LiPo Battery Safety

Lithium polymer (LiPo) batteries power almost every combat robot. They pack a lot of energy into very little weight – which is exactly why they're dangerous when they're damaged, overcharged or run too low. A LiPo fire is violent, hard to put out, and gives off toxic smoke. This page covers how to charge, store, travel with and dispose of LiPos safely, and what to do if one catches fire. For choosing batteries, see Batteries.

Know your voltages

Chemistry	Nominal per cell	Fully charged	Don't go below	Storage
LiPo	3.7 V	4.2 V	3.0 V (about 3.3–3.5 V is a safer stopping point)	about 3.8 V
LiHV (high-voltage LiPo)	3.8 V	4.35 V	3.0 V	about 3.8–3.85 V
LiFePO4 (LiFe)	3.2–3.3 V	3.6–3.65 V	2.5–2.8 V	about 3.3 V

Sources: Oscar Liang's LiPo guide, Battery University, FRA Build Rules. Check your battery maker's specifications.

Taking a LiPo below about 3.0 V per cell can permanently damage it, and a damaged cell is more likely to fail later. Many ESCs have a low-voltage cutoff, but in combat robots it's usually disabled, so watch your run times.

Charging

Use a proper LiPo charger in balance mode, plugged into the battery's balance lead, so every cell is charged to the same voltage. Many events, such as NHRL and FRA events, require balance charging.[1]
Charge at 1C or less unless the manufacturer says otherwise – for example, 1.5 A for a 1500 mAh pack.
Never leave a charging battery unattended. Events take this seriously: NHRL gives demerits for it, and the FRA treats it as a serious breach that can get you removed from an event.
Charge in a fire-resistant container on a non-flammable surface, away from anything that can burn. Take the battery out of the robot to charge it, and check it first.
Check before you charge: if a pack is below its safe minimum voltage, puffy, dented, hot or has damaged wires, don't charge it.
If a battery gets hot or starts to swell while charging, stop, disconnect it, and move it somewhere safe in the open.[2]

LiPo bags and ammo cans

LiPo bags and metal boxes can slow down a fire and contain some of it, but they won't fully contain a serious LiPo failure – they buy you time, not safety.[3] Many builders use a steel ammo can for charging and storage. A tightly sealed box can build up pressure if a battery fails inside it, so builders commonly remove the rubber seal or add a vent.

Storage

Store LiPos at storage charge (about 3.8 V per cell, roughly 40–50% charged) if you won't use them for more than a few days. Most chargers have a "storage" mode. Batteries stored fully charged lose capacity much faster, especially when warm.[4]
Keep them somewhere cool and dry, in a fire-resistant container, away from anything flammable.
Protect the terminals and wires so they can't short.

In your robot

Mount batteries securely, with padding, where weapons and sharp edges can't reach them.
Protect the terminals and wiring against short circuits (required by the SPARC Robot Construction Specifications).
After a hard match, check your batteries for dents, puffing and damaged wires before you charge or use them again. Some events stop a match if a battery is exposed.

Damaged and puffy packs

A swollen ("puffed") LiPo can't be fixed. Retire it – don't charge it, use it, or puncture it.
At an event, put a damaged or swollen pack in a LiPo bag and move it somewhere ventilated, preferably outdoors, and let the event staff know.
Disposal: discharge the pack fully (for example with a discharger, or a resistor or bulb wired to it), then take it to a battery recycler or household hazardous waste site. Never put lithium batteries in the trash or curbside recycling.[5] In the US, The Battery Network (formerly Call2Recycle) lists drop-off sites; call ahead, since not every site accepts damaged batteries.
The old advice of soaking batteries in salt water is now generally discouraged as slow and unreliable.[6]

If a LiPo catches fire

Get people away and avoid breathing the smoke – LiPo smoke is harmful.
At events, the standard approach in the SPARC Arena Construction Best Practices is: put out any fires around the robot, then get the burning robot outdoors immediately, carried in a metal container with sand or on a fire-resistant cart, using an extinguisher to stop the fire spreading. Only once it's outside and no longer burning should anyone try to separate the battery.
SPARC recommends CO2 extinguishers at events because they don't damage robots like powder extinguishers do. Be aware that an extinguisher protects the surroundings but won't stop the burning cells themselves.
Outside of events, fire safety agencies advise cooling burning lithium batteries with large amounts of water if it's safe to do so – and calling the fire department for anything you can't control.[7]
Ventilate the area afterwards.

Travel

In the US, the FAA rules for air passengers (as of 2026) are:[8]

Spare LiPo batteries go in carry-on bags only, never checked bags, with the terminals protected from short circuits (for example in a LiPo bag or with taped connectors).
Batteries up to 100 Wh each are allowed.
Batteries from 101 to 160 Wh need airline approval, with a limit of two spares per person. Batteries over 160 Wh aren't allowed.
Damaged or recalled batteries aren't allowed.
Airlines can have stricter rules, so check with yours.

To work out watt-hours, multiply the battery's voltage by its capacity in amp-hours. For example, a 4S 1300 mAh pack is 14.8 V × 1.3 Ah ≈ 19 Wh. Most insect-class batteries are well under 100 Wh.

LiPo Battery Safety

Contents