# LiPo battery safety for robots: charging, storage and fires

> Learn LiPo battery safety on a robot: charge and store packs, read cell voltages, fuse and wire safely, choose a BMS and know what to do when a pack swells.

Canonical: https://roboticsengineering.in/lipo-battery-safety-charging-robots/ | Published: 2026-10-28 | Updated: 2026-10-28 | Author: Jignesh Panchal

**A lithium polymer pack stores enough energy to weld a wrench to its terminals.** Treat it as a fuel tank with wires, not as a battery you can abuse like a AA cell. LiPo battery safety starts with treating the pack as a fuel tank with wires, not as a battery you can abuse.

LiPo packs are the default choice for mobile robots because they deliver high current from a light package. They also fail violently when overcharged, over-discharged, punctured or charged cold. This guide covers the numbers you need, the charging routine that keeps a lab safe, the wiring that prevents shorts and the warning signs that mean a pack should be retired.

## The four numbers that matter

| Term | Meaning | Rule of thumb |
| --- | --- | --- |
| Nominal voltage | 3.7 V per cell | A 3S pack is 11.1 V nominal |
| Full charge | 4.2 V per cell | 12.6 V for a 3S pack |
| Storage voltage | About 3.8 V per cell | Best state for a pack sitting idle |
| Cut-off | 3.0 to 3.3 V per cell | Stop the robot before this, under load |
| Capacity | mAh | 2200 mAh delivers 2.2 A for one hour |
| C rating | Discharge multiple | 2200 mAh at 20C is 44 A continuous claim |

C ratings from hobby suppliers are optimistic. Use half the printed figure for continuous load and check temperature after a run. A pack that is hot to the touch after normal driving is being asked for more current than it can give comfortably.

## Charging routine that does not burn the lab

- Use a balance charger, never a dumb power supply, and connect the balance lead every time.
- Charge at 1C or less. A 2200 mAh pack charges at 2.2 A maximum, and slower is kinder.
- Charge on a non-flammable surface, away from paper, solvent and the robot.
- Never charge unattended, and never charge overnight.
- Charge at room temperature. A pack that came in from a hot vehicle should cool first.
- Check cell voltages before charging. A pack with one cell far below the others should not be charged until you understand why.

A balance charger brings every cell to the same voltage, which is what keeps the pack healthy. A pack charged without balancing drifts over time, and the weakest cell goes below the cut-off first while the total voltage still looks fine. LiPo battery safety depends on a fixed charging routine, so follow the same one every time.

## Discharge limits and the cut-off trap

Running a pack flat damages it permanently. Cells that sit below about 3.0 V per cell lose capacity and can develop internal shorts. The trap is measuring voltage under load: a pack at 3.3 V per cell under load may recover to 3.6 V when the robot stops, so design the cut-off for the loaded voltage, not the resting voltage.

```
// voltage divider on an Arduino, 3S pack example
const float R1 = 100000.0;
const float R2 = 22000.0;
const float CELLS = 3.0;

float packVoltage() {
  int raw = analogRead(A0);
  float v = raw * (5.0 / 1023.0);
  return v * ((R1 + R2) / R2);
}

bool batteryLow() {
  return (packVoltage() / CELLS) < 3.35;   // loaded threshold
}
```

Calibrate the divider against a multimeter before trusting it. Resistor tolerance of one percent plus the ADC reference error can easily be a few hundred millivolts on a 3S pack, which is the difference between a safe cut-off and a damaged cell.

## Wiring that prevents a dead short

- Fuse the main positive lead close to the pack. A 30 A blade fuse costs little and saves the harness.
- Use connectors rated for the current, and never run a pack through a breadboard.
- Keep the balance lead capped and never let it touch the main terminals.
- Add a precharge resistor before big capacitor banks, or the connector will spark and pit.
- Route the pack so a crash cannot crush the cells against a sharp chassis edge.
- Fit an emergency stop that cuts motor power, not just the logic supply.

Most robot fires start with a short across the main leads, not with a charger fault. A fused lead and a strain-relieved connector solve a large share of the risk.

## Single cell, protected pack, or BMS

| Option | Protection | Where it fits |
| --- | --- | --- |
| Bare LiPo pack | None in the pack | Robots with a balance charger and cut-off in firmware |
| Protected 18650 pack | Per-cell board, over-current and over-discharge | Simple robots, low current |
| BMS with 3S or 4S board | Cell balancing, cut-off, sometimes temperature | Custom packs, ebike style builds |
| Tool battery with adapter | Built-in BMS and charger | Quick prototypes, heavier robots |

A BMS protects the pack but does not replace a balance charger. Many cheap BMS boards only cut off and never balance, which is why packs with them still drift out of balance over months.

## Storage, heat and monsoon humidity

Store packs at about 3.8 V per cell in a cool dry place, inside a fire-resistant bag, away from sunlight. In Indian summer, a closed car or a metal workshop shelf in direct sun can exceed the safe storage temperature, and heat accelerates the chemical ageing that leads to swelling.

- Check storage voltage every two to three months and top up to 3.8 V per cell.
- Do not store a fully charged pack for weeks; it ages faster at 4.2 V.
- Keep packs out of humid cabinets where condensation forms on the tabs.
- Never store a pack that is swollen, even if it still works.

## LiPo battery safety warning signs and retirement

| Sign | What it means | Action |
| --- | --- | --- |
| Puffed or soft pack | Gas from cell damage | Retire, discharge safely, do not charge |
| One cell much lower | Weak or damaged cell | Investigate, likely retire |
| Hot during gentle use | High internal resistance | Retire, it is near end of life |
| Charger terminates early | Cell imbalance or charger fault | Check per-cell voltages |
| Sweet or solvent smell | Electrolyte venting | Move to open air, do not charge |

Retire packs by discharging them slowly to about 1 V per cell with a resistive load in a safe place, then take them to an e-waste collection point. Do not puncture a pack to discharge it, and do not put a lithium pack in household waste.

## If a pack starts smoking

Move away and let it burn in a safe place if it is already venting. A small LiPo fire produces dense smoke and hydrogen fluoride, so do not breathe the fumes and do not handle the pack with bare hands. A fire-resistant bag or a bucket of dry sand is the practical response in a workshop without a lithium fire extinguisher.

Water is not the right answer for a live pack, and neither is a standard CO2 extinguisher, which cools but does not stop the reaction. Prevention, spacing and a bag are worth more than any extinguisher in a small lab.

Related: [robot battery guide](https://roboticsengineering.in/robot-battery-guide/), [battery management systems](https://roboticsengineering.in/battery-management-system-robotics/), [how to power a robot](https://roboticsengineering.in/how-to-power-a-robot/), [buck converters for robots](https://roboticsengineering.in/buck-converter-12v-to-5v-robots/), [mobile robot docking](https://roboticsengineering.in/mobile-robot-charging-docking/).

**Can I charge a LiPo with a normal 12 V adapter?**

No. A LiPo needs a charger that limits current, stops at 4.2 V per cell and balances the cells. A fixed adapter will overcharge the pack and can start a fire.

**What voltage should I stop the robot at?**

Around 3.3 to 3.5 V per cell measured under load, which protects the pack from deep discharge. Check the loaded voltage, because the pack recovers several tenths of a volt when the motors stop.

**Is a swollen LiPo still usable?**

No. Swelling means gas has formed inside the cell, which is a sign of damage. Retire the pack, discharge it slowly in a safe place and take it to e-waste collection.

**Do I need a BMS if I use a balance charger?**

Not for charging, but a BMS adds over-discharge and short-circuit protection for the robot. Many cheap boards do not balance, so keep using the balance charger either way.

**How do I store packs between projects?**

At about 3.8 V per cell, in a cool dry place inside a fire-resistant bag. Check the voltage every few months and never store a fully charged or swollen pack.

Charge rate and storage voltage affect usable capacity, covered alongside the [pack sizing maths](https://roboticsengineering.in/robot-battery-sizing-calculation/).

## Sources and further reading

- [Battery University](https://batteryuniversity.com/article/bu-205-types-of-lithium-ion) — lithium cell chemistry, charging and ageing behaviour.
- [OSHA](https://www.osha.gov/) — battery handling and fire safety guidance for workplaces.
- [Arduino analogRead reference](https://docs.arduino.cc/language-reference/en/functions/analog-io/analogRead/) — ADC behaviour used in the voltage divider example.