voltage is held fixed and the current gradually falls almost to zero. At that point the cell is considered full.

In DIY this is handled by a dirt-cheap module built on the TP4056 chip (often with USB-C): it implements CC/CV up to 4.2 V on its own and frequently comes paired with DW01 protection (against over-discharge/short circuit). That is for a single cell (1S). Multi-section packs need a charger for the right number of S — you cannot charge a 2S/3S pack with an ordinary USB module.

How to get 5 V for a hotspot

A Raspberry Pi and an MMDVM hotspot run on 5 V, while a single 18650 gives 3.0–4.2 V — "wandering" volts both below and above five. You cannot connect them directly. The options:

Option 1: 1S + a boost converter

One cell → charge/protection board (TP4056+protection) → a boost DC-DC that turns 3.0–4.2 V into a stable 5 V. A popular module is the MT3608 (input 2–24 V, output up to 28 V, current up to 2 A, comfortably ~1 A in practice). You must set the output with the trimmer to exactly 5.0 V on a multimeter before connecting the Pi — otherwise you can fry the board.

Option 2: a ready-made UPS module for the Pi

There are all-in-one boards: a holder for 1–4 18650 cells, CC/CV charging, protection and a 5 V boost straight to the Pi pins/USB. Convenient for a stationary setup and for an uninterruptible supply, but module quality varies — look at the output current and the presence of protection.

Safe-build checklist

1. Only branded cells with honest capacity (2.0–3.5 Ah), verified with a tester.
2. In the pack — identical, matched cells.
3. A mandatory BMS/protection board for the right number of S, with a balancer for multi-section packs.
4. CC/CV charging, no higher than 4.2 V per cell; for multi-section packs — a proper balancing charger.
5. A 5 V converter set with a multimeter, with current headroom for your Pi.
6. Insulated terminals, short-circuit protection, heat monitoring; a damaged cell goes in the trash.