Safety and storage
A drone battery is charged and stored inside a LiPo bag, a fire resistant pouch usually lined with woven glass fibre, and the bag's job is containment rather than prevention. The bag does not stop a cell failing. It keeps the flame and the burning debris off the carpet, so a fire in a bag stays a fire in a bag. That is a modest promise and the correct one.
The bag is one part of a routine with three parts: the pack is stored part charged rather than full, kept where it cannot set anything alight, and inspected before it is charged. Those three habits prevent almost every lithium polymer incident a flyer will be near. Drone batteries explains what the pack is; this page is about keeping it.
What a storage routine is protecting the pack from
The storage routine protects a lithium polymer pack from three things: its own state of charge, temperature, and mechanical damage. The first surprises people, because a pack left fully charged in a drawer degrades the whole time it sits there. A cell at its top voltage runs reactions a resting cell does not, and loses capacity whether it flies or not.
Temperature works both ways. Heat accelerates that ageing, so a pack in a car boot in July spends its life fast. Cold raises internal resistance, so a cold pack sags hard under throttle, and charging one is worse than flying it. A pack in from an unheated shed waits until it reaches room temperature.
The three numbers that decide how a pack is kept
The numbers that matter are three voltages per cell, and they describe a lithium polymer pack's whole life. Every storage decision is made against them.
Multiply by the cell count: a 3S is 12.6 volts full and about 11.4 volts at storage charge, a 4S is 16.8 volts full, a 6S is 25.2 volts full. Storage charge sits a little over half capacity, which is why these packs are kept part charged rather than empty. A flat pack is not a safe pack: a cell taken too low is damaged as surely as one taken too high.
How long a pack can sit full before it matters is a question of days rather than hours, and the direction is all that needs stating: the sooner it comes down to storage charge the better. Chargers have a storage mode that charges up to that level or discharges down to it, and Charging covers the settings. A 6S pack holds far more energy than a 1S micro pack and deserves more room around it.
Putting a pack away properly after a flight
To put a pack away properly after a flight, follow the sequence below rather than dropping it in the bag warm.
- Let the pack cool, and check the voltage while it does. A pack flown down hard needs bringing back up before it sits, since a low cell keeps drifting down.
- Inspect the pouch, corners and leads. Run a thumb over the flat faces: invisible swelling is often obvious to a hand.
- Run storage mode, if the pack will not fly within a day or two.
- Cap or tape the main connector so it cannot touch another connector or a set of keys. A shorted lead is the fastest way to start a fire in a bag.
- Stand packs upright and separated, not stacked where one failing cell heats its neighbours.
- Keep the container cool, dry and clear of anything that burns. A garage shelf away from paint and fuel beats a bedroom.
Caution
Do not store packs on or under something flammable, and do not seal them in an airtight metal box. A cell in thermal runaway produces gas as well as heat, and a container that cannot vent becomes a pressure vessel. A bag, an ammunition box with the seal removed, or a ceramic pot are all sensible. A biscuit tin on a wooden shelf is not.
What goes wrong, seen and unseen
What goes wrong divides in two, and the visible damage is the easy half. A swollen pouch, a dent, a scuff through the foil, a lead pulled at the root, a balance plug with a pin pushed back: any of these means the pack is finished. Swelling is never reversible, since the gas inside is the electrolyte breaking down.
The damage that is not visible is what makes the routine worth following. Four failures show no outward sign until well advanced, and all four are found by reading cell voltages before and after flights.
- Internal resistance rising with age, so the pack sags under throttle and lands warm while looking perfect.
- A single cell drifting from its neighbours, which shows only as a charger that keeps having to balance.
- A crush injury from a heavy landing that damaged the layers inside without breaking the foil outside.
- Lithium plating from charging the pack cold, which does its damage over many cycles rather than at once.
Caution
A pack that has swollen, been punctured or crashed hard goes outside, away from the building, in something that will not burn. Leave it a day, since a damaged cell can fail hours after the impact. A pack fire is not smothered and not reliably put out, because the cell releases its own oxygen as it burns: get people clear and call 999. Lithium packs never go in household waste.
Choosing a bag, a box or a tin
A fireproof bag is the common answer and it is adequate for the pack sizes a hobby flyer uses, provided it is treated as containment rather than insurance. These bags are a standard item, sold through model shops, general hardware retailers and online, in the single figures to low tens of pounds. Buy one big enough that the pack does not strain the seams.
| Container | Where it is good | Where it is not |
|---|---|---|
| Fire resistant pouch | Charging and storage of one to three packs | Large packs, or many together. The seams are the weak point |
| Steel ammunition box, seal removed | A collection on a hard floor | Useless if left airtight, and heavy |
| Unglazed ceramic pot | Isolating one suspect pack outdoors | No lid, no shelter from rain |
What is worth arranging is somewhere the container itself can stand: a hard, non flammable surface with space around it. A cheap bag on concrete beats an expensive box on a carpet. Care and cleaning covers the rest of the between flights routine.
Transport has its own rule and it is not ours to write. Lithium packs carried on a passenger flight go in cabin baggage, terminals protected, and airlines set a size limit above which a pack is refused. Those limits are published by the Civil Aviation Authority and by the airline, they change, and they are checked there rather than here.
Does your drone battery need a bag?
Yes, if the battery is a bare pouch with a balance lead, whatever its size. A 1S micro pack holds little energy, but it fails the same way as a large one and costs nothing to bag. A 4S or 6S pack should never be charged outside one.
Yes, with one adjustment, if the pack is a sealed unit for a ready to fly camera drone. Those have protection electronics inside and fail far less often, but the chemistry is the same: most discharge themselves towards storage charge after a set number of days. Box them regardless.
No, only in the sense that a bag is not the whole answer. A bag holding a fully charged pack, on a wooden shelf next to a can of solvent, is worse than no bag, because it feels like a precaution. LiPo against lithium ion sets out which chemistry you hold.