When a pack is done
Oscar Liang lists the signs that a pack should be retired: swelling, physical damage, one or more cells that regularly end a flight far from the others, and a pack that gets extremely hot after flying. A pack that simply holds less charge and sags more can keep flying if you are happy with it. A puffed or damaged one should not be charged or flown again.
Discharge it slowly, to near zero
The goal is a pack with almost no energy left. Oscar aims for close to 0 V. A guide published by RC Battery (rcbattery.com) sets the minimum at below 2 V per cell, preferably 1 V per cell or lower. Tests by Empa, described further down, put the remaining energy of a lithium-ion cell below 2 V in the single-digit percent range.
Do it slowly. The same guide recommends a current well below 1C, ideally no more than 0.5C. For a 1500 mAh pack, 0.5C is 0.75 A. Some people go even slower, at 0.1C to 0.2C. Higher currents make swelling, cell rupture or fire more likely. Expect the pack to puff a little or get warm towards the end, which the guide describes as normal.
Wherever you do it, put the pack outdoors on something that cannot burn, RC Battery mentions a cinder block, a barbecue or a gravel path, somewhere a burning pack would do no serious damage.
With a charger
A hobby charger's LiPo discharge mode usually stops at around 3.0 V per cell. Oscar points out that this is too high for disposal. RC Battery describes a way around it on multi-chemistry chargers: disconnect the balance lead, choose the NiMH or NiCd discharge program, which goes down to about 1 V per cell, and set the current to 0.5C or less. Not every charger accepts this.
With a light bulb or resistors
An incandescent or halogen bulb is a simple load, and the bulb going dark tells you when it is nearly done. RC Battery gives the example of a 21 W, 12 V car tail light bulb, which draws about 1.7 A from a 3S pack. Oscar uses a halogen bulb and warns against LED bulbs: they stop conducting below a certain voltage and leave charge in the pack.
Power resistors work too. Oscar suggests 150 ohm, 2 W resistors for 2S and 3S packs and 47 ohm, 5 W ones for 4S and 6S.
After the bulb goes out, RC Battery suggests waiting another half hour and then measuring. Use a multimeter: Oscar notes that most battery checkers do not read reliably below 3 V per cell.
DJI and other smart batteries
DJI's battery safety guidelines say to put a battery in a recycling box only after a complete discharge, and never in normal household waste. If the battery's power button no longer works and it cannot be discharged, DJI says to contact a professional battery disposal or recycling company.
Salt water: slow and unreliable
The salt water bath is still common advice, but both guides argue against it. Salt water conducts, so the pack starts to discharge, but the current also corrodes the metal in the connector and the tabs inside the pack. RC Battery describes this as a race: if the tabs corrode through first, the discharge stops while the cells still hold charge, and a voltmeter on the main lead reads zero anyway. Oscar calls the method slow and ineffective.
Empa, the Swiss materials research institute, tested how used pouch cells from phones and laptops, some deliberately damaged, behave in water. Over 48 hours, intact cells in plain tap water dropped from about 4.0 V to between 0.4 and 1.75 V. Cells in salt water were on average less discharged, because their contacts corroded faster. The water also released gases and ended up as hazardous waste. That test describes workshop quarantine of damaged cells, not a method to use at home.
Do not puncture, cut open or smash a pack. Oscar calls that the most dangerous method of all, both because of the fire risk and because of the fumes.
Packs you cannot discharge
If the main lead reads no voltage on an intact pack, an internal connection may be broken. RC Battery suggests measuring each pair of neighbouring balance pins. If the cells show voltage there, you can discharge through the balance connector. If a cell is visibly damaged or some balance pins read zero, do not try to discharge it at all. Put the pack in a strong sealed plastic bag and take it to a hazardous waste collection point.
Before you hand it in
The US EPA recommends covering the terminals with non-conductive tape, ideally electrical tape, or putting each battery in its own plastic bag, so that loose batteries cannot short against each other or against metal. Do that even with a pack you have discharged.
Where to take it in Poland
The Polish act on batteries and accumulators treats a drone LiPo as a portable battery: sealed, can be carried by hand, and is not industrial or automotive. These rules apply:
- Not in household waste. Article 12 bans putting used batteries in the same container as other waste, and Article 78 sets a fine for doing it. The Ministry of Climate and Environment adds: not in the yellow bin for metal and plastic, and not in a street bin.
- PSZOK. Municipalities run selective waste collection points (PSZOK), and some also run mobile ones. Opening hours are on the municipality's website.
- Shops. A retailer selling portable batteries with a sales area over 25 m² must take used ones back free of charge, using a container. The law does not make this depend on buying something. Wholesalers and businesses that replace batteries have the same duty.
- Free of charge. Under Article 46, collection points accept used portable batteries without payment.
Smaller shops may take batteries back, but they are not required to. If you are not sure whether a damaged pack will be accepted, call the PSZOK first and say it is a puffed lithium polymer battery.
One more note: guides from abroad sometimes say that a fully discharged LiPo can go into normal household waste where local rules allow it. Polish rules do not allow it.
The EU Battery Regulation 2023/1542 has applied since 18 February 2024. Poland is preparing a new act on batteries and waste batteries to implement it. Until it passes, the 2009 act described above sets the rules for collection.