Using LiPo and Li-ion batteries safely in DIY builds

A single lithium cell, whether a soft pouch (LiPo) or a hard can (Li-ion), works between roughly 3.0 and 4.2 V. It is safe to use if you do three things: charge it with a proper lithium charger, use a cell or module with a protection circuit, and stop at once if it swells or runs hot. This article collects the numbers you need and how to fit a battery into a case you printed yourself.

This site stays at low voltage. A small cell has a low voltage but stores enough energy to get hot enough to start a fire if it is shorted or damaged, so treat it with the same respect as a hot tool on the bench.

What each voltage means

Voltage (per cell)	Meaning
4.2 V	Full. Never charge past this.
3.7 V	Nominal: the average during use, printed on the cell
About 3.4 V	Considered empty; recharge
About 3.0 V	The protection circuit disconnects
Below 2.0 V for over a week	Battery University says discard it; do not recharge

These figures are for ordinary 3.7 V / 4.2 V cells. Other chemistries exist, such as cells that charge to 4.35 V or older 4.1 V types, and each needs a matching charger. Adafruit warns that the wrong charger voltage can damage a cell permanently or, at worst, start a fire. Check the label or datasheet every time.

Soft LiPo pouch vs hard Li-ion can

	LiPo (pouch)	Li-ion (hard can, e.g. cylindrical)
Casing	Thin, soft foil pouch	Hard shell
Weight and size	Light, thin, many shapes	Heavier, usually more capacity
Durability	Easily punctured, pressed or cut by sharp edges	Harder to puncture thanks to the shell
Good for	Small, thin, light toys	Builds that need more capacity

Same chemistry, same voltages, same charging rules. The difference is how much physical abuse they take.

Protection circuit and charger are not the same thing

• Protection circuit: a small board usually soldered to the top of the cell and taped in place. It disconnects on overcharge (about 4.2 V), over-discharge (about 3.0 V) and overcurrent (around 1–2C). Raw cells sold bare are not protected; check the datasheet or look for that board at the top of the cell.
• Charger: charges in two stages. First a constant current, then it holds the voltage at 4.20 V while the current tapers off, stopping when the current falls to about 3–5% of capacity. Lithium cells cannot take a trickle charge; the charger must cut off when full.

For DIY work, pick a charger module that states it has both a lithium charger and protection on the same board, or a microcontroller board with a battery connector and charger built in.

The charger and the protection circuit are two separate layers. If one fails, the other can still save you. Don’t remove either.

Setting charge current with C

C is the current equal to the capacity number. For a 1,000 mAh cell, 1C = 1,000 mA and 0.5C = 500 mA. Battery University gives 0.5C–1C as the usual charge rate, with manufacturers recommending 0.8C or less for longer life. Many charger modules set their current with a resistor on the board; check that module’s table.

Worked example:

1. A 400 mAh LiPo goes into a toy car.
2. The charger module ships set to 1,000 mA → 1,000 ÷ 400 = 2.5C, above the recommended range.
3. Target 0.5C = 400 × 0.5 = 200 mA. Swap the current-set resistor using the module’s table to get as close to 200 mA as possible without going over.
4. The constant-current stage takes roughly 400 ÷ 200 = 2 hours, plus a while longer for the constant-voltage tail.

One more number: energy = voltage × capacity. A 3.7 V, 1,000 mAh cell stores 3.7 × 1 = 3.7 Wh. Released as heat in a short time, even that small amount is enough to burn.

While charging

• Charge at room temperature. Never charge at or below freezing. Adafruit gives roughly 0–50 °C as the safe range.
• Battery University: the cell should stay cool while charging. A rise of about 5 °C near full is normal; if it climbs more than 10 °C, stop.
• Set it on a non-flammable surface, not on cushions, cloth or paper, and stay nearby while it charges.

Swelling, heat or hissing: stop

Battery University describes a failing cell as one that may hiss, bulge and leak a highly flammable electrolyte. What to do:

1. Stop charging and using it. Move it away from anything flammable onto a non-combustible surface.
2. Don’t assume unplugging the charger ends it. A damaged cell can keep going.
3. If there is smoke or a gas smell, ventilate and leave the room; the fumes are toxic. If there is fire or heavy smoke, call the fire service.
4. Never puncture, squeeze or try to “let the air out” of a swollen cell.

Disposing of a dead cell: tape over the terminals, bag each one separately, and take it to a household hazardous-waste or battery drop-off point. Never put it in ordinary rubbish (US EPA guidance).

Long-term storage

Battery University recommends storing at about 40% charge (40–50% is fine), which reads about 3.82 V per cell after 90 minutes of rest. Keep it cool; about 15 °C is recommended, and heat speeds up ageing. In Battery University’s table, a cell kept at 25 °C for a year retains about 96% capacity at 40% charge, but about 80% when stored full. Don’t leave batteries in a car in the sun.

Batteries in a 3D-printed case

A sealed box traps heat and leaves no room for a cell to expand. Good design habits:

• Leave space around the cell. Don’t clamp it tight with walls or the lid; a pouch cell must not be pressed.
• No sharp edges or screw tips pointing at it. Chamfer the inside corners and check that lid screws don’t reach the battery.
• Strain-relieve the wires so nothing pulls on the cell’s tabs; the joint to the protection board is fragile.
• Don’t charge inside a sealed case unattended. If it must charge in the case, add vents and stay close.
• Case material: some printed plastics soften at fairly modest temperatures. See the filament types overview, and size the clearances using printing a case that fits.

Common mistakes

• Using a raw cell with no protection and letting the board drain it below 3.0 V.
• Charging a small cell at a current meant for a big one (over 1C).
• Reversing polarity, or leaving bare wire ends that touch: an instant short.
• Carrying on with a swollen cell because “it still works”.
• Storing cells full in a hot place for months.

Sources

• Li-Ion & LiPoly Batteries: Voltages — Adafruit Learning System
• Li-Ion & LiPoly Batteries: Protection Circuitry — Adafruit Learning System
• BU-409: Charging Lithium-ion — Battery University
• BU-304a: Safety Concerns with Li-ion — Battery University
• BU-702: How to Store Batteries — Battery University
• Used Lithium-Ion Batteries — US EPA